U.S. patent number 3,772,701 [Application Number 05/114,451] was granted by the patent office on 1973-11-13 for satellite antenna autotrack system permitting error signals to appear at the earth station.
This patent grant is currently assigned to Communications Satellite Corporation. Invention is credited to Ernest James Wilkinson.
United States Patent |
3,772,701 |
Wilkinson |
November 13, 1973 |
SATELLITE ANTENNA AUTOTRACK SYSTEM PERMITTING ERROR SIGNALS TO
APPEAR AT THE EARTH STATION
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.
Inventors: |
Wilkinson; Ernest James
(Sudbury, MA) |
Assignee: |
Communications Satellite
Corporation (Washington, DC)
|
Family
ID: |
22355292 |
Appl.
No.: |
05/114,451 |
Filed: |
February 11, 1971 |
Current U.S.
Class: |
342/353; 342/359;
342/354; 342/365; 342/367 |
Current CPC
Class: |
G01S
3/146 (20130101) |
Current International
Class: |
G01S
3/14 (20060101); G01s 001/02 () |
Field of
Search: |
;343/1CS,117,1PE,107,1ST,109 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Benjamin A.
Assistant Examiner: Berger; Richard E.
Claims
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 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.
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 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
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.
Description
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 satellite's antenna axis in 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 satellite's
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
satellite's 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 band 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, U.S. Pat. No. 3,060,425 issued Oct. 23, 1962 and U.S.
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 satellite's 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 satellite's 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.degree. 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, corresponding to 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 corresponding beams being squinted away
from the focal axis in opposite directions. The vertical and
horizontal 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 received position 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 system 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. These probes are designated A.sub.H, A.sub.V,
B.sub.H, B.sub.V, C.sub.H, C.sub.V, D.sub.H and D.sub.V for horns
A, B, C and D respectively. The four horns are located in the focal
plane of a large parabolic reflector on board the satellite.
Terminals A.sub.H, A.sub.V, B.sub.H, B.sub.V, C.sub.H, C.sub.V,
D.sub.H and D.sub.V 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.sub.1 is applied to the linear probes
of the horns A and C in a manner to result in an f.sub.1 beacon of
right hand circular polarization radiated from horn A and an
f.sub.1 beacon of left hand circular polarization radiated from
horn C. The horizontally positioned probes, A.sub.H and C.sub.H
receive the beacon frequency signal f.sub.1 directly while the
vertically aligned probes A.sub.V and C.sub.V receive the beacon
frequency signal f.sub.1 in phase quadrature.
The beacon frequency signal f.sub.1 is applied through a delay
line, 4, which may be any conventional device resulting in a
90.degree. phase shift of the input signal to delay line 4. The
output of delay line 4 is applied to the difference terminal,
.DELTA., of a conventional hybrid circuit 6 resulting in output
signals at terminals A.sub.V and C.sub.V of equal amplitude and
frequency, but of opposite phase. The f.sub.1 signal at A.sub.V
will have the same phase as the input to the terminal .DELTA.,
whereas the f.sub.1 signal at C.sub.V will be 180.degree. out of
phase with the input to terminal .DELTA.. Both of the latter
mentioned f.sub.1 signals will be 90.degree. out of phase, or in
phase quadrature, with the corresponding f.sub.1 signals applied to
probes A.sub.H and C.sub.H.
Since the signals at frequency f.sub.1 which are applied to horn A
are in space quadrature (two electric field vectors oriented
90.degree. apart in space) and phase quadrature, the resulting
f.sub.1 beacon radiated by horn A will be circularly polarized. The
same is true for the f.sub.1 beacon radiated by horn C. However,
since in one horn the f.sub.1 signal applied to the horizontal
probe phase leads the f.sub.1 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.sub.2, to
distinguish it from f.sub.1, and the two horns B and D are in a
plane 90.degree. 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 .SIGMA. of
hybrids 6 and 10 respectively to probes A.sub.V, B.sub.V, C.sub.V
and D.sub.V, only.
Consequently, the down link communications signal is vertically
polarized. It will be noted that the signal applied to the
summation terminal .SIGMA. 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 is designated 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
distinguishable 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. 4B 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 at 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 develops error signals for each of
said planes will be more fully understood in connection with the
description of FIG. 2.
Referring to FIG. 2, at the ground station, the single horn ground
feed 14 stands ready to receive the signals from the four horn
cluster 2. The single horn 14 is of conventional construction and
contains probes E.sub.V and E.sub.H and a 90.degree. phase shifter
16. This single horn configuration produces dual circularly
polarized outputs of opposite sense. Probes E.sub.V and E.sub.H are
coupled to hybrid junction 18. The difference terminal .DELTA. of
hybrid 18 is coupled to diplexer 20 while the summation terminal
.SIGMA. is coupled to diplexer 22 through coupler 19. The summation
terminal .SIGMA. 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.sub.V will be vertically polarized. The phase shifter 16,
positioned at 45.degree. to the plane of polarization will split
the signal into its space quadrature components with one component
lagging the other in phase by 90.degree.. The result is a left hand
circularly polarized signal.
Considering the horizontal probe E.sub.H 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.sub.V, into its space quadrature vectors and assigning
a -90.degree. phase to the quadrature component aligned with the
phase shifter 16, and doing the same for a horizontal vector,
representing a signal applied to E.sub.H.
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.sub.H, whereas a circularly polarized
signal having left hand sense will result in an output signal at
probe E.sub.V. A plane polarized signal will result in an output at
E.sub.V and E.sub.H.
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.sub.H is equal to the energy of the output signal
at probe E.sub.V, resulting in a zero output at the difference
terminal .DELTA. 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.sub.H or E.sub.V 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 .DELTA. 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.sub.V and E.sub.H reflect this difference in received
intensity, producing a signal at the .DELTA. terminal of hybrid 18,
the polarity of which is determined by whether the satellite
antenna's 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 .DELTA. 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 .DELTA.
terminal of hybrid 18, separates the difference signals, with the
signal corresponding to the vertical misalignment, that is, the
signal at frequency f.sub.1, 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 .SIGMA. 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 station's communication
equipment (not shown) and diplexer 22. At diplexer 22 the sum
signal is separated into signals at frequencies f.sub.1 and f.sub.2
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 International 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 antenna's 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 correcting 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 illustrate
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 system 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.
* * * * *