WO2005093462A1

WO2005093462A1 - Impulsive multi-channel ground penetrating radar

Info

Publication number: WO2005093462A1
Application number: PCT/IB2004/000878
Authority: WO
Inventors: Guido Manacorda; Mario Miniati
Original assignee: Ids Ingegneria Dei Sistemi S.P.A.
Priority date: 2004-03-24
Filing date: 2004-03-24
Publication date: 2005-10-06

Abstract

A method for detecting, by means of radar techniques, objects immersed in a material, which uses an antenna, or an array of antennas, driven by a first and a second circuit that generate respectively reception and transmission triggering signals in a digital domain and then convert them into analog pulses respectively of periods T and T+S. Two identical circuits (10' and 10') are used for generating respectively transmission and reception triggering pulses (20' and 20'). They are operated in parallel by a microcontroller (21) that administers said first and second circuit supplying as input to them respectively a first counting duration (11') and a second counting duration (11') that are different from each other for a numerical value derived from said periods T and T+S. This way, the periodic signal (17') resulting from said first circuit (10') and the periodic signal (17') resulting from said second circuit (10') have respectively a first and a second frequency f(l) and f(2) that are different from each other for a frequency value derived from said range S. The timing circuit carrying out a sample-conversion of the frequency of the received signals not subject to misadjustment of the time scale and much more flexible concerning the programmation of time setting.

Description

TITLE IMPULSIVE MULTI-CHANNEL GROUND PENETRATING RADAR DESCRIPTION Field of the invention The present invention relates to a method for detecting, by means of radar techniques, objects immersed in a material, in particular in the underground or in walls of buildings . Furthermore, the invention relates to a device that carries out this method. Description of the prior art Inspecting devices are known that provide the use of radio frequency energy for searching, with radar techniques, objects present in the underground, in walls of buildings, or in other hidden sites, with various applications in the field of building engineering, geology, archaeology. These devices have the merit of not affecting the physical, chemical and mechanical characteristics of the structures and of the materials that surround them. Among such devices, known as geo-radar or with the acronyms GPR (Ground Penetrating Radar) or SPR (Surface Penetrating Radar) , many of them provide at least one RF receiving/transmitting antenna and a remote control unit comprising a PC and an antenna (s) interface board. The operation of these devices is typical of a radar, emitting a RF signal for a very short time (a few nanoseconds) and analysing a returning signal reflected by the objects hit by the emitted signal (echo) . The antenna is displaced next to the surface of the investigated material, and, once started the transmission, the received and suitably filtered returning signals are generally displayed in the form of bidimensional images, representing, with suitable coding colours, the amplitude (ordinate) and space (abscissa) versus time. The principles of operation of these ground penetrating radars are described for example in Daniels, D. J. (1996) , Surface penetrating radar, The Institution of Electrical Engineers, Stevenage (UK) and in Manacorda , Guido. IDS Radar Products For Utilities Mapping And Ground Classification - Proceedings of 20th International NO-DIG Conference and Exhibition Copenhagen Denmark. May 28-31 , 2002" available also in www. ids-spa . it. Normally, in the devices of this type the returning RF broadband signal is subject to conversion with substantial frequency lowering. In other words, each single signal as received, which has a band of about hundreds of MHz or GHz, is brought to KHz frequencies, i.e. encoded as an audiofrequency band signal, by sampling it and memorizing it on a computer using analog-to-digital converters of easy construction which are also available on the market. As diagrammatically shown in figure 1, the methodology used for translating the frequency is that of using different repetitions of the transmitted signal, taking at a transmitting instant a sample from each signal received (echo) , by a "sample and hold" circuit (S/H) , and maintaining it until the next sample is taken. In figure 1, in particular, as 1, 1 ' , 1 ' ' the input signals of the S/H and with 2, 2 ' , 2 ' ' the output signals of the S/H have been indicated. This technique, known in literature as sample conversion, requires that the delay, with which the "sample and hold" circuit of the receiving antenna is activated with respect to the instant of transmission, is gradually increased, up to the end of the conversion. In figure 1 it is shown how, with a period T between two successive receiving and transmitting steps, the period T' between two sample-taking steps is higher than T for a factor δ. This way, the succession of signals 1, 1', V ' , etc., allows a reconstruction of the starting signal "expanded" versus time, i.e. with a decreased frequency. In ground penetrating radars of prior art, the generation of transmitting and receiving pulses (trigger) is carried out by an electronic board located in the control unit, so-called "time-base" or "timing circuit". Normally, it uses the so-called principle of the double ramp (figure 2) that employs analog integrating circuits for making a succession of two ramps 4 and 5 with different slopes and a comparator, whose signal is used for delaying suitably the reception trigger signal with respect to the transmission trigger signal. In substance, if the TX pulses 6 are obtained always by the intersection of the ramp 4 of higher slope with a signal 7, so-called signal position, the RX pulses 8 are obtained always by the intersection of the raps 4 of higher slope with the ramp 5 of lower slope. An alternative technique is that of using delay programmable lines which can be controlled by means of a Resistance-Condenser unit. Sampling with analog integrators is rather common; however, the wear of the components and the intrinsic unsteadiness of the integrators can cause relevant errors in the conversion and then in the linearity and correct adjustment of the timing scale. The solution of programmable delay lines, even if better from this point of view, is not enough flexible owing to the resolution that can be set of the delay δ (typically 10 psec) . Another aspect in some of the known ground penetrating radars is the use of an array of antennas, for increasing the investigated area. The used architectures provide a system of distribution of the TX and RX triggers known as Time Division Multiplexing (TDM) . However, the use of the TDM increases the computing time in a way directly proportional to the number of antennas connected and then limits practically the maximum number of them. Summary of the invention. It is therefore a feature of the present invention to provide a method for detecting, by means of radar techniques, objects immersed in a material, which uses a driver board (timing circuit) carrying out a sample conversion of the frequency of the received signals not subject to misadjustment of the time scale and much more flexible concerning the progra mation of time setting. It is another feature of the present invention to provide an implementation of this method that provides the serial distribution (time division multiplexing - TDM) of the trigger signals to the antennas that make up the array. It is another feature of the present invention to provide an implementation of this method that provides the simultaneous generation of the trigger signals of the antennas that make up the array (parallelization) , in order to reduce drastically the scanning time and to cause them to be practically independent from the number of antennas . It is a further feature of the invention to provide a ground penetrating radar apparatus that carries out such methods . In a first aspect of the invention, a method for detecting, by means of radar techniques, objects immersed in a material, using a driver board (timing circuit) and at least one antenna, provides the steps of: - transmitting RF signals from said at least one antenna through said material by driving said signals to said antenna by means of transmission triggering pulses spaced by a first period; — receiving RF signals from said at least one antenna reflected by said objects immersed in said material and sample-converting them through driving said antenna by means of reception triggering pulses spaced by a second period, wherein said second period differs from said first period for a predetermined interval; - analysing said reflected signals for recognizing said objects immersed in said material; and characterised in that said transmission and reception triggering pulses are obtained separately as respective periodic signals in a digital domain and then converted into analog pulses spaced respectively of said first and second period. Preferably, said transmission and reception triggering pulses are obtained in a digital domain through the following steps: - prearranging an oscillator that generates a clock signal; - setting a counting duration; - carrying out a phase accumulation responsive to said counting duration and said clock signal to generate a phase-modulated periodic digital signal; - carrying out a phase/amplitude conversion of said phase-modulated periodic signal and providing an amplitude-modulated periodic digital signal, the resulting amplitude-modulated periodic digital signal being encoded into a quantized sinusoidal analog signal responsive to said clock signal and then transformed into analog triggering pulses. Preferably, said transmission and reception triggering pulses are obtained in parallel supplying respectively a first counting duration and a second counting duration that are different from each other for a predetermined numerical value derived from said interval, whereby the respective resulting periodic signals have respectively a first and a second frequency that are different from each other for a frequency value derived from said numerical value. Advantageously, said quantized analog periodic signal is filtered by means of a low-pass filter to obtain a periodic sinusoidal signal before generating said triggering pulses for the sender or the receiver of the antenna. Advantageously, said sinusoidal signals, which are periodic and have a double infinity of zero crossings, are compared with a reference .signal to generate said triggering pulses of the antennas at each of said zero crossing. Advantageously, said reception triggering pulses generate, through division by a predetermined factor, a clock signal used for digitalizing the audio-frequency signal, resulting from the sample conversion of the radio- frequency original signal. Advantageously, said phase accumulation is reset after a predetermined cycle for starting again at each cycle with phase zero. In particular, said predetermined cycle corresponds to the time necessary for carrying out the sample frequency conversion. Advantageously, at each resetting step a signal is generated called "Start of Scan", which represents the beginning of each conversion cycle. In a second aspect of the invention, a method for detecting, by means of radar techniques, objects immersed in a material, using a plurality (array) of antennas and a serial distribution (TDM) of the triggering pulses, said transmission and reception trigger signals are obtained by a single timing circuit as above defined that carries out a sample conversion of the frequency of the received signals based on the use of digital components . A multiplexer directs to a first antenna the respective transmission and reception pulses up to the end of the sample conversion and receives from it the relative audio-frequency signal, for then driving another antenna, and so forth, up to driving the last antenna of the array. Said multiplexer queues the many audio-frequency signals received by said antennas and drives them to a single A/D conversion circuit that digitalizes the signals so that it is possible to store them on a computer. In a third aspect of the invention, a method for detecting, by means of radar techniques, objects immersed in a material, using a plurality (array) of antennas and a simultaneous generation of the transmission and reception triggering pulses, provides that each antenna has an own digital timing circuit, as above defined, synchronised with the other antennas and an own A/D conversion circuit. In this case, the sender and the receiver of the second antenna are activated by the synchronising circuit as soon as the

S/H circuit of the first antenna has taken the first sample of the received RF signal; then, the sender and the receiver of the third antenna are activated, and so forth, up to driving the last antenna of the array. The cycle is repeated up to the collection of the last sample and up to taking the received RF signal by the last antenna of the array. Advantageously, each antenna has an own A/D conversion circuit, which digitalizes each signal value collected by the S/H circuit and sends it to a concentrator (hub) , which reconstructs the many signals already digitalized and suitable for being recorded on a computer. In another aspect of the invention, a ground penetrating radar apparatus, having at least one antenna, for detecting objects immersed in a material, comprises: - means for driving to said at least one antenna of transmission triggering pulses spaced by a first period for transmitting RF signals towards said objects and said material through said antenna; - means for driving to said at least one antenna of reception triggering pulses spaced by a second period for receiving RF signals reflected by said objects and said material through said antenna, wherein said second period differs from said first period for a predetermined interval; - means for sample-converting said reflected signals on said interval obtaining a low frequency reflected signal, - means for analysing said low frequency reflected signal for recognizing said objects from said material; characterised in that said means for driving comprise a first and a second circuit to generate respectively reception and transmission triggering signals in a digital domain and then converting them into analog pulses spaced respectively of said first and second period. Preferably, said first and second circuit comprise each: - a phase accumulator controlled numerically and having predetermined input counting duration and input clock frequency and having a phase-modulated periodic output digital signal; - a ROM for phase/amplitude conversion of the phase- modulated periodic digital signal and providing an amplitude-modulated periodic digital signal, - a D/A converter for converting said amplitude- modulated periodic digital signal into a quantized analog periodic signal, - means for transforming said analog periodic signal into analog pulses. Preferably, said means for transforming comprise a comparator that detects the "zero crossing" of said analog periodic signal and generates driving pulses for each of said "zero crossing". Advantageously, upstream from said comparator a filter is provided for transforming said quantized analog periodic signal into a sinusoidal signal. Preferably, a clock generator is provided and a microcontroller is provided as well that administers in parallel said first and second circuit supplying as input to them respectively a first counting duration and a second counting duration that are different from each other for a numerical value derived from said interval, whereby the periodic signal resulting from said first circuit and the periodic signal resulting from said second circuit have respectively a first and a second frequency that are different from each other for a frequency value derived from said interval. In another aspect of the invention, advantageously, the radar apparatus provides a plurality (array) of antennas, each of which receives respective transmission and reception triggering pulses. In a first exemplary embodiment, said transmission and reception triggering pulses are obtained by a single timing circuit that contains said first and second circuit in association to said microcontroller, a time division multiplexer being provided that directs to a first antenna the respective transmission and reception pulses up to the end of the sample conversion and receives from it the relative audio-frequency signal, for then driving another antenna, and so on, up to driving the last antenna of the array, said multiplexer queuing the many audio-frequency signals received by said antennas and addressing them to a A/D conversion circuit that digitalizes the signals so that it is possible to store them on computer. In a second exemplary embodiment, said transmission and reception triggering pulses are obtained for each antenna by a respective timing circuit that contains said first and second circuit in association to said microcontroller, and a respective A/D conversion circuit. The sender and the receiver of the second antenna are activated by a synchronising circuit as soon as the S/H circuit of the first antenna has taken the first sample of the received RF signal; then, the sender and the receiver of the third antenna are activated, and so on up to driving the last antenna of the array. The cycle is repeated up to the collection of the last sample and up to taking the received RF signal by the last antenna of the array. Preferably, each antenna has an own A/D conversion circuit that digitalizes each signal value collected by the S/H circuit and sends it to a concentrator (hub) , which reconstructs the many signals already digitalized and suitable for being recorded on computer. Brief description of the drawings The invention will now shown with the following description of an exemplary embodiment thereof, exemplifying but not limitative, with reference to the attached drawings wherein:

- figure 3 shows the succession of the driving pulses of the antennas transmission and reception (trigger) of a first and second frequency;

- figure 4 shows a diagrammatical view of a circuit synthesizer belonging to a circuit for making directly a periodic signal used for generating pulses of trigger for an antenna;

- figure 5 shows a diagrammatical view of two circuits according to the invention for making directly a periodic signal for generating transmission and reception triggering pulses, which use in parallel two respective synthesizing circuits as that of figure 4;

- figure 6 shows a first type of layout of an array of antennas of a ground penetrating radar with the device of figure 5; — figure 7 shows a second type of layout of an array of antennas of a ground penetrating radar with the device of figure 5. Description of the preferred exemplary embodiment As described in the inventor' s article cited in the introductory part, here incorporated for reference, a radar apparatus for detecting objects immersed in a material comprises means for driving at least one antenna, but preferably of a plurality (array) of antennas, by means of transmission and reception triggering pulses. The received signals are sent to an analysing circuit for recognizing such objects with respect to the material where they are immersed. Once irradiated a signal in a material to investigate, the radar is switched for detecting the echo generated by the objects in it present for a certain time (called full scale or range) after the transmission. As already said in the introductory part, it is known to expand temporally the signal received by the antenna of a ground penetrating radar when in reception modality in order to convert it in an audio-frequency band signal, which is suitable for being analysed for measuring the presence of objects immersed in a material. According to the invention, as shown in figure 3, the instants 9 and 9' when respectively the transmission and reception triggering pulses 8 and 8' are obtained are chosen in numerically. Therefore, if T is the period of the transmissions carried out by the transmitting antenna (TX) , N_s samples are taken in reception (RX) at the instants 0, T+δ, 2T+2δ, ...etc. , where δ= full scale/N_s, using as input values responsive to them. With respect to the prior art, which provides analog integrators or delay lines, according to the present invention an antenna is driven by a first and a second circuit that generate respectively reception and transmission triggering signals in a digital domain and then convert them into analog pulses respectively of periods T and T+δ. With reference to figure 4, the first and second circuit comprise each: - a phase accumulating circuit 11 controlled numerically having in input a counting duration 12 and a clock frequency 13 and having in output a phase- modulated periodic digital signal 14; - a ROM 15 for phase/amplitude conversion of the phase-modulated periodic digital signal 14, capable of generating an amplitude-modulated periodic digital signal 16, - a D/A converter 17 for conversion, responsive to the clock frequency 13, of the amplitude-modulated periodic digital signal 16 into a quantized analog periodic signal 18. With reference to figure 5, the first and second circuit 10' and 10'' are the same as the circuit 10 of figure 4 and generate respectively transmission and reception triggering pulses 20' and 20' A They are operated in parallel by a microcontroller 21 that administers said first and second circuit supplying as input to them respectively a first counting duration 11' and a second counting duration 11' ' that are different from each other for a numerical value derived from said periods T and T+δ of figure 3. This way, the periodic signal 17' resulting from said first circuit 10' and the periodic signal 17'' resulting from said second circuit 10'' have respectively a first and a second frequency f_x and f₂ that are different from each other for a frequency value derived from said range δ. The signals 17' and 17'' are transformed by a filter 22' and 22'' into respective sinusoidal signals 23' and 23'' and then arrive to a comparator 24' and 24'' that detects the "zero crossing" of the sinusoidal signals 23' and 23'' and generates driving pulses in transmission and in reception 20' and 20'' for each of said "zero crossing". The microcontroller 21, at the beginning of each scanning step, resets the circuits 10' and 10' 'in order to cause them to start again with phase 0° and generates the signal "Start of Scan" necessary to the A/D converting board, whereas the relative sampling reference, or clock audio 25, is obtained directly for division in a divider 26. The clock audio 25 is used for digitalizing the audiofrequency signal (KHz) , resulting from the sample conversion of the radio-frequency original signal (MHz) . Example 1 Always with reference to figure 5, the frequency of the sinusoid 17' and 17'' generated is given by: M *F_rtr F = ^■ CIK OUT - yN wherein M= tuning word = counting duration of the counter FC_K ⁼ frequency of the clocking system N= length in bit of the phase accumulator

The microcontroller 21 programs the two circuits 10' and 10'' to generate the sinusoids at the frequency /ι = PRFx and

where full _ scale S = N _ sample The clock audio 25 is obtained directly for division of the PRF_RX by the divider 26. A possible size is the following: If

it is possible to generate sinusoids that are different for

Δ _min=^= 0.018Hz

Therefore if PRF_TX=400 ΗZ the δ_min (resolution) obtainable is the same as

<A = -— = 0.11ps PRF_TX-AF^ PRF_TX to which corresponds (if N_sample = 2048) the full scale: full _ scale = N _ sample * δ^ = 0.2ns Example 2 Another possible example is the following: If f.s. = 128ns, PRF =400KHz, N_sample=2048 then „ fondo scala _rr . δ = = = 62.5 w N_ sample

PRFxx

and the "tuning word" 11' and 11'' is: 21.474.836 (0147AE14hex)

M^-INT PRF*x^λ * 2³² = 21.474.299 (0147 AB FB hex) V ^CLK J the filters 22' and 22'' are of type "low-pass" and provide a one pole RC circuit with frequency of cut of about 1,5MHz. Ground penetrating radar formed by Array of antennas The following are two exemplary embodiments of ground penetrating radars formed by array of antennas, each of which receives respective transmission and reception triggering pulses. With reference to figure 6, in a first exemplary embodiment, the transmission and reception triggering pulses are obtained by a single timing circuit 40 made as in Figure 5. The embodiment provides a time division multiplexer 50 that directs in turn to the various antennas 51 the respective transmission and reception triggering pulses and that receives from said antennas 51 respective audiofrequency signals, queues them and drives them to a single A/D conversion circuit 52 that digitalizes the signals so that it is possible to store them on a computer. This solution exploits the advantages of the timing circuit according to the invention, but requires an A/D conversion circuit 52 whose power is as high as the number of the antennas, with a maximum number of antennas that cannot be exceeded, except from adversely affecting strongly the performances of the ground penetrating radar. With reference to figure 7, in a second exemplary embodiment, said transmission and reception triggering pulses are obtained for each antenna by a respective timing circuit 40' made as in Figure 5 and synchronised with the other antennas. Each antenna, moreover, has an own A/D conversion circuit 52' . A synchroniser 54 activates the sender and the receiver of the second antenna 51 as soon as the S/H circuit of the first antenna 51 has taken the first sample of the RF signal received; then, the sender and the receiver of the third antenna are activated, and so on up to driving the last antenna of the array. The cycle is repeated up to the collection of the last sample and up to taking the received RF signal by the last antenna of the array. The A/D conversion circuit 52' of each antenna, digitalizes each signal taken by the S/H circuit and sends it to a concentrator 55 (hub) , which queues all signals already digitalized and provides them for being recorded on a computer. The exemplary embodiment of figure 7 is particularly advantageous since there is not an upper limit to the number of antennas, as instead in the example of figure 6.

Furthermore, it cannot be equipped with the timing circuits known in the art, based on analog integrators. With respect to the solution of figure 6, in the solution of figure 7, under a same antenna there are savings, since a multiplexer and a powerful A/D conversion circuit are not necessary. The saving is balanced by the fact that in the solution of figure 7 many timing circuits 40' and corresponding circuits of conversion A/D 52' are necessary, which however can be sized to the minimum, since their speed does not adversely affect the start of driving the following antenna. The invention above shown, in addition to providing a ground penetrating radar with an array of antennas according to the diagrammatical view of figure 7, gives further advantages among which: - the timing circuit 40 of Figure 5 is "auto-calibrated" and steady versus time, because the errors of conversion are due to the steadiness of the Clocking reference (typically <100ppm) , then much steadier of the known solutions (which provided analog integrators or delay lines) . - There are no limits on the maximum driving rate of the sender and the receiver, being virtually possible to set frequencies up to ten MHz and more;

- The timing circuit is very flexible, obtaining wide freedom concerning PRF setting, full scale and number of samples, with resolutions of a tenth of picosecond;

- The timing circuit can be reprogrammed "in real time" in order to use antennas at different frequencies by setting different temporisations. The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

1. A method for detecting, by means of radar techniques, objects immersed in a material, using a driver board (timing circuit) and at least one antenna, provides the steps of: - transmitting RF signals from said at least one antenna through said material, by driving said signals to said antenna by means of transmission triggering pulses spaced by a first period; — receiving RF signals from said at least one antenna reflected by said objects immersed in said material and sample-converting them through driving said antenna, by means of reception triggering pulses spaced by a second period, wherein said second period differs from said first period for a predetermined interval; - analysing said reflected signals for recognizing said objects immersed in said material; and characterised in that said transmission and reception triggering pulses are obtained separately as respective periodic signals in a digital domain and then converted into analog pulses spaced respectively of said first and second period.

2. Method according to claim 1, wherein said transmission and reception triggering pulses are obtained in a digital domain through the following steps: - prearranging an oscillator that generates a clock signal; - setting a counting duration; - carrying out a phase accumulation responsive to said counting duration and said clock signal to generate a phase-modulated periodic digital signal; - carrying out a phase/amplitude conversion of said phase-modulated periodic signal and providing an amplitude-modulated periodic digital signal, the resulting amplitude-modulated periodic digital signal being encoded into a quantized sinusoidal analog signal responsive to said clock signal and then transformed into analog pulses (trigger) .

3. Method according to claim 1, wherein said transmission and reception triggering pulses are obtained in parallel supplying respectively a first counting duration and a second counting duration that are different from each other for a predetermined numerical value derived from said interval, whereby the respective resulting periodic signals have respectively a first and a second frequency that are different from each other for a frequency value derived from said numerical value.

4. Method according to claim 1, wherein said quantized analog periodic signal is filtered by means of a low- pass filter to obtain a periodic sinusoidal signal before generating said driving pulses of the sender or of the receiver of the antenna.

5. Method according to claim 1, wherein said sinusoidal signals, which are periodic and have a double infinity of zero crossing, are compared with a reference signal to generate said triggering pulses of the antennas at each of said zero crossing.

6. Method according to claim 1, wherein said reception triggering pulses generate, through division by a predetermined factor, a clock signal used for digitalizing the audio-frequency signal, resulting from the sample conversion of the radio-frequency original signal.

7. Method according to claim 1, wherein said phase accumulation is reset after a predetermined cycle for starting again at each cycle with phase zero, said predetermined cycle corresponding in particular to the time necessary for carrying out the sample frequency conversion.

8. Method according to claim 1, wherein at each resetting step a signal is generated called "Start of Scan", which represents the beginning of each conversion cycle.

9. A method for detecting, by means of radar techniques, objects immersed in a material, using a plurality (array) of antennas and a serial distribution (TDM) of triggering pulses, transmission and reception trigger signals being obtained by a single timing circuit according to claims 1-9 that carries out a sample conversion of the frequency of the received signals based on the use of digital components, a multiplexer being provided that directs to a first antenna the respective transmission and reception pulses up to the end of the sample conversion and receives from it the relative audio-frequency signal, for then driving another antenna, and so on up to driving the last antenna of the array, said multiplexer queuing the many audio-frequency signals received by said antennas and addressing them to a single A/D conversion circuit that digitalizes the signals so that it is possible to store them on computer.

10. A method for detecting, by means of radar techniques, objects immersed in a material, using a plurality (array) of antennas, transmission and reception triggering pulses being obtained by a timing circuit according to claims 1-9 for each antenna, and the timing circuit of each antenna being synchronised with the other antennas and having each antenna an own A/D conversion circuit, a concentrator being furthermore provided for reconstruction of the signals recorded by the various antennas starting from the samples producted by the A/D circuits.

1. A radar apparatus, having at least one antenna, for detecting objects immersed in a material, comprising: - means for driving, to said at least one antenna, transmission triggering pulses spaced by a first period for transmitting RF signals towards said objects and said material through said antenna; - means for driving, to said at least one antenna, reception triggering pulses spaced by a second period for receiving RF signals reflected by said objects and said material through said antenna, wherein said second period differs from said first period for a predetermined interval; - means for sample-converting said reflected signals on said interval obtaining a low frequency reflected signal, - means for analysing said low frequency reflected signal for recognizing said objects from said material; characterised in that said means for driving comprise a first and a second circuit to generate respectively reception and transmission triggering signals in a digital domain and then converting them into analog pulses spaced respectively of said first and second period.

12. Radar apparatus according to claim 11, wherein said first and second circuit comprise each: - a phase accumulator controlled numerically and having predetermined input counting duration and input clock frequency and having a phase-modulated periodic output digital signal; - a ROM for phase/amplitude conversion of the phase- modulated periodic digital signal and providing an amplitude-modulated periodic digital signal, - a D/A converter for converting said amplitude- modulated periodic digital signal into a quantized analog periodic signal, - means for transforming said analog periodic signal into analog pulses.

13. Radar apparatus according to claim 11, wherein said means for transforming comprise a comparator that detects the "zero crossing" of said analog periodic signal and generates driving pulses for each of said "zero crossing".

14. Radar apparatus according to claim 11, wherein upstream from said comparator a filter is provided for transforming said quantized analog periodic signal into a sinusoidal signal.

15. Radar apparatus according to claim 11, wherein a clock generator is provided, and a microcontroller is provided as well that administers in parallel said first and second circuit supplying as input to them respectively a first counting duration and a second counting duration that are different from each other for a numerical value derived from said interval, whereby the periodic signal resulting from said first circuit and the periodic signal resulting from said second circuit have respectively a first and a second frequency that are different from each other for a frequency value derived from said interval.

16. Radar apparatus according to claim 11, wherein a plurality is provided (array) of antennas, each of which receives respective transmission and reception triggering pulses, wherein said transmission and reception triggering pulses are obtained by a single timing circuit that contains said first and second circuit in association to said microcontroller, a time division multiplexer being provided that directs to a first antenna the respective transmission and reception pulses up to the end of the sample conversion and receives from it the relative audio-frequency signal, for then driving another antenna, and so on up to driving the last antenna of the array, said multiplexer queuing the many audio-frequency signals received by said antennas and addressing them to an A/D conversion circuit that digitalizes the signals so that it is possible to store them on computer.

17. Radar apparatus according to claim 11, wherein a plurality is provided (array) of antennas, each of which receives respective transmission and reception triggering pulses, wherein said transmission and reception triggering pulses are obtained for each antenna by a respective timing circuit that contains said first and second circuit in association to said microcontroller, and a respective A/D conversion circuit, a concentrator being provided suitable for reconstructing the signals recorded by the various antennas starting from the samples produced by said A/D circuits .