US3772701A

US3772701A - Satellite antenna autotrack system permitting error signals to appear at the earth station

Info

Publication number: US3772701A
Application number: US00114451A
Authority: US
Inventors: E Wilkinson
Original assignee: Comsat Corp
Current assignee: Comsat Corp
Priority date: 1971-02-11
Filing date: 1971-02-11
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13
Also published as: GB1352085A; FR2125394B1; AU3879072A; FR2125394A1; CA998458A; AU457885B2; IT949052B

Abstract

The invention pertains to satellite antenna autotrack systems which permit error signals to appear at the ground station rather than at the satellite''s antenna. A conventional four horn cluster is used to produce pairs of circularly polarized position signals which are detected and processed at the ground station to produce vertical and horizontal error signals. The four horn cluster is simultaneously used to transmit the satellite''s down link communications signals. The error signals are transmitted from the ground station to the satellite to reposition the satellite''s vertical and horizontal axis to correspond to the line of sight to the ground station.

Description

United States Patent [1 1 Wilkinson i 5] Nov. 13,1973

[ SATELLITE ANTENNA AUTOTRACK SYSTEM PERMITTING ERROR SIGNALS TO APPEAR AT THE EARTH STATION [76] Inventor: Ernest James Wilkinson, 3812 Willis Rd., Sudbury, Mass. 01776 [75] Inventor: Ernest James'Wilkinson,Sudbury,

Mass.

[73] Assignee: Communications Satellite Corporation, Washington, DC.

22 Filed: Feb.l1, 1971 211 Appl. No.: 114,451

[52] US. Cl.343/100 CS, 343/100 PE, 343/100 ST,

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-Richard E. Berger [57] ABSTRACT The invention pertains to satellite antenna autotrack systems which permit error signals to appear at the ground station rather than at the satellite's antenna. A conventional four horn cluster is used to produce pairs of circularly polarized position signals which are detected and processed at the ground station to produce vertical and horizontal error signals. The four horn cluster is simultaneously used to transmit the satellite's down link communications signals. The error signals are transmitted from the ground station to the satellite to reposition the satellites vertical and horizontal axis to correspond to the line of sight to the groun d station.

8 Claims, SDrawing Figures iDOWN LINK COMMUNICATIGNS SIGNAL 2 HYB A lo PAIENTEDRUHSSISTS 3.772.701 SHEET 1 CF 3 *DOWN LINK COMMUNICATIONS SIGNAL HYB A DOWN LINK coumumczmows OUTPUT I is I HYB l 22 2 1 20 2 A TRACKING TRACKING RECEIVER RECENER INVENTOR ERNEST J. WILKINSON \ERRTICAL HORIZgNTSL BY Z K R 0R RR R :M M a SIGNAL T0 SATE IGNAL LLITE comm STEM ATTORNEYS PATENTEDKUV Is 1973 3772.701

' SHEET 2 EF 3 COMMAND TONES W 1 54 I 56 BANDPASS BANDPASS FILTER FILTER VERTICAL 50 52 HORIZONTAL ERROR ERROR VOLTAGE LQWPASS LOWPASS VOLTAGE FILTER '

FILTER

46 ,48 DISCRIMINATOR DISCRIMINATOR AMPLIFIER 4o SATELLITE r42 COMMUNICATIONS DIRECTIONAL SIGNAL REPEATER F T R (LOW LEVEL) E SATELLITE ANTENNA Y GROUND TRANSMITTING ANTENNA COMMAND CARRIER HORIZQNTAL ERRoR VOLTAGE I I30 Y 36 FREQUENCY DEVIATOR s4, COMMAND DIRECTIONAL TONES FILTER FREQUENCY VERTICAL ERRoR VOLTAGE DEVIATOR COMMUNICATIONS CARRIERS COMMAND CARRIER FIGS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is in the field of satellite antenna autotrack systems.

2. Description of the Prior Art It is desirable to have the satellites antenna axisin coincidence with the line of sight to the ground station. A prior system for accomplishing line of sight correspondence includes equipment for generating the error signals at the terminals of the satellites antenna. Such a system necessitates the placing of the autotrack receiving and signal processing equipment, which often requires maintenance, on the satellite itself. Thus, maintenance of the receiving and processing equipment becomes most difficult and often impossible.

The advantages of placing the autotrack receiving and signal processing equipment at the ground station rather than at the satellites antenna terminals were realized in a prior device which used a special three antenna array on the satellite. Each-antenna was supplied with suppressed carrier, double side hand signals. The signals associated with each of the antennas are distinguished from each other by being modulated with a different low frequency signal. The ground station receives and demodulates the signals to generate error signals. Such systems are exemplified by the patents to- Cutler, US. Pat. No. 3,060,425 issued Oct. 23, 1962 and US. Pat. No. 3,088,697 issued May 7, 1963. These systems require the use of special positioning antennas selectively arranged on board the satellite in addition to the satellites communication antenna. Further, such systems require the use of complex modulating equipment on board the satellite. The need for this equipment on board'the satellite again presents maintenance problems.

SUMMARY OF THE INVENTION The instant invention provides a satellite antenna autotrack system which permits error signals to be generated at the ground station rather than at the satellites antenna terminals thereby reducing the equipment on board the satellite.

A pair of horns are displaced equally on opposite sides of the focal axis of a parabolic reflector. A second pair of horns are similarly arranged, but in a plane 90 from the first pair. Each pair provides signals to the ground station which enables the ground station to determine the offset of the satellite antenna pointing direction in the plane containing that pair of horns. Error signals are generated, correspondingto the offsets and are transmitted to the satellite to control the antenna servo motors to point the antenna in a direction to reduce the errors to zero. For ease of description, the first and second pair of horns will be referred to herein as producing horizontal and vertical position signals.

All four horns are excited by the same communications signal to provide a down link antenna lobe pattern whose axis is the pointing direction of the antenna.

Beacon frequencies are applied to the horns in such a manner that opposite sense circularly polarized signals are radiated by opposite horns with the corre sponding beams being squinted away from the focal axis in opposite directions. The vertical and horizontal quadrature.

error signals are distinguished by different beacon frequencies. The down link communications signal is at a third frequency and is radiated with identical polarization from all four horns.

If the satellite antenna pointing direction (focal axis) is coincident with the radio line of sight between the satellite and the ground station, equal intensity left and right circularly polarized signals are received at the ground station for each of the two beacon frequencies.

If the satellite is misaligned the signal radiating from one horn of each corresponding pair of horns will appear at the earth station with greater intensity than the signal from the opposite horn.

' The ground station which includes a single horn antenna with dual circularly polarized outputs of opposite sense receives the position signals and the downlink communications signals. Suitable processing equipment generates vertical and horizontal error signals in response to the receivedposition signals.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the satellite antenna feed excitation system of this invention for producing vertical and horizontal position signals,

FIG. 2 illustrates the ground antenna feed excitation system of this invention for producing vertical and horizontal error signals proportional 'to the received intensity of the position signals, and

FIG. 3 represents one example of a command system adapted to transmit the error signals to the satellite, receive the signals at the satellite and correct the satellite antenna position, and

FIG. 4 illustrates in simplified form how the pointing axis offset is seen at the ground station.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows the satellite antenna feed excitation systern for use with this invention. The four horn cluster 2 comprises horns A, B, C and D. Each horn is excited with a pair of orthogonal, linearly polarized probes. Theseprobes are designated A A B B C C D H and Dy for horns A, B, C and D respectively. The four hornsare located in the focal plane of a large parabolic reflector on board the satellite.

Terminals A A B By, C Cy, D and D, are each connected to their correspondingly designated probes. Horns A and C may be viewed as operating in conjunction to develop vertical position signals with horns B and D operating in conjunction to develop horizontal position signals.

A vertical beacon frequency f is applied to the linear probes of the horns A and C in a manner to result in an f, beacon of right hand circular polarization radiated from horn A and an f beacon of left hand circular polarization radiated from horn C. The horizontally positioned probes, A and C receive the beacon frequency signal f directly while the vertically aligned probes A and Cy receive the beacon frequency signal f, in phase The beacon frequency signal f is applied through a delay line, 4, which may be any conventional device resulting in a phase shift of the input signal to delay line 4. The output of delay line 4 is applied to the difference terminal, A, of a conventional hybrid circuit 6 resulting in output signals at terminals A and Cy of equal amplitude and frequency, but of opposite phase. The f signal at A will have the same phase as the input to the terminal A, whereas the f, signal at Cy will be 180 out of phase with the input to terminal A. Both of the latter mentioned f signals will be 90 out of phase, or in phase quadrature, with the corresponding f signals applied to probes A and C Since the signals at frequency f, which are applied to horn A are in space quadrature (two electric field vectors oriented 90 apart in space) and phase quadrature, the resulting f beacon radiated by horn A will be circularly polarized. The same is true for the f, beacon radiated by horn C. However, since in one horn the f signal applied to the horizontal probe phase leads the f signal applied to the vertical probe, and in the other horn the opposite is true, the two circularly polarized beacons will be of opposite sense.

The above explanation applies equally to the generation of opposite sense circularly polarized beacons from horns B and D, with the only difference being that the beacon frequency is f,, to distinguish it from f and the two horns B and D are in aplane 90 from the plane of horns A and C.

In addition to radiating position signals, the four horn cluster 2 radiates the down link communications signal from the satellite to the ground station. This is accomplished by applying the down link communications signal through the summation terminals 2 of

hybrids

6 and 10 respectively to probes A By, Cy and D only.

Consequently, the down link communications signal is vertically polarized. It will be noted that the signal applied to the summation terminal 2 of either

hybrid

6 or 10 results in equal amplitude, equal phase signals at the input frequency.

In order to appreciate how the ground station can see the angular offset of the satellite pointing direction from the radio line of sight, a simplified pictorial representation of a two dimensional system will now be presented in connection with FIGS. 4A and 4B. In FIGS. 4A and 4B the numeral 43 designates the ground station antenna and it is assumed that it is pointing at the satellite. Ground station autotrack systems are well known for accomplishing the latter function. The satellite antenna isdesignated by the numeral 41 and as shown in FIG. 4A the satellite antenna pointing direction 51 is substantially coincident with the line of sight path 49. The

lobe patterns

45 and 47 represent two radiation beams in the same plane, e.g., vertical, and as explained above are radiated by the horns, such as horns A and C in FIG. 1, on opposite sides of the antenna pointing direction. The two beams are distin guishable since they are circularly polarized in opposite sense. The ground station will detect substantially the same amount of energy from both radiation beams indicating that the satellite antenna is properly pointing in the vertical plane.

FIG. 48 indicates the case of the satellite pointing axis 51 being angularly offset in the vertical plane from the line of sight path 49. This condition will be detected junction 18. The difference terminal A of hybrid 18 is coupled to diplexer 20 while the summation terminal 2 I is coupled to diplexer 22 through coupler 19. The sumat the ground station by receiving a greater amount of radiation from the lobe 47 than from the lobe 45. The ground station then generates a vertical error signal which is transmitted to the satellite to cause angular movement of the satellite antenna to bring'the pointing axis 51 in line with the line of sight 49 as in FIG. 4A.

The manner in which the ground station detects the differing amounts of radiation received from the two radiation lobes in each of the coordinate planes and demation terminal 2 output signal is also coupled through coupler 19. to the ground station's communications equipment (not shown).

One output of diplexer 20 is coupled to one input of a two channel tracking receiver 24 with the second output of the diplexer 20 being coupled to one input of two channel tracking receiver 26. Similarly, one output from diplexer 22 is coupled to a second input of the two channel tracking receiver 24 with the second output of diplexer 22 being coupled to the other input of two channel tracking receiver 26. The signal at the output of tracking receiver 24 will be the horizontal error signal while the signal at the output of receiver 26 will be the vertical error signal. It is, understood by those skilled in the art that the vertical error signal may be generated at the output of receiver 24 while the horizontal error signal may be generated at the output of receiver 26 merely by rearranging the inputs to the receivers from the

diplexers

20 and 22.

Operation of the ground antenna feed excitation system will now be described.

The single horn 14 with phase shifter 16 may be best understood by describing the device as a transmitter and remembering that it will have reciprocal operation as a receiver. A signal applied to E will be vertically polarized. The phase shifter 16, positioned at 45 to the plane of polarization will split the signal into its space quadrature components with one component lagging the other in phase by The result is a left hand circularly polarized signal.

Considering the horizontal probe E with the phase shifter 16, the same result will occur except that the final signal will have right hand circular polarization. The opposite sense polarization is due to a reversal in the relative phase shifts in the space quadrature components. This difference can be conceptually visualized by dividing a vertical vector, representing a signal applied to E into its space quadrature vectors and assigninga 90 phase to the quadrature component aligned with the phase shifter 16, and doing the same for a horizontal vector, representing a signal applied to Considering the device as a receiver, and remembering that there is reciprocity between receiver and transmitter operation, a circularly polarized signal having right hand sense will result in an output signal at probe E whereas a circularly polarized signal having left hand sense will result in an output signal at probe E A' plane polarized signal will result in an output at E and E If the right and left hand circularly polarized signals received by horn 14 are of equal energy levels then the energy of the output signal at probe E is equal to the energy of the output signal at probe E resulting in a zero output at the difference terminal A of hybrid 18. Similarly, if the circularly polarized signals are received with different intensities, as'would be the case if the satellite antenna was misaligned, the output signal on one of the probes E or B, would be of a greater magnitude than the signal on the other. Under such conditions a signal proportional to the difference in received intensity of the circularly polarized signals appears at difference terminal A of hybrid 18.

Thus, if the satellite pointing direction is misaligned along the horizontal axis one of the signals from horns B and D appears at horn 14 with greater intensity than the other. The outputs from probes E and E reflect this difference in received intensity, producing a signal at the A terminal of hybrid 18, the polarity of which is detennined by whether the satellite antennas pointing direction is offset to the right or left of the radio line of sight between the satellite and the ground station.

Similarly, if the satellite antenna is offset up or down a signal appears at the A terminal of hybrid 18 of a magnitude and polarity indicative of the vertical misalignment.

The vertical and horizontal difference signals are frequency distinguishable. Diplexer 20, which is coupled to the A terminal of hybrid 18, separates the difference signals, with the signal corresponding to the vertical misalignment, that is, the signal at frequency f being fed to tracking receiver 26 while the signal corresponding to the horizontal misalignment of the antenna being applied to the tracking receiver 24.

At the summation terminal 2 or hybrid 18 appears the sum of the circularly polarized signals for each of the beacon frequencies as well as the down link communications signal. The sum signal is applied through coupler 19 to the ground stations communication equipment (not shown) and diplexer 22. At diplexer 22 the sum signal is separated into signals at frequencies f and f, to provide reference signals to

receivers

24 and 26.

Tracking receivers

24 and 26 are known in the art. One example of a receiver which can be used as

receivers

24 and 26 is the ITT Model 4004 Pulse Tracking Receiver manufactured by lnternational Telephone and Telegraph Corporation. In a manner known in the art the tracking receivers process the inputs thereto and produce error signals proportional to the antennas misalignment.

These error signals are coupled to the ground station's command system which modulates and transmits them to the satellite via the ground to satellite communications link. At the satellite the error signals are detected and separated from the communications and command signals and used to drive suitable antenna position correcting apparatus. Ground station and satellite command systems as well as antenna position cor-. recting apparatus as known in the art and a description thereof is not necessary for a full understanding of this invention. However, in order to more fully appreciate the operation of the instant invention, a brief description of a known command system will be given to illus- .trate how the error signals can be transmitted from the ground station to the satellite.

It is understood by those skilled in the art that particular means for transmitting the ground developed satellite error signals to the satellite depends upon the particular type of command system used.

With respect to FIG. 3 there is illustrated one command system which utilizes audio tones sent out in bursts, the number of bursts being sent representing the command. These command tones are sent to the satellite via a frequency modulating system. Each of the error signals is applied to a frequency deviator simultaneously with the command tones and command carrier signals.

For example, the horizontal error signals may be applied to one input of frequency deviator 30 with the vertical error signals being applied to one input of frequency deviator 32. By applying a command carrier signal to the

frequency deviators

30, 32, the carrier is frequency modulated in response to the error voltages. The modulated command signals are then combined with the ground to satellite communications signals in directional filter 34. The composite signal is transmitted to the satellite by means of the ground station transmitting antenna 36.

The error signals, being at a much lower frequency than the audio command tones, can be easily separated from the audio command tones by a simple low pass filter after detection in the satellite.

The composite ground to satellite signal is received at the satellite 's receiving antenna 38. To insure proper orientation between the ground station transmitting antenna and the satellite, the ground antenna has a separate conventional autotrack system of its own (not shown) operating to keep the ground antenna pointed at the satellite. The signal received at the satellite's antenna 38 passes through a repeater 40 which reduces the center frequency of the carrier signals. In the systern being described, the transmitted carrier may have a center frequency at 6GHZ; with the output signal of the repeater having a center frequency at 4GHZ.

The modulated composite signal passes through directional filter 42 wherein the communications signal is separated from the command signal. The communications signal is processed by communications signal processing equipment on board the satellite (not shown). Such equipment and its operation is not part of the invention and a description thereof is not necessary for a full understanding thereof.

The modulated command carrier is passed through a conventional mixer amplifier 44 to

discriminators

46 and 48. The discriminators remove the command carriers to produce the command tones and the horizontal and vertical error signals. Since the frequency of the error signals is much lower than the frequency of the tone signals, low pass filters 50 and 52 effectively separate the vertical and horizontal error signals from the tone signals. These error signals are applied to suitable servo systems to drive the satellite antenna into alignment.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an antenna autotrack system for aligning a transmitter antenna pointing direction to correspond to the line of sight between a receiver antenna of a receiver and said transmitter antenna, the receiver producing error signals proportional to the angular offset of the transmitter antenna pointing direction with respect to the receiver, the improvement comprising:

a. means at said transmitter for generating electromagnetic beams, said means including first radiator means, displaced in a first plane on opposite sides ofthe focal axis of said transmitter antenna for generating a first pair of distinguishable, directional electromagnetic beams and second radiator means, displaced in a second plane on opposite sides of said focal axis for generating a second pair of distinguishable, directional electromagnetic beams, said second pair of beams being distinguishable from each other and from said first pair of beams; and

b. means, at said receiver, for receiving said beams and generating error signals proportional to the difference in the received intensities of said first pair of beams and said second pair of beams.

2. The antenna autotrack system of claim 1 wherein said first and second planes are orthogonal.

3. The antenna autotrack system of claim 2 wherein said means for receiving and generating includes means for distinguishing between said first and second pairs of beams and for generating error signals corresponding 7 to said angular offset of the pointing direction in said first and second planes, said error signals being proportional to the difference in the intensities of said received beams which comprise a pair of beams.

4. The antenna autotrack system of claim 3 wherein said distinguishable, directional beams are circularly polarized, with the beams in a pair of beams having opposite sense polarization.

5. The antenna autotrack system of claim 4 wherein said means for generating said electromagnetic beams comprises:

a. beacon means for generating beacon signals of different frequencies;

b. a first pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to beacon signals of a first frequency, for generating a first pair of circularly polarized beams of opposite sense; and i c. a second pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to beacon signals of a second frequency, for radiating circularly polarized beams of opposite sense. 7

6. The antenna autotrack system of claim wherein said means for receiving and generating includes:

a. a single horn radiator responsive to said pairs of di- 7. In a satellite antenna autotrack system of the type v wherein a satellite generates distinguishable, directional beams on opposite sides of the focal axis of an antenna, a ground station for producing error signals proportional to the offset of the antenna pointing direction with respect to the ground station comprising:

a. means for receiving said distinguishable, directional beams, said receiving means including a single horn radiator means for generating output signals proportional to the difference in the intensities of said received beams;'and

b. means, responsive to the difference in the intensity of said received beams, for generating error signals proportional to the difference in the intensities of said received beams.

8. A transmitting system including a transmitting antenna for generating electromagnetic beams to enable a distant ground receiving station to determine the offset of the pointing direction of the transmitting system antenna from the line of sight between the transmitting system and the ground receiving means comprising:

a. beacon means for generating beacon signals of different frequencies;

b. a first pair of radiators, located on opposite sides of the focal axis of said transmitting antenna, each radiator including a pair of orthogonal, linearly polarized probes responsive to a beacon signal of a first frequency, for generating a first pair of circularly polarized beams of opposite sense; and

a second pair of radiators, displaced on opposite sides of said focal axis in a plane distinct from said first pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to a beacon signal of a second frequency for generating a second pair of circularly polarized beams of opposite sense.

Claims

1. In an antenna autotrack system for aligning a transmitter antenna pointing direction to correspond to the line of sight between a receiver antenna of a receiver and said transmitter antenna, the receiver producing error signals proportional to the angular offset of the transmitter antenna pointing direction with respect to the receiver, the improvement comprising: a. means at said transmitter for generating electromagnetic beams, said means including first radiator means, displaced in a first plane on opposite sides of the focal axis of said transmitter antenna for generating a first pair of distinguishable, directional electromagnetic beams and second radiator means, displaced in a second plane on opposite sides of said focal axis for generating a second pair of distinguishable, directional electromagnetic beams, said second pair of beams being distinguishable from each other and from said first pair of beams; and b. means, at said receiver, for receiving said beams and generating error signals proportional to the difference in the received intensities of said first pair of beams and said second pair of beams.

3. The antenna autotrack system of claim 2 wherein said means for receiving and generating includes means for distinguishing between said first and second pairs of beams and for generating error signals corresponding to said angular offset of the pointing direction in said first and second planes, said error signals being proportional to the difference in the intensities of said received beams which comprise a pair of beams.

5. The antenna autotrack system of claim 4 wherein saId means for generating said electromagnetic beams comprises: a. beacon means for generating beacon signals of different frequencies; b. a first pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to beacon signals of a first frequency, for generating a first pair of circularly polarized beams of opposite sense; and c. a second pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to beacon signals of a second frequency, for radiating circularly polarized beams of opposite sense.

6. The antenna autotrack system of claim 5 wherein said means for receiving and generating includes: a. a single horn radiator responsive to said pairs of directional beams for generating dual, opposite sense circularly polarized output signals of a magnitude proportional to the difference in intensity of the beams in each of said pairs of beams; and b. tracking receiver means, responsive to said single horn radiator generated signals, for producing error signals proportional to the offset of the transmitter antenna pointing direction with respect to the receiver antenna.

7. In a satellite antenna autotrack system of the type wherein a satellite generates distinguishable, directional beams on opposite sides of the focal axis of an antenna, a ground station for producing error signals proportional to the offset of the antenna pointing direction with respect to the ground station comprising: a. means for receiving said distinguishable, directional beams, said receiving means including a single horn radiator means for generating output signals proportional to the difference in the intensities of said received beams; and b. means, responsive to the difference in the intensity of said received beams, for generating error signals proportional to the difference in the intensities of said received beams.

8. A transmitting system including a transmitting antenna for generating electromagnetic beams to enable a distant ground receiving station to determine the offset of the pointing direction of the transmitting system antenna from the line of sight between the transmitting system and the ground receiving means comprising: a. beacon means for generating beacon signals of different frequencies; b. a first pair of radiators, located on opposite sides of the focal axis of said transmitting antenna, each radiator including a pair of orthogonal, linearly polarized probes responsive to a beacon signal of a first frequency, for generating a first pair of circularly polarized beams of opposite sense; and c. a second pair of radiators, displaced on opposite sides of said focal axis in a plane distinct from said first pair of radiators, each radiator including a pair of orthogonal, linearly polarized probes responsive to a beacon signal of a second frequency for generating a second pair of circularly polarized beams of opposite sense.