WO2006094510A1

WO2006094510A1 - Fm-cw radar

Info

Publication number: WO2006094510A1
Application number: PCT/DK2006/000139
Authority: WO
Inventors: Peder Richardt Pedersen; Fernando Casanova Galeano
Original assignee: Weibel Scientific A/S
Priority date: 2005-03-11
Filing date: 2006-03-10
Publication date: 2006-09-14

Abstract

There is provided a method and a system for radar detection of one or more objects. One or more types of radar signals are transmitted for successive signal time periods, with at least a first set of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal. Radar signals reflected from one or more objects present in a detection range of the radar system are received and corresponding transmitted and reflected radar signals are mixed to produce corresponding beat signals. The Fourier transform of the obtained beat signals is taken to thereby obtain a corresponding set of frequency spectrum data, and a number of peak frequencies are determined based on the obtained frequency data. At least three determined peak frequencies including at least one peak frequency from each of the first set of three successive signal time periods are being paired or associated to thereby obtain a first set of associated peak frequencies. It is preferred that each of the successive signal time periods holds only one type of transmitted radar signals, and that the types of transmitted radar signals comprises both FM-CW signals and CW signals.The transmission of radar signals may include a plurality of sets of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal, and a plurality of sets of associated peak frequencies may be obtained, where each set of associated peak frequencies include at least one determined peak frequency from each of three successive signal time periods belonging to a set of three successive signal time periods. One or more object track records may be initiated, where the initiation of an object track record is at least partly based on information relating to three peak frequencies belonging to a selected set of peak frequencies and representing all three signal time periods of the corresponding set of three successive signal time periods. Furthermore, an initial object velocity and/or an initial object distance or range may be determined based on the information of a set of three associated peak frequencies being used for initiating a corresponding object track record.

Description

FM-CW RADAR

FIELD OF THE INVENTION

The present invention relates to a method and a system for radar detection of an object. More particularly, the invention relates to radar detection of an object by transmitting one or more types of radar signals for successive signal periods of time, with at least a first set of three successive signal time periods comprising a signal time period wherein the transmitted signal is a frequency modulated continuous wave, FM-CW, radar signal. The type of transmitted radar signals may also include continuous wave, CW, radar signals.

DESCRIPTION OF THE PRIOR ART

A number of radar detection systems are known, especially within the area of providing automobiles with radars for road traffic control.

In U.S. Pat. Nos. 5,731 ,778 and 5,751 ,240 is described an FM-CW radar, which is suitable for automotive anti-collision systems. This radar outputs a radar signal in the form of a triangular wave whose frequency is increased at a given rate and decreased at a given rate. Two receivers receive a wave reflected from the same target to produce corresponding beat signals and take the Fourier transforms of the beat signals to determine peak frequency components thereof showing peaks in a frequency spectrum. The receivers also determine phases of the peak frequency components and selects at least one from the peak frequency components in a frequency-rising range wherein the frequency of the radar signal is increased and at least one from the peak frequency components in a frequency-falling range wherein the frequency of the radar signal is decreased, which selected peak frequencies show substantially the same phase to pair them for determining the distance to and relative speed of the target based on the frequency of the paired peak frequency components. However, for the system described in US Pat. Nos. 5,731,778 and 5,751 ,240 there is no transmission of a CW radar signal, whereby the system relies on the pairing of peak frequency components from the frequency-rising range and frequency-falling range, which might leave the system without any signal from the tracked object in conditions of heavy clutter. In U.S. Pat. No. 5,325,097 is described a road vehicle radar system for discriminating between hazard and non-hazard targets within a predetermined zone. The described system uses a pair of frequency modulated continuous wave radar cycles, FM-CW, and a single continuous wave cycle, CW, in the generation of radar quantities for measuring target range and apparent target velocity. It is preferred to use a triangular FM-CW radar wave with an increase in frequency during a first cycle and a decrease in frequency during a second cycle. A FM-CW Doppler quantity may be determined from the received FM-CW radar signals and a CW Doppler quantity may be determined from the received CW radar signal. The Doppler quantities correspond to target velocities, and from the obtained FM-CW and CW Doppler quantities it is determined whether the target is a hazard or non-hazard target. In U.S. Pat. No. 5,325,097, both a CW signal, which gives information on the target velocity only, and a FM-CW, which holds information relating to the target velocity and distance, are used, thereby reducing the ambiguity when determining target velocity and distance. The FM-CW and CW signals are transmitted at different time-cycles, and the target observation is divided into three time-cycles. However, the radar system described in U.S. Pat. No. 5,325,097 does not make use of more than one receiver and there is no determination of phase differences or phases of peak frequency components. Furthermore, the issue of determining the range and velocity of multiple objects simultaneously is not addressed either, since the system is intended to provide detection only of the most prominent object in sight.

Thus, there is a need for an improved radar detection system capable of providing unambiguous tracking information of one or more objects also in heavy clutter conditions. A solution to such a radar detection system is provided by the system of the present invention.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of radar detection of one or more objects, said method comprising: transmitting one or more types of radar signals for successive signal time periods, with at least a first set of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal; receiving reflected radar signals reflected from one or more objects present in a detec- tion range of the radar system; mixing corresponding transmitted and reflected radar signals to produce corresponding beat signals; taking the Fourier transform of the obtained beat signals to thereby obtain a corresponding set of frequency spectrum data; determining a number of peak frequencies based on the obtained frequency data; and pairing or associating at least three determined peak frequencies including at least one peak frequency from each of the first set of three successive signal time periods, thereby obtaining a first set of associated peak frequencies.

Here, each of the successive signal time periods may hold only one type of transmitted radar signals. However, it is also within an embodiment of the invention that the one or more types of transmitted radar signals comprise FM-CW signals and CW signals.

It should be understood that for the present invention the wording "signal time period" or "signal period of time", refers to a time period in which only one type of radar signal is transmitted.

It is preferred that the transmitted signal of at least one of said first set of three successive signal time periods is an up or down modulated ramp FM-CW signal.

It is within an embodiment of the invention that the transmission of radar signals includes a plurality of sets of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal, and wherein a plurality of sets of associated peak frequencies are obtained, each set of associated peak frequencies including at least one determined peak frequency from each of three successive signal time periods belonging to a set of three successive signal time periods.

It is preferred that a set of associated peak frequencies comprises one and only one determined peak frequency from each of three signal time periods of a set of succes- sive signal time periods.

The method of the invention may further comprise initiating one or more object track records, said initiation of an object track record being at least partly based on information relating to three peak frequencies belonging to a selected set of peak frequencies and representing all three signal time periods of the corresponding set of three succes- sive signal time periods. Here, an initial object velocity and/or an initial object distance or range may be determined based on the information of a set of three associated peak frequencies being used for initiating a corresponding object track record.

It is within an embodiment of the method of the invention that an object track record is updated for a selected signal time period based on a determined peak frequency corresponding to said selected signal time period. Here, the radar signal being transmitted within said selected signal time period for which the object track record is updated may be a FM-CW signal or a CW signal. Preferably, an initiated object track record may be updated by use of a tracking algorithm incorporating a Kalman filter. It is also within an embodiment of the method of the invention that an object velocity and/or an object distance or range is determined based on the information of an updated object track record.

It is within an embodiment of the invention that the reflected radar signals are received via a plurality of radar signal receivers arranged in the same plane or along a line, said plurality of receivers having at least two receivers arranged along a first receiver direction. Here, the plurality of receivers may have at least two receivers arranged along the first receiver direction and at least two receivers arranged along a second receiver di- rection, said first receiver direction being different to the second receiver direction. Preferably, the first and second receiver directions are substantially perpendicular to each other.

Preferably, the method of the invention further comprises the step of detecting, based at least partly on corresponding radar signals received by the receivers along the first receiver direction, one or more time or phase differences relating to a first object angular direction. The method of the invention may also comprise the step of detecting, based at least partly on corresponding radar signals received by the receivers along the second receiver direction, one or more time or phase differences relating to a sec- ond object angular direction.

According to an embodiment of the invention, the detection of a phase difference may comprise determining a phase difference based on at least two Fourier transformed outputs representing received radar signals corresponding to at least two receivers arranged along the same receiver direction, said received radar signals corresponding to the same transmitted radar signal. It is preferred that the detection of phase differences may comprise determining a number of phase differences for Fourier transformed outputs corresponding to a number of determined peak frequencies.

It is within an embodiment of the invention that for the one or more object track records having information relating to one or more determined peak frequencies, the method further comprises holding information of detected phase differences corresponding to the one or more detected peak frequencies, thereby holding information relating to first and/or second object angular directions corresponding to the detected phase differ- ences. It is preferred that for a selected track record, information relating to one or more determined peak frequencies are hold as a function of time. It is also preferred that information relating to detected phase differences corresponding to one or more determined peak frequencies are hold as a function of time.

According to an embodiment of the invention, then for a set of three successive signal time periods, the transmitted types of radar signals are FM-CW signals, with each of the three successive signal time periods holding an up or down modulated ramp FM- CW signal, and wherein a first pair of associated peak frequencies are selected based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal. Here, the third associated peak frequency may be determined based on the determined first pair of associated peak frequencies and determined peak frequencies of the remaining signal time period of said set of three successive signal time periods.

According to another embodiment of the invention, then for at set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with the set of three successive signal time periods having two signal time periods of CW signals and one signal time period of an up or down ramp FM-CW signal, and wherein a first pair of associated peak frequencies are selected based on deter- mined peak frequencies of two periods of the set of three successive signal time periods having a CW signal. Here, the third associated peak frequency may be determined based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a ramp FM-CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

According to a further embodiment of the invention, then for at set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with the set of three successive signal time periods having one signal time period with a CW signal and two signal time periods of a similar type up or down ramp FM-CW signal, and wherein a first pair of associated peak frequencies are selected based on determined peak frequencies of two periods of the three successive signal time periods having a similar ramp type FM-CW signal. Here, the third associated peak frequency may be determined based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining time period of said set of three successive signal time periods having a CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

According to an even further embodiment of the invention, then for a set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with three successive signal time periods having one signal time pe- riod with a CW signal and two signal time periods of opposite type up or down ramp FM-CW signals. Here, the method may further comprise determining FM-CW object velocities, Vy, corresponding to each pair or a number of pairs of determined FM-CW up ramp peak frequencies, f_fm-upi, and FM-CW down peak frequencies, f_fm-_dwj, comparing the obtained FM-CW object velocities, vy, to determined CW object velocities, said de- termined CW object velocities being determined based on determined CW peak frequencies, matching FM-CW object velocities, Vy, and CW object velocities, and forming a set of associated peak frequencies corresponding to the three peak frequencies of the matching FM-CW object velocity, Vy, and CW object velocity.

According to embodiments of the method of the invention for which three associated frequencies have been determined, it is preferred that the determined associated three peak frequencies are used for forming an object track record. For methods of the invention comprising the use of a plurality of receivers, it is preferred that beat signals are produced for each of the plurality of receivers. Here, the Fourier transform of beat signals representing the same signal time period are summed for each receiver, to thereby obtain the corresponding set of frequency spectrum of data.

According to an embodiment of the invention, the receivers along the first receiver direction are vertically arranged and a time or phase difference detected by the receivers along the first receiver direction relates to an elevation phase difference corresponding to the angle of elevation of an object, and wherein the receivers along the second receiver direction are horizontally arranged and a time or phase difference detected by the receivers along the second receiver direction relates to an azimuth phase difference corresponding to the azimuth angle of an object.

For methods of the invention comprising the use of a plurality if receivers, it is within an embodiment of the invention that the plurality of radar signal receivers comprises at least four receivers with at least a first and a second of said receivers arranged along the first receiver direction and with at least a third and a fourth of said receiver arranged along a line being parallel to the first receiver direction, and with the first and third of said receivers arranged along the second receiver direction and with the second and the fourth of said receivers arranged along a line in parallel to the second receiver direction. Here, the detection of a first direction phase difference may comprise determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and third receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the second and fourth receivers. Furthermore, the detection of a second direction phase difference may comprise determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and second receivers and a summation of the Fourier trans- formed outputs representing received radar signals corresponding to the third and fourth receivers.

According to an embodiment of the invention the transmitted radar signals are transmitted via at least a first radar signal transmitter, said first transmitter being arranged at a distance in relation to the plurality of radar receivers. According to the present invention there is also provided a radar system for detection of one or more objects, said system comprising: one or more radar wave transmitter for transmitting one more types of radar signals for successive signal periods of time, with at least a first set of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal; one or more radar wave receivers for receiving reflected radar signals reflected from one or more objects present in a detection range of the radar system; one or more signal mixers for mixing corresponding transmitted and reflected radar signals to produce corresponding beat signals; one or more signal transformers for taking the Fourier transform of the obtained beat signals to thereby obtain a corresponding set of frequency spectrum data; one or more peak detectors for detecting or determining a number of peak frequencies based on the obtained frequency data; and pairing or associating means for pairing or associating at least three determined peak frequencies including at least one peak frequency from each of the first set of three successive signal time periods, thereby obtaining a first set of associated peak frequencies.

Here, the transmitter(s) may be adapted to transmit only one type of radar signals. However, it is also within an embodiment of the present invention that the transmitter(s) is/are adapted to transmit FM-CW signals and CW signals.

It is within a preferred embodiment that the radar wave transmitter(s) is/are adapted for transmitting an up or down modulated ramp FM-CW signal for at least one of said first set of three successive time periods.

It is within an embodiment of the system of the invention that the transmitter(s) is/are adapted for transmission of radar signals including a plurality of sets of three successive signal time periods with each set comprising a signal time period with the transmitted signal being a FM-CW radar signal, and wherein the peak detectors are adapted for detecting a plurality of sets of associated peak frequencies, each set of associated peak frequencies including at least one determined peak frequency from each of three successive signal time periods belonging to a set of three successive signal time periods.

According to an embodiment of the system of the invention the pairing or associating means may be adapted for obtaining a set of associated peak frequencies comprising one and only one determined peak frequency from each of the three signal time periods of a set of successive signal time periods.

The system of the invention may further comprise means for initiating one or more ob- ject track records, said initiation of an object track record being at least partly based on information relating to three peak frequencies belonging to a selected set of peak frequencies and representing all three signal time periods of the corresponding set of three successive signal time periods. Here, the system may further comprise means for determining an initial object velocity and/or an initial object distance or range based on the information of a set of three associated peak frequencies being used for initiating a corresponding object track record. The system of the invention may also further comprise means for updating an object track record for a selected signal time period based on a determined peak frequency corresponding to said selected signal time period. Here, the transmitters) may be adapted for transmitting a FM-CW signal or a CW sig- nal within said selected signal time period for which the object track record is updated. It is preferred that the system of the invention further comprises means for updating an initiated object track record by use of a tracking algorithm incorporating a Kalman filter. It is also preferred that the system of the invention further comprises means for determining an object velocity and/or an object distance or range based on the information of an updated object track record.

According to an embodiment of the invention the system of the invention may further comprise a plurality of radar signal receivers for receiving the reflected radar signals, said plurality of radar signal receivers being arranged in the same plane or along a line, and said plurality of receivers having at least two receivers arranged along a first receiver direction. Here, the plurality of receivers may have at least two receivers arranged along the first receiver direction and at least two receivers arranged along a second receiver direction, said first receiver direction being different to the second receiver direction. Preferably, the first and second receiver directions are substantially perpendicular to each other. It is within an embodiment of the invention that the system of the invention further comprises one or more phase detectors for detecting, based at least partly on corresponding radar signals received by the receivers along the first receiver direction, one or more time or phase differences relating to a first object angular direction. The system may also comprise one or more phase detectors for detecting, based at least partly on corresponding radar signals received by the receivers along the second receiver direction, one or more time or phase differences relating to a second object angular direction.

For systems of the invention having one or more phase detectors, it is preferred that a phase detector is adapted for determining a phase difference based on at least two Fourier transformed outputs representing received radar signals corresponding to at least two receivers arranged along the same receiver direction, said received radar signals corresponding to the same transmitted radar signal. Here, a phase detector may be adapted for determining a number of phase differences for Fourier transformed outputs corresponding to a number of determined peak frequencies.

For systems of the invention with one or more object track records having information relating to one or more determined peak frequencies, the radar system may further comprise means for holding information of detected phase differences corresponding to the one or more determined peak frequencies, said information holding means thereby holding information relating to first and/or second object angular directions corresponding to the detected phase differences.

For systems of the invention with one or more track record, it is preferred that the system comprises means for holding information relating to one or more determined peak frequencies corresponding to a track record as a function of time. It is also preferred that the information means for holding information of phase differences is adapted to hold information relating to detected phase differences corresponding to one or more determined peak frequencies as a function of time.

For systems of the invention having one or more phase detectors, it is preferred that a phase detector is adapted for detecting a time or phase difference based on corre- sponding reflected radar signals being received simultaneously by at least two receivers arranged along the first receiver direction or by two receivers arranged along the second receiver direction.

According to an embodiment of the system of the invention the radar wave transmit- ter(s) is/are adapted for transmitting a set of three successive signal time periods having FM-CW signals only, whit each of the set of three successive signal time periods holding an up or down modulated ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak fre- quencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal. Here, the peak frequency associating means may be adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies and determined peak frequencies of the remaining signal time period of said set of three successive time periods.

According to another embodiment of the system of the invention the radar wave transmitters) is/are adapted for transmitting a set of three successive signal time periods having both CW and FM-CW types of radar signals, with the set of three successive signal time periods having two signal time periods of CW signals and one signal time period having an up or down ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak frequencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a CW signal. Here, the peak frequency associating means may be adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a ramp FM-CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three succes- sive signal time periods.

According to a further embodiment of the system of the invention, the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having both CW and FM-CW types of radar signals, with the set of three succes- sive signal time periods having one signal time period with a CW signal and two signal time periods of a similar type up or down ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak frequencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal. Here, the peak frequency associating means may be adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

According to an even further embodiment of the system of the invention, the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having both CW and FM-CW types of radar signals, with the set three succes- sive signal time periods having one signal time period with a CW signal and two signal time periods of opposite type up or down ramp FM-CW signals. Here, the radar system may further comprise means for determining FM-CW object velocities, Vy, corresponding to each pair or a number of pairs of determined FM-CW up ramp peak frequencies, f_fm-upi. and FM-CW down peak frequencies, ff_m-dwj. and for comparing the obtained FM- CW object velocities, Vy, to determined CW object velocities, said determining means further being adapted for determining CW object velocities based on determined CW peak frequencies, and for matching FM-CW object velocities, Vy, and CW object velocities, and for forming a set of associated peak frequencies corresponding to the three peak frequencies of the matching FM-CW object velocity, Vy, and CW object velocity.

For systems of the invention for which three associated peak frequencies have been determined, it is preferred that the means for initiating or establishing an object track record is adapted for using said determined associated three peak frequencies for forming an object track record.

According to an embodiment of the system of the invention, the system may comprise signal mixers for producing beat signals for each of the plurality of receivers. Here, the radar system may further comprise means for summing for each receiver the Fourier transform of beat signals representing an equal signal time period, to thereby obtain the corresponding set of frequency spectrum of data. For systems of the invention having receivers along the first and/or second receiver directions, it is preferred that the receivers along the first receiver direction are vertically arranged and a time or phase difference detected by the receivers along the first receiver direction relates to an elevation phase difference corresponding to the angle of elevation of an object. It is also preferred that the receivers along the second receiver direction are horizontally arranged and a time or phase difference detected by the receivers along the second receiver direction relates to an azimuth phase difference corresponding to the azimuth angle of an object.

It is within an embodiment of the system of the invention that the plurality of radar signal receivers comprises at least four receivers with at least a first and a second of said receivers arranged along the first receiver direction and with at least a third and a fourth of said receivers arranged along a line being parallel to the first receiver direc- tion, and with the first and third of said receivers arranged along the second receiver direction and with the second and the fourth of said receivers arranged along a line in parallel to the second receiver direction. Here, the phase detector for detection of a first direction phase difference may be adapted for determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and third receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the second and fourth receivers. Furthermore, the phase detector for detection of a second direction phase difference may be adapted for determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corre- sponding to the first and second receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the third and fourth receivers.

The system of the present invention also covers radar systems wherein the transmitted radar signals are transmitted via at least a first radar signal transmitter, said first transmitter being arranged at a distance in relation to the plurality of radar receivers.

Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1a, 1b and 1c are block diagrams illustrating a transmitter and a receiver of a combined CW and FM-CW radar system according to an embodiment of the present invention,

Fig. 2 shows a radar wave receiver according to an embodiment of the invention having 4 receive antenna channels,

Figs. 3a and 3b are graphs showing the relative frequency spectrum of transmitted and received radar wave according to an embodiment of the invention,

Figs. 4 shows examples of radar transmission signal combinations according to the present invention,

Figs. 5a and 5b are graphs showing relations between waves transmitted and received by a radar system and beat signals for a static target and a moving target, when the transmitted signal is an up or down ramp modulated FW-CW radar signal,

Figs. 6a, and 6b are block diagrams illustrating the processing of received radar signals using a radar system with 4 receive antennas according to an embodiment of the present invention,

Figs. 7a and 7b illustrate peak frequencies for received CW signals and FM-CW signals obtained by the processing illustrated in Fig. 6,

Fig. 8 illustrates a phase comparator for determining elevation and azimuth phase differences from received radar signals according to an embodiment of the present inven- tion,

Fig. 9 is a flowchart illustrating a tracking routine for a track record holding information relating to paired or associated peak frequencies of a radar detected object, Fig. 10 is a flowchart illustrating a routine for obtaining paired or associated peak frequencies of a radar detected object, and

Fig. 11 shows an object velocity table being part the processing of received FM-CW radar waves for a triangular shaped FM-CW waveform according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1a is a block diagram showing a transmitter 110 and Fig. 1b is a block diagram showing a receiver 120 of a combined CW and FM-CW radar system according to an embodiment of the present invention. The transmitter 110 has a modulator, MOD, controlling the output frequency of the voltage controlled oscillator, VCO, 101. Output from the modulator, MOD, is either an up-ramp, a down-ramp or a fixed value. The voltage controlled oscillator VCO outputs a corresponding signal close to 10 GHz, which is either frequency modulated or fixed in frequency. The frequencies of the output signal are sweeping over a range of 1-150 MHz in the frequency modulated mode. The VCO signal has a sweeping time or ramp repetition period T_r in the range of 10-100 mS. The frequency modulated output signal of the VCO may be a ramp modulated signal having equal up-ramp and down-ramp time periods, or the VCO signal may have different up- ramp and down-ramp time periods. According to a first preferred embodiment, the VCO signal is a ramp modulated signal or a fixed frequency signal having a repetition period T_r of 20 mS, and a bandwidth BW of 50 MHz.

Output from the voltage controlled oscillator, VCO, is fed into a band pass filter, BPF, in order to remove unwanted frequency components. The output signal of BPF, LO, is either a continuous wave, CW, radio frequency signal or a frequency modulated- continuous wave, FM-CW, radio frequency signal, which output signal is amplified by an amplifier, 102, and emitted as a CW or FM-CW radar signal via an antenna 103.

The receiver channel 120 includes an antenna 121 for receiving reflected radar signals with the output of the antenna being fed to a band pass filter 122, where the output of the filter 122 is amplified via an amplifier 123. The output signal of the amplifier 123 is a radio frequency signal, RF, about the 10 GHz, which is fed to a mixer, IQ MIX. Here, IQ MIX mixes the transmitted CW signal, represented by the signal LO, with the received RF signal during periods of time of CW signal transmission. The low frequency components of the output contain the beat signals relating to the velocities of the objects reflecting the CW signal. Other frequency components are removed by the following band pass filters 124, 125. The IQ MIX mixes the transmitted FM-CW signal, repre- sented by the signal LO, with the received RF signal during periods of time of FM-CW signal transmission. The low frequency components of the output contain the beat signals relating to the distance and velocity of the objects reflecting the FM-CW signal. Other frequency components are removed by the band pass filters 124, 125.

An example of an IQ mixer is shown in more details in Fig. 1c. Here, the IQ mixer 130 has two mixers 131, 132, each of which has as input the received RF signal and the transmitted CW or FM-CW signals, represented by the signal LO. Via a phase shifter 133 the LO input is phase shifted about 90 degrees before being input to the mixer 132, when compared to the in-phase LO input to mixer 131. The resulting outputs are the in-phase output I from mixer 131 and the quadrature output Q from mixer 132. The mixers 131 and 132 shifts the input signals from the radio frequency range down to low frequencies, so that the output signals I and Q are in the 0 to 1 MHz range.

For the IQ mixer 130 of the receiver 120, the outputs are filtered by corresponding band pass filters, 124, 125 to thereby obtain the outputs I, Q.

When the transmitted signal is a CW signal, each of the frequency components of the signals at the outputs I and Q corresponds to a beat signal resulting from the frequency difference between the transmitted CW signal and the signal reflected by an object with a radial velocity v with respect to the antenna.

This frequency difference is originated by the Doppler effect induced by the velocity of the object, being the relation between its radial velocity relative to the sensor and the frequency of the beat signal given by: /_^ = 2— (1)

where f_cw is the the frequency of the beat signal (the Doppler shift), and λ_cw the wavelength of the transmitted CW signal. A spectral analysis of the signals at the outputs I and Q serves to determine the amount and frequency values of the beat signals contained in them. This is achieved in the preferred embodiment of the invention by digitising the signals at the outputs I and Q, grouping them into blocks, and performing an FFT (Fast Fourier Transform) to each block of samples. Each sample block must be aligned to beginning of the corresponding waveform segment, and be of size equal to that of the segment. The frequency peaks observed in the resulting discrete spectra correspond to each of the beat signals present.

When the transmitted signal is an FM-CW signal, each of the frequency components of the signals at the outputs I and Q corresponds to a beat signal resulting from the frequency difference between the transmitted FM-CW signal and the signal reflected by an object with a radial velocity v and a distance R with respect to the antenna.

This frequency difference is originated by the linear frequency modulation of the transmitted signal and the Doppler effect induced by the velocity of the object, being the relation between the velocity and range of the object and the frequency of the beat signal given by:

for an up-ramp frequency modulation, and:

for a down-ramp frequency modulation, where f_fm__up and f_fm-dw are the frequencies of the beat signals with respectively up-ramp and a down-ramp frequency modulation, BW/T_r is the sweep rate of the ramp, and Λ_FM is the wavelength corresponding to the centre frequency of the generated sweep.

A spectral analysis of the signals at the outputs I and Q serves to determine the amount and frequency values of the beat signals contained in them. This is achieved in the preferred embodiment of the invention by digitising the signals at the outputs I and Q, grouping them into blocks, and performing an FFT (Fast Fourier Transform) to each block of samples. Each sample block must be aligned to beginning of the corresponding waveform segment, and be of size equal to that of the segment. The frequency peaks observed in the resulting discrete spectra correspond to each of the beat signals present.

If up-ramp and down-ramp FM-CW segments are alternatively transmitted, it is possible to calculate the distance and radial velocity of an object if the frequencies of the FM-CW beat signals originated by this object in an up-ramp and a down-ramp segment of the transmitted signal are known. In such case, the radial velocity and range of the object may be evaluated as: λ 'F.M

V = ' \J fii-up ^"■" J fin-dw I (4)

T c I \ R = ~r~ ^• — ^■ [fβt-up ^* f fin-dw J (5)

Expressions 4 and 5 assume that the radial velocity and range of the object remain constant for both ramps.

If only a CW segment and an FM-CW segment are available, and the frequencies of the beat signals originated by this object in both segments are available, the slant range to the object must be evaluated by making use of the expression:

when up-ramp FM-CW segments are employed, or:

in the case of down-ramp FM-CW segments. The radial velocity of the object relative to the sensor is then calculated as:

for both kinds of ramp.

The above expressions are especially suitable for a system like the one of the present invention, where a CW and an FM-CW signal are simultaneously transmitted, received and processed, and yield significantly more accurate results when the objects under observation present very high radial velocities with respect to the system. It should be noted that transmitter 110 and the receiver 120 may have separate antennas 103, 121 , but they may also share a single, common antenna. In Fig. 1 there is shown one receive antenna channel 120 having a receive antenna 121 , but by having several receive antenna channels, with the corresponding receive antennas arranged in the same plane, it is possible to detect phase differences between corresponding reflected radar signals received by different receive antennas. For a preferred radar system according to the present invention, there are two receive antennas arranged horizontally besides each other and two receive antennas arranged vertically above each other. This requires 3 receive antenna channels. However, in another preferred embodiment there are 4 receive antenna channels with the receive antennas 1 , 2, 3, 4 arranged as illustrated in Fig. 2. In Fig. 2, azimuth phase differences can be detected from the signals received by the horizontally arranged antennas 1 and 2 and similarly from the signals received by antennas 3 and 4. Accordingly, elevation phase differences can be detected from the signals received by the vertically arranged antennas 1 and 3 and similarly from the signals received by antennas 2 and 4. The 4 receive antennas 1, 2, 3, 4 in Fig. 2 may be used as a single transmit antenna 103 for the transmitter 110.

It should be understood that when performing a radar detection of an object according to the present invention using a radar system having a transmit antennae and one or more receive antennas arranged in the same plane and being arranged relatively close to each other, then an object velocity determined by the use of such a radar system relates to a radial velocity of the object. Furthermore, if the radar system is moving, then the object velocity relates to the relative, radial velocity of the object. The radial object velocity may be given as the velocity substantially in the direction of a line going from the centre of the object to the centre of the antenna system.

The relationship between the angle of arrival of a reflected signal, and the phase difference between the signals received by two antennas separated by a known distance, is given by the expression: φ = ^- d - sin(ø) (9)

A where φ is the phase difference between the signals received by each antenna, d is the distance between the antennas, and θ is the angle of incidence of the incoming signal with respect to the axis along which the antennas are aligned. The phase difference between the signals received by two sets of antennas arranged horizontally will yield the azimuth location with respect to the system of the object originating such signal. The phase difference between two sets of antennas arranged vertically will provide the elevation location with respect to the system of the referred object.

Figs. 3a and 3b are graphs showing the relative frequency spectrum of the radar signal transmitted by the transmitter 110 and received by the receiver 120 for a static target or object, see Fig. 3a, and a moving target or object, see Fig. 3b. The transmitted spectrum consists of two signals, a CW and a FM-CW signal, whose centre frequencies are denoted by f 1 and f2 in Figs. 3a and 3b. The CW signal is a sinusoid of constant amplitude and frequency. When this signal impinges on a moving target, the reflected signal collected by the receiver will be shifted in frequency with respect to the transmitted signal by an amount f_d related to the radial velocity of the target with respect to the system. This is indicated by the dashed tone depicted in Fig. 3b. The FM-CW signal is sinusoid of constant amplitude whose frequency is modulated by either a saw-tooth or a triangle shaped signal, as shown in Figs. 4 and 5. If the product of the swept frequency range and the sweep time is sufficiently high, the frequency spectrum of the transmitted signal approximates a pedestal whose width is the swept frequency range. When the signal impinges on a moving target, the spectrum of the received signal will also be shifted in frequency by an amount similar to the one experienced by the CW signal. In Fig. 3b, the transmitted FM-CW spectrum is outlined by a solid line, while that of the received signal is outlined by a dashed line.

Figs. 4 shows examples of radar transmission signal combinations according to the present invention. It is preferred that the transmission time period is equal, T_r, for each signal period. It is noted that according to the present invention, once the range and velocity of the objects being tracked have been acquired, any combination of transmitted signals can be employed, since, as explained later, the frequency information of any single period is sufficient to update the state of the corresponding track. Tracking of an object can still be performed if only CW segments are transmitted. However, both valid range and velocity measurements can only be obtained if at some point a FM-CW modulated signal is transmitted. In general, the preferred combination of transmitted signals is a combination consisting of three periods containing an FM-CW up-ramp, a CW segment and an FM-CW down-ramp. Processing bandwidth considerations may make necessary the usage of only one kind of FMCW ramp (either up-ramp or down- ramp), while the presence of heavy clutter may require switching to a purely CW mode in order to avoid track loss.

Figs. 5a and 5b are graphs showing relations between waves transmitted and received by the radar system 110, 120 and beat signals when the transmitted segment is an up- ramp modulated FW-CW signal, see Fig. 5a, and when the transmitted segment is an down-ramp modulated FW-CW signal. For the moving target situation there is a Dop- pler frequency as indicated by f_d and the frequency of the beat signals f_fm._up and f_fm-dw is changed in accordance with the Doppler frequency f_d. In Figs. 5a and 5b the bandwidth of the FM-CW signal is indicated by BW and the repetition period is indicated by T_r.

When a single target or object is detected, a single frequency component of the beat signal appears for the CW signal and a single frequency component of the beat signal appears for each of the frequency rising and frequency falling ranges of the FM-CW signal. However, when a plurality of objects are detected, beat signal frequency components of a number equal to the number of objects appear for the CW signal and for both the frequency rising and frequency falling ranges of the FM-CW signal.

Fig. 6 is a block diagrams illustrating the processing of received radar signals using a radar system with 4 receive antennas and 4 corresponding receiver channels, with the 4 antennas being arranged as illustrated in Fig. 2. In Fig. 6, the signals Ch1 I and Ch1 Q are the I and Q outputs from the first receiver channel, having frequency components corresponding to beat signals resulting from the frequency difference between the transmitted CW or FM-CW signal and reflected CW or FM-CW signals received by the first radar signal receiver. In the same way, the signals Ch2 I and Ch2 Q are the I and Q outputs from the second receiver channel, the signals Ch3 I and Ch3 Q are the I and Q outputs from the third receiver channel, and the signals Ch4 I and Ch4 Q are the I and Q outputs from the fourth receiver channel. Each signals of the 4 pairs of I and Q signals are digitised by corresponding A/D converters, and the digitised Q signal is brought on the imaginary form jQ by a corresponding "x j" multiplication unit. Each pair of corresponding digital I and jQ signals are summed via a summation unit, and a FFT (Fast Fourier Transform) is performed on each of the 4 summed I + jQ signals, to thereby obtain 4 corresponding FFT signal outputs. These 4 FFT outputs are summed, and the summed FFT signal, which contains information of summed CW or FM-CW beat frequencies, is fed to a peak detector. The output of the peak detector, which cor- responds to the frequency peaks of the CW or FM-CW beat signals, is stored in a storage unit.

The 4 FFT outputs for the CW or FMCW radar signals are further fed into a phase comparator see Fig. 8, in order to determine azimuth and elevation phase differences from the radar signals received by the 4 receive antennas corresponding to the detected and stored CW or FM-CW frequency peaks.

In Fig. 7a are shown peak frequencies corresponding to received CW signals, while in Fig. 7b peak frequencies corresponding to received FM-CW signals are shown.

The operation of the phase comparator of Fig. 6 is illustrated in Fig. 8. Here, the outputs of the 4 FFT channels are denoted Ch1 FFT, Ch2 FFT, Ch3 FFT and Ch4 FFT, respectively. The Ch1 FFT and Ch2 FFT signals are summed 81, with the summed signal providing a first input to an elevation phase comparator 83, and the Ch3 FFT and Ch4 FFT signals are summed 82, thereby providing a second input to the elevation phase comparator 83. The comparator subtracts the arguments of the added FFT results of each input corresponding to the same frequency bin, and obtains for each frequency a phase difference which is related to the measured angle of incidence in ele- vation of each received signal according to equation (9) (being the distance between the two antenna sets along the vertical axis whose outputs are summed). The output of the elevation phase comparator is fed to a peak selection unit 84, which further has as input the frequency location of the corresponding CW peaks or FM-CW peaks. From the unit 84 elevation phase differences corresponding to the frequencies, for which a peak has been detected, are being outputted, which elevation phase differences are then stored in a storage unit. In the same way, the Ch1 FFT and Ch3 FFT signals are summed 85, with the summed signal providing a first input to an azimuth phase comparator 87, and the Ch2 FFT and Ch4 FFT signals are summed 86, thereby providing a second input to the azimuth phase comparator 87. The comparator subtracts the ar- guments of the added FFT results of each input corresponding to the same frequency bin, and obtains for each frequency a phase difference which is related to the measured angle of incidence in azimuth of each received signal according to equation (9) (being the distance between the two antenna sets along the horizontal axis whose outputs are summed). The output of the azimuth phase comparator is also fed to the peak selection unit 84, from which unit 84 elevation phase differences corresponding to the detected peak frequencies are being outputted to be stored in a storage unit.

From the signal processing illustrated in Fig. 6 and 8, stored values are obtained for the CW or FM-CW peak signals and the corresponding elevation and azimuth phase differences. However, the range of or distance to detected objects and their radial velocity still needs to be determined.

This is performed by the tracking algorithm described by the block diagram in Fig. 9, which starts in step 901. Each of the objects successfully processed by the system gives rise to a track record which is defined by a state vector x(t) that contains the values of the calculated range, velocity and acceleration of the object at an instant t. The state vector summarizes the information of the peaks that have been identified to be originated by the object represented by the track record. In the preferred embodiment of the invention, a Kalman filter is employed as a means of propagating the state vector of the track to a later time instant and updating it with newly found frequency peaks.

A track record with valid range and velocity measurements can only be initiated based on the frequency information of three peaks from three different signal segments, in which at least one of the segments is of the FM-CW type, as explained later in the text. Once the track record is initiated, the Kalman filter associated to a track allows updating its state vector with individual peak frequencies from either CW or FM-CW segments on a one-by-one basis. A track can therefore be maintained with whatever combination of transmitted CW and FM-CW signals.

In addition to the state vector, the tracking Kalman filter is defined by the following elements:

• A state vector x(t), which contains the values of the magnitudes to be estimated (range, velocity and acceleration) at an instant t. • A covariance matrix P(t), which describes the estimated covariance of the estimates contained in the state vector at an instant t.

• A propagation matrix Φ(ΔT), which serves to estimate the values of the state vector after a given time ΔT.

• A measurement matrix H, which relates the expected measured frequencies to a given value of the state vector. • A measurement error covariance R, which represents the expected mean square errors of the measured frequencies.

• A plant noise covariance matrix Q, which accounts for the unexpected manoeuvres of the target. Given the state vector corresponding to a track record at a particular instant t, the expected values of the state vector after a time ΔT are calculated as: x(t + AT-)= φ{AT)- x{t) (10) where the superscript ^" indicates that the estimated values of the state vector are calculated before the addition of the possible new measurements incorporated to the track record at time t+ΔT. The estimated covariance of the propagated state vector is calculated as: p(t + ΔΓ^~ ) = Φ(ΔΓ) ^• p{t) ^■ Φ^T (ΔΓ> + Q (11 ) where the superscript ^τ denotes matrix transposition and the propagation matrix is defined as:

1 AT AT¹ Il Φ(ΔT) = o i AT (12)

0 0 1

In the block diagram of Fig. 9, when a new signal segment is processed and its peaks identified, the state vector of each of the active tracks in the system, step 902, is propagated in step 903 up to the time of this new segment.

From the propagated state vector, the values of the expected measured frequencies and their variance are estimated as: z(t + AT-)= H- x(t + AT-) s(t + AT-) = H -p(t + AT-)- H^T +R where z represents the expected frequency value and S its variance. By using these two values, it is possible to define a gate, whose centre and size are determined by z and S, that specifies the range of frequencies within which the next frequency peak to be assigned to the track record is expected to appear. This range of frequencies is defined as:

where k will take the value 2.6 if measurement is to be found inside the gate with a 99% probability when a Gaussian distribution is assumed for the estimated variable. If a new measurement is found inside the association gate, it will be incorporated to the current state of the filter by use of: x{t + AT) = x(t + AT^' )+ p(t + AT^' ) • H^τ ■ S^{"1 ■} (m - H ^■ x(t + AT^~ )) P{t + AT) = p(t + AT^~)-P(t + AT^' )• H^τ ■ S^'x • H ^■ p(t + AT^~ )

(15) where m represents the vector containing the newly found peak frequencies which are incorporated to the track record and which serves to update the range and velocity estimates in the state vector.

If the segment to be processed is CW, steps 904, 905 and 906 in Fig. 9, the measurement matrix to be used in equations (14) and (15) must be of the form:

H = 0 (16)

A 'CW

If the segment to be processed is FM-CW, steps 907 and 908 in Fig. 9, then the measurement matrix becomes:

where the sign of the first element of H will be that corresponding to the sign of the next transmitted frequency ramp, according to equations (2) and (3).

There will be in general two kinds of track records maintained by the tracking routine: tentative and confirmed. Tentative track records are those corresponding to objects that have recently been detected by the system for which some time will be given to allow them receiving some predefined amount of updates before the algorithm declares them as firm (confirmed) tracks and their current state shown to the user in step 911.

In step 909 in Fig. 9, after the tracks have been updated with the latest frequency peaks detected by the system, the confirmed tracks that have not received any new update in the current iteration of the algorithm must undergo a maintenance test. In the preferred embodiment of the system, this test consists in determining for how long the track has not received any new update, and if this time turns out to be longer than some predefined deletion time, the track record is deleted from the system.

It must be then determined which tentative tracks must be maintained, deleted or declared as new confirmed tracks. This is performed in step 910 in Fig. 9. The tentative tracks in the system are checked for their time extent and number of updates received in that time. If the time extent of a tentative track is longer than some specified confirmation time, and the number of updates received by it bigger than some specified confirmation threshold, the tentative track is confirmed and its range and velocity shown henceforward. If the time extent of the track is longer than the specified confirmation time, but the number of received updates smaller than the confirmation threshold, the tentative track is deleted. In the rest of the cases the tentative tracks are kept as tentative in the system until a decision about their confirmation or deletion can be taken when the extent of their existence in the system reaches the specified confirmation time.

The peaks that are not assigned to any of the active tracks in the system are then used to (possibly) initiate new tracks in the system. Step 912 in Fig. 9 marks the end of the tracking algorithm and the beginning of the track initiation algorithm. The block diagram of Fig. 10 describes the algorithm employed to initiate new tentative tracks in the system, which starts in step 1000.

In principle, all frequency peaks that have not been assigned to any track are candidates to initiate a new tentative track. Since the state vector of a track is defined by three magnitudes (range, velocity and acceleration), a minimum of three peaks, in which at least one must be generated by an FM-CW segment, are necessary to initiate a new tentative track.

At any given point, the system stores the frequency peaks found in the last two fre- quency segments that were not assigned to any track. When the peaks of new frequency segment are received by the initiation routine, they are processed together with the peaks of the two previous segments stored in the system, step 1001 in Fig. 10. After the peaks have been processed and new tentative tracks (possibly) generated, the peaks from the oldest segment are removed and those from the latest segment stored, steps 1017 and 1018 in Fig. 10.

The aim of the initiation routine is finding a set of three peaks, each of them located in a different segment, which allows initiating a new tentative track according to the procedure described later in the text. The method employed to associate the three initial peaks differs attending to the particular combination of waveforms found in the three processed segments.

As shown in Fig. 10, all methods determine first whether there is any kind of waveform (CW, up-ramp FM-CW or down-ramp) that appears more than once, steps 1002, 1003, 1004, 1007 and 1013. Pairs of peaks from the first two segments with the same waveform are associated on the basis of their frequencies. Around the frequency of each peak from the first segment, an association gate is opened with its width determined by the maximum velocity and acceleration expected for the targets that the system is in- tended to track. Being these maximum values designated as v_max and a_max, and the time separation between the segments as T₃ (either T_r or 2-I_x) the range of frequencies covered by the association gate is given by:

for FM-CW segments, steps 1008 and 1012 in Fig. 10, where f^-™ represents the fre- quency of the peak from the first segment, or:

for CW segments, step 1005 in Fig. 10, where f_cw represents the frequency of peak from the first segment. If the frequency of a peak from the second segment falls into the defined gate, it is associated to the first peak for further processing. If more than one peak falls into a gate, only the peak closest to the centre of the gate is selected for association.

The pairs of peaks resulting from the previous procedure are then correlated with the peaks of the third segment.

If the third segment is of the same kind as the first two processed segments, new association gates can be generated based on the frequencies of the peaks of the pairs located in the second segment in the same fashion as in equations (18) and (19). This is performed in step 1015 in Fig. 10.

If the three segments are of the FM-CW type, but the third segment has as slope with opposite sign to that of the first two processed segments, step 1014 in Fig. 10, a first velocity estimate can be calculated as:

where f_f^ and f_fm2 respectively represent the frequencies of the peaks from the first and second segments and where the positive sign is used when the first two processed segments are of the up-ramp type and the negative sign when they are of the down- ramp type.. From this a range estimate for the first segment can be formed as:

In the previous derivation it is assumed that the velocity remains approximately constant in both segments. From the estimates, a new association gate can be formed for the third segment whose frequency range is given by:

+2.6-σ_r

(22)

where σ_r is the standard deviation of the FM-CW frequency measurements, T_s3 is the time separation between the first and the third segment (again either T_r or 2T_r) and the signs are the opposite ones to those used in equations (20) and (21).

If the third segment is of the CW type and the two first segments of the FM-CW type, or the third segment is of the FM-CW type and the two first of the CW type, the association between CW and FM-CW must be done on the basis of the monopulse angle measurements of the peaks, steps 1006 and 1009 in Fig. 10. From the first pair of peaks, a monopulse measurement is extracted as the average of the monopulse measurements of its peaks. From that average, a monopulse association gate can be built which covers the range of monopulse measurements specified by: {φ- 2.6 - σ_m , φ + 2.6 - σ_m) (23) where φ represents the monopulse measurement extracted from the first pair of peaks, and σ_m the expected standard deviation of the monopulse measurements, which may be calculated from the signal-to-noise ratio of the peaks in the pair. It must be noticed here, that the above association method requires the use of a receiver capable of measuring monopulse angles in at least one direction. If the receiver can measure mo- nopulse angles in two directions, two different monopulse gates must be generated for each of the directions, and only those peaks from the third segment whose monopulse measurements satisfy the two monopulse gates accepted for association.

If the three segments contain the three possible kinds of waveforms in the system, the association procedure between CW and FM-CW peaks can be made in a third and more robust way, steps 1010 and 1011 in Fig. 10. Fig. 11 shows a velocity table, which on the horizontal scale has peak frequencies f_fm-_up corresponding to the FM-CW up- ramp frequencies, and on the vertical scale has peak frequencies f_fm-dw corresponding to the FM-CW down-ramp frequencies. For each pair of f_fm-upi and f_fm-dwJ frequencies, a corresponding object velocity Vy is determined by use of equation (4). The velocities Vg are then compared with the velocities determined from the frequencies of the peaks in the CW segment by use of equation (8). When there is a match between a FM-CW velocity Vy and a CW velocity, the up and down ramp peak frequencies f_fm-Upi and f_fm-dwj corresponding to the matched velocity Vy, are then regarded as originated by the same target, which is the target giving rise to the matching CW velocity. The associated peaks form a triad that can initiate a new tentative track, as explained below.

Given three peaks located in three different segments, being at least one of the seg- ments of the FM-CW type (either up-ramp or down-ramp), that have been associated by means of any of the association procedures described above, it is possible, step 1016, to initiate a new tentative track in the system in the following way.

First a measurement vector m is built which contains the measured frequencies of the three associated peaks:

where f|, f₂ and f₃ respectively represent the frequencies of the peaks from the first, second and third segment.

Then, a measurement matrix H is built whose rows are defined such that the meas- urement vector and the estimated initial state vector of the track at the time corresponding to the centre of the third segment relate as:

where H₁, H₂, and H₃ represent the three rows of the matrix H. In order to achieve this, the first row the matrix H must be of the form:

H₁ = ±

(26) if the first segment is of the FM-CW type, the positive signs used when it is an up-ramp and negative for a down-ramp, or:

if the first segment is of the CW type.

In a similar way the second and third rows of the measurement matrix are built as:

H₂ = +

(28) if the second segment is of the FM-CW type or:

2

H-, = - - [O 1 -T_r] (29)

"cw if it is of the CW type, and

if the third segment is of the FM-CW type or:

2

H₅ = ^■• [o i o] (30)

"CW if it is of the CW type.

A least square estimation of the initial track state may be then calculated as: x{T_i ) = H^~ϊ - m (31) where T₃ represents the time at the centre of the third segment, and the covariance matrix of this estimate calculated as:

P[T₃) = (H^T - R-¹ - H)^"1 (32) where R is a diagonal matrix with its elements corresponding to the expected variances of the frequency measurements.

The estimated state vector and associated covariance can then be used to initialize the corresponding Kalman filter that will be in charge of processing the measurement up- dates for the track.

While the invention has been particularly shown and described with reference to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and it is intended that such changes come within the scope of the following claims.

Claims

1. A method of radar detection of one or more objects, said method comprising:

transmitting one or more types of radar signals for successive signal time periods, with at least a first set of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal,

receiving reflected radar signals reflected from one or more objects present in a detec- tion range of the radar system,

mixing corresponding transmitted and reflected radar signals to produce corresponding beat signals,

taking the Fourier transform of the obtained beat signals to thereby obtain a corresponding set of frequency spectrum data,

determining a number of peak frequencies based on the obtained frequency data, and

pairing or associating at least three determined peak frequencies including at least one peak frequency from each of the first set of three successive signal time periods, thereby obtaining a first set of associated peak frequencies.

2. A method according to claim 1 , wherein each of the successive signal time peri- ods holds only one type of transmitted radar signals.

3. A method according to claim 1 , wherein the one or more types of transmitted radar signals comprises FM-CW signals and CW signals.

4. A method according to any one of the claims 1-3, wherein the transmitted signal of at least one of said first set of three successive signal time periods is an up or down modulated ramp FM-CW signal.

5. A method according to any one of the claims 1-4 wherein the transmission of ra- dar signals includes a plurality of sets of three successive signal time periods compris- ing a signal time period with the transmitted signal being a FM-CW radar signal, and wherein a plurality of sets of associated peak frequencies are obtained, each set of associated peak frequencies including at least one determined peak frequency from each of three successive signal time periods belonging to a set of three successive signal time periods.

6. A method according to any one of the claims 1-5, wherein a set of associated peak frequencies comprises one and only one determined peak frequency from each of three signal time periods of a set of successive signal time periods.

7. A method according to any one of the claims 1-6, further comprising: initiating one or more object track records, said initiation of an object track record being at least partly based on information relating to three peak frequencies belonging to a selected set of peak frequencies and representing all three signal time periods of the corresponding set of three successive signal time periods.

8. A method according to claim 7, wherein an initial object velocity and/or an initial object distance or range is determined based on the information of a set of three associated peak frequencies being used for initiating a corresponding object track record.

9. A method according to claim 7 or 8, wherein an object track record is updated for a selected signal time period based on a determined peak frequency corresponding to said selected signal time period.

10. A method according to claim 9, wherein the radar signal being transmitted within said selected signal time period for which the object track record is updated is a FM- CW signal or a CW signal.

11. A method according to any one of the claims 7-10, wherein an initiated object track record is updated by use of a tracking algorithm incorporating a Kalman filter.

12. A method according to claim 10 or 11, wherein an object velocity and/or an object distance or range is determined based on the information of an updated object track record.

13. A method according to any one of the claims 1-12, wherein the reflected radar signals are received via a plurality of radar signal receivers arranged in the same plane or along a line, said plurality of receivers having at least two receivers arranged along a first receiver direction.

14. A method according to claim 12, wherein the plurality of receivers has at least two receivers arranged along the first receiver direction and at least two receivers arranged along a second receiver direction, said first receiver direction being different to the second receiver direction.

15. A method according to claim 14, wherein the first and second receiver directions are substantially perpendicular to each other.

16. A method according to any one of the claims 1-15, further comprising: detecting, based at least partly on corresponding radar signals received by the receivers along the first receiver direction, one or more time or phase differences relating to a first object angular direction.

17. A method according to any one of the claims 14-16, further comprising: detecting, based at least partly on corresponding radar signals received by the receivers along the second receiver direction, one or more time or phase differences relating to a second object angular direction.

18. A method according to claim 16 or 17, wherein the detection of a phase differ- ence comprises determining a phase difference based on at least two Fourier transformed outputs representing received radar signals corresponding to at least two receivers arranged along the same receiver direction, said received radar signals corresponding to the same transmitted radar signal.

19. A method according to claim 18, wherein said detection of phase differences comprises determining a number of phase differences for Fourier transformed outputs corresponding to a number of determined peak frequencies.

20. A method according to claim 7 and 19, wherein for the one or more object track records having information relating to one or more determined peak frequencies, the method further comprises holding information of detected phase differences corresponding to the one or more detected peak frequencies, thereby holding information relating to first and/or second object angular directions corresponding to the detected phase differences.

21. A method according to any one of the claims 7-20, wherein for a selected track record, information relating to one or more determined peak frequencies are hold as a function of time.

22. A method according to claim 20 or 21 , wherein information relating to detected phase differences corresponding to one or more determined peak frequencies are hold as a function of time.

23. A method according to any one of the claims 1-22, wherein for a set of three suc- cessive signal time periods, the transmitted types of radar signals are FM-CW signals, with each of the three successive signal time periods holding an up or down modulated ramp FM-CW signal, and wherein a first pair of associated peak frequencies are selected based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal.

24. A method according to claim 23, wherein the third associated peak frequency is determined based on the determined first pair of associated peak frequencies and determined peak frequencies of the remaining signal time period of said set of three successive signal time periods.

25. A method according to claims 3-22, wherein for at set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with the set of three successive signal time periods having two time periods of CW signals and one time period of an up or down ramp FM-CW signal, and wherein a first pair of associated peak frequencies are selected based on determined peak frequencies of two periods of the set of three successive signal time periods having a CW signal.

26. A method according to claim 25, wherein the third associated peak frequency is determined based on the determined first pair of associated peak frequencies, the de- termined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a ramp FM-CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

27. A method according to claims 3-22, wherein for at set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with the set of three successive signal time periods having one signal time period with a CW signal and two signal time periods of a similar type up or down ramp FM-CW signal, and wherein a first pair of associated peak frequencies are selected based on determined peak frequencies of two periods of the three successive signal time periods having a similar ramp type FM-CW signal.

28. A method according to claim 27, wherein the third associated peak frequency is determined based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining time period of said set of three successive signal time periods having a CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

29. A method according to claims 3-22, wherein for a set of three successive signal time periods, the transmitted types of radar signals are CW signals and FM-CW signals, with three successive signal time periods having one signal time period with a CW signal and two signal time periods of opposite type up or down ramp FM-CW signals.

30. A method according to claim 29, further comprising: determining FM-CW object velocities, Vy, corresponding to each pair or a number of pairs of determined FM-CW up ramp peak frequencies, f_fm-UPi, and FM-CW down peak frequencies, f_fm-dwj, comparing the obtained FM-CW object velocities, Vy, to determined CW object velocities, said determined CW object velocities being determined based on determined CW peak frequencies, matching FM-CW object velocities, Vy, and CW object velocities, and forming a set of associated peak frequencies corresponding to the three peak frequen- cies of the matching FM-CW object velocity, Vy, and CW object velocity.

31. A method according to any one of the claims 24, 26, 28 or 30, wherein said determined associated three peak frequencies are used for forming an object track record.

32. A method according to any one of the claims 13-31 , wherein beat signals are produced for each of the plurality of receivers.

33. A method according to claim 32, wherein the Fourier transform of beat signals representing the same signal time period are summed for each receiver, to thereby obtain the corresponding set of frequency spectrum of data.

34. A method according to any one of the claims 14-33, wherein the receivers along the first receiver direction are vertically arranged and a time or phase difference de- tected by the receivers along the first receiver direction relates to an elevation phase difference corresponding to the angle of elevation of an object, and wherein the receivers along the second receiver direction are horizontally arranged and a time or phase difference detected by the receivers along the second receiver direction relates to an azimuth phase difference corresponding to the azimuth angle of an object.

35. A method according to any one of the claims 14-34, wherein the plurality of radar signal receivers comprises at least four receivers with at least a first and a second of said receivers arranged along the first receiver direction and with at least a third and a fourth of said receiver arranged along a line being parallel to the first receiver direction, and with the first and third of said receivers arranged along the second receiver direction and with the second and the fourth of said receivers arranged along a line in parallel to the second receiver direction.

36. A method according to claim 35, wherein the detection of a first direction phase difference comprises determining a phase difference based on a summation of the

Fourier transformed outputs representing received radar signals corresponding to the first and third receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the second and fourth receivers, and wherein the detection of a second direction phase difference comprises deter- mining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and second receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the third and fourth receivers.

37. A method according to any one of the claims 1-36, wherein the transmitted radar signals are transmitted via at least a first radar signal transmitter, said first transmitter being arranged at a distance in relation to the plurality of radar receivers.

38. A radar system for detection of one or more objects, said system comprising:

one or more radar wave transmitter for transmitting one more types of radar signals for successive signal periods of time, with at least a first set of three successive signal time periods comprising a signal time period with the transmitted signal being a FM-CW radar signal,

one or more radar wave receivers for receiving reflected radar signals reflected from one or more objects present in a detection range of the radar system,

one or more signal mixers for mixing corresponding transmitted and reflected radar signals to produce corresponding beat signals,

one or more signal transformers for taking the Fourier transform of the obtained beat signals to thereby obtain a corresponding set of frequency spectrum data,

one or more peak detectors for detecting or determining a number of peak frequencies based on the obtained frequency data, and

pairing or associating means for pairing or associating at least three determined peak frequencies including at least one peak frequency from each of the first set of three successive signal time periods, thereby obtaining a first set of associated peak frequencies.

39. A radar system according to claim 38, wherein the transmitter(s) are adapted to transmit only one type of radar signals.

40. A radar system according to claim 38, wherein the transmitter(s) is/are adapted to transmit FM-CW signals and CW signals.

41. A radar system according to any one of the claims 38-40, wherein the radar wave transmitter(s) is/are adapted for transmitting an up or down modulated ramp FM-CW signal for at least one of said first set of three successive time periods.

42. A radar system according to any one of the claims 38-41 , wherein the transmitters) is/are adapted for transmission of radar signals including a plurality of sets of three successive signal time periods with each set comprising a signal time period with the transmitted signal being a FM-CW radar signal, and wherein the peak detectors are adapted for detecting a plurality of sets of associated peak frequencies, each set of associated peak frequencies including at least one determined peak frequency from each of three successive signal time periods belonging to a set of three successive signal time periods.

43. A radar system according to any one of the claims 38-42, wherein the pairing or associating means is adapted for obtaining a set of associated peak frequencies comprising one and only one determined peak frequency from each of the three signal time periods of a set of successive signal time periods.

44. A radar system according to any one of the claims 38-43, further comprising means for initiating one or more object track records, said initiation of an object track record being at least partly based on information relating to three peak frequencies belonging to a selected set of peak frequencies and representing all three signal time periods of the corresponding set of three successive signal time periods.

45. A radar system according to claim 44, further comprising means for determining an initial object velocity and/or an initial object distance or range based on the informa- tion of a set of three associated peak frequencies being used for initiating a corresponding object track record.

46. A radar system according to claim 44 or 45, further comprising means for updating an object track record for a selected signal time period based on a determined peak frequency corresponding to said selected signal time period.

47. A radar system according to claim 46, wherein the transmitter(s) is/are adapted for transmitting a FM-CW signal or a CW signal within said selected signal time period for which the object track record is updated.

48. A radar system according to any one of the claims 44-47, further comprising means for updating an initiated object track record by use of a tracking algorithm incorporating a Kalman filter.

49. A radar system according to claim 47 or 48, further comprising means for determining an object velocity and/or an object distance or range based on the information of an updated object track record.

50. A radar system according to any one of the claims 38-49, further comprising a plurality of radar signal receivers for receiving the reflected radar signals, said plurality of radar signal receivers being arranged in the same plane or along a line, and said plurality of receivers having at least two receivers arranged along a first receiver direction.

51. A radar system according to claim 50, wherein the plurality of receivers has at least two receivers arranged along the first receiver direction and at least two receivers arranged along a second receiver direction, said first receiver direction being different to the second receiver direction.

52. A radar system according to claim 51 , wherein the first and second receiver directions are substantially perpendicular to each other.

53. A radar system according to any one of the claims 38-52, further comprising: one or more phase detectors for detecting, based at least partly on corresponding ra- dar signals received by the receivers along the first receiver direction, one or more time or phase differences relating to a first object angular direction.

54. A radar system according to any one of the claims 51-53, further comprising: one or more phase detectors for detecting, based at least partly on corresponding radar signals received by the receivers along the second receiver direction, one or more time or phase differences relating to a second object angular direction.

55. A radar system according to claim 53 or 54, wherein a phase detector is adapted for determining a phase difference based on at least two Fourier transformed outputs representing received radar signals corresponding to at least two receivers arranged along the same receiver direction, said received radar signals corresponding to the same transmitted radar signal.

56. A radar system according to claim 55, wherein a phase detector is adapted for determining a number of phase differences for Fourier transformed outputs corresponding to a number of determined peak frequencies.

57. A radar system according to claim 44 and 56, wherein for the one or more object track records having information relating to one or more determined peak frequencies, the radar system further comprises means for holding information of detected phase differences corresponding to the one or more determined peak frequencies, said information holding means thereby holding information relating to first and/or second object angular directions corresponding to the detected phase differences.

58. A radar system according to any one of the claims 44-57, wherein a track record is holding information relating to one or more determined peak frequencies as a function of time.

59. A radar system according to claim 57 or 58, wherein the information means for holding information of phase differences holds information relating to detected phase differences corresponding to one or more determined peak frequencies as a function of time.

60. A radar system according to any one of the claims 57-59, wherein a phase detector is adapted for detecting a time or phase difference based on corresponding reflected radar signals being received simultaneously by at least two receivers arranged along the first receiver direction or by two receivers arranged along the second receiver direction.

61. A radar system according to any one of the claims 38-60, wherein the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having FM-CW signals only, whit each of the set of three successive signal time periods holding an up or down modulated ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak frequencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal.

62. A radar system according to claim 61 , wherein the peak frequency associating means is adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies and determined peak frequencies of the remaining signal time period of said set of three successive time periods.

63. A radar system according to claims 53-60, wherein the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having both CW and FM-CW types of radar signals, with the set of three successive signal time periods having two signal time periods of CW signals and one signal time period having an up or down ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak frequencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a CW signal.

64. A radar system according to claim 63, wherein the peak frequency associating means is adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a ramp FM-CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three succes- sive signal time periods.

65. A radar system according to claims 53-60, wherein the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having both CW and FM-CW types of radar signals, with the set of three successive signal time periods having one signal time period with a CW signal and two signal time periods of a similar type up or down ramp FM-CW signal, and wherein the peak frequency associating means is adapted for selecting a first pair of associated peak frequencies based on determined peak frequencies of two periods of the set of three successive signal time periods having a similar ramp type FM-CW signal.

66. A radar system according to claim 65, wherein the peak frequency associating means is adapted for determining the third associated peak frequency based on the determined first pair of associated peak frequencies, the determined peak frequencies of the remaining signal time period of said set of three successive signal time periods having a CW signal, and on information relating to determined phase differences corresponding to the determined peak frequencies of said set of three successive signal time periods.

67. A radar system according to claims 38-60, wherein the radar wave transmitter(s) is/are adapted for transmitting a set of three successive signal time periods having both

CW and FM-CW types of radar signals, with the set three successive signal time periods having one signal time period with a CW signal and two signal time periods of opposite type up or down ramp FM-CW signals.

68. A radar system according to claim 67, further comprising means for determining FM-CW object velocities, Vy, corresponding to each pair or a number of pairs of determined FM-CW up ramp peak frequencies, f_fm-upi, and FM-CW down peak frequencies, f_fm-_dwi, and for comparing the obtained FM-CW object velocities, Vy, to determined CW object velocities, said determining means further being adapted for determining CW object velocities based on determined CW peak frequencies, and for matching FM-CW object velocities, Vy, and CW object velocities, and for forming a set of associated peak frequencies corresponding to the three peak frequencies of the matching FM-CW object velocity, Vy, and CW object velocity.

69. A radar system according to any one of the claims 62, 64, 66 or 68, wherein the means for establishing an object track record is adapted for using said determined associated three peak frequencies for forming an object track record.

70. A radar system according to any one of the claims 50-69, said system comprising signal mixers for producing beat signals for each of the plurality of receivers.

71. A radar system according to claim 70, further comprising means for summing for each receiver the Fourier transform of beat signals representing an equal signal time period, to thereby obtain the corresponding set of frequency spectrum of data.

72. A radar system according to any one of the claims 51-71 , wherein the receivers along the first receiver direction are vertically arranged and a time or phase difference detected by the receivers along the first receiver direction relates to an elevation phase difference corresponding to the angle of elevation of an object, and wherein the receiv- ers along the second receiver direction are horizontally arranged and a time or phase difference detected by the receivers along the second receiver direction relates to an azimuth phase difference corresponding to the azimuth angle of an object.

73. A radar system according to any one of the claims 51-72, wherein the plurality of radar signal receivers comprises at least four receivers with at least a first and a second of said receivers arranged along the first receiver direction and with at least a third and a fourth of said receivers arranged along a line being parallel to the first receiver direction, and with the first and third of said receivers arranged along the second receiver direction and with the second and the fourth of said receivers arranged along a line in parallel to the second receiver direction.

74. A radar system according to claim 73, wherein the phase detector for detection of a first direction phase difference is adapted for determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and third receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the second and fourth receivers, and wherein the phase detector for detection of a second direction phase difference is adapted for determining a phase difference based on a summation of the Fourier transformed outputs representing received radar signals corresponding to the first and second receivers and a summation of the Fourier transformed outputs representing received radar signals corresponding to the third and fourth receivers.

75. A radar system according to any one of the claims 38-74, wherein the transmitted radar signals are transmitted via at least a first radar signal transmitter, said first transmitter being arranged at a distance in relation to the plurality of radar receivers.