As I describe in my TVRO page, this
invention was entirely my own work, but all of the principal engineers are
credited. John Ma, the project director, is listed first on all patents.
| United States Patent |
4,801,940 |
|
Ma
, et al.
|
January 31, 1989
|
Satellite seeking system for earth-station antennas for TVRO systems
Abstract
A TVRO earth station having a satellite seeking system comprising at least
one controllable motor for adjusting the position of an antenna for
receiving signals from a satellite having multiple transponders
transmitting signals at prescribed nominal center frequencies and with
different polarizations, control means for energizing the motor to move
said antenna along a predetermined satellite-searching path, a receiver
for receiving the incoming signals from the antenna and successively
tuning to the center frequencies at each of a succession of intervals
along the searching path, means responsive to the signals detected by the
receiver for producing a signal or value representing the quality of the
detected signals at each of the successive intervals along the searching
path, and means responsive to the quality-representing signal or value for
identifying the locations along the searching path at which the antenna
receives signals from a satellite.
| Inventors: |
Ma; John Y. (Milpitas, CA), McCracken; David H. (San Jose, CA), Weiss; Steven (Los Gatos, CA), Houston, III; Albert C. (Santa Cruz, CA) |
| Assignee: |
Capetronic (BSR) Ltd.
(Kowloon,
HK)
|
| Family ID:
|
25158066
|
|---|
| Appl. No.:
|
06/792,786 |
|---|
| Filed:
|
October 30, 1985 |
|---|
| Current U.S. Class: |
342/359 ; 342/356 |
| Current International Class: |
H01Q 1/12 (20060101); H01Q 3/00 (20060101); H01Q 003/00 () |
| Current CPC Class: |
H01Q 1/1257 (20130101); H01Q 3/005 (20130101) |
| Field of Search: |
343/352,359,356,362-364
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Blum; Theodore M.
Assistant Examiner: Hellner; Mark
Attorney, Agent or Firm: Rudisill; Stephen G.
Claims
We claim:
1. A TVRO earth station having a satellite seeking system comprising
at least one controllable motor for adjusting the position of an
antenna for receiving signals from a satellite having multiple
transponders transmitting signals at prescribed nominal center
frequencies and with different polarization,
control means for energizing said motor to move said antenna along a predetermined satellite-searching path,
a receiver for receiving the incoming signals from said antenna
and successively tuning to said center frequencies at each of a
succession of intervals along said searching path,
means responsive to the signals detected by said receiver for
producing signals or values representing the quality of the detected
signals at each of said successive intervals along said searching path,
at least one of said signals representing
the noise level associated with the detected signals and not
representing the signal level associated with the detected signals, and
means responsive to said quality-representing signals or values
for identifying the position along said searching path at which the
antenna receives the detected signals with a minimum noise figure.
2. The TVRO earth station of claim 1 wherein at least one of
said quality-representing signals or values represents the strength of
the incoming signals within a bandwidth of about 3 MHz centered on each
of said center frequencies.
3. The TVRO earth station of claim 1 wherein said receiver
includes a demodulator producing a video baseband output, and said
quality-representing signal or value represents the noise level in said
video baseband output.
4. The TVRO earth station of claim 3 wherein said noise
level-representing signal or value represents the noise level at about
23 MHz in said video baseband output.
5. The TVRO earth station of claim 1 wherein said receiver
includes a tuner for converting the incoming signals at said prescribed
nominal center frequencies to an IF frequency, and at least one of said
quality-representing signals or values
represents the strength of the resulting IF signal within a narrow
bandwidth at the IF center frequency.
6. The TVRO earth station of claim 1 which includes a
controllable polarizer for feeding the receiver incoming signals with
different selected angles of polarization, and control means for
adjusting said polarizer to a plurality of different
angles of polarization at each of said center frequencies, and wherein
said means for producing said quality-representing signals or values
produces said signal or value at each of said angles of polarization.
7. The TVRO earth station of claim 1 which includes means
responsive to said quality-representing signal or value produced at each
of said different angles of polarization for determining the optimum
angles of polarization for the signals
received from a given satellite.
8. The TVRO earth station of claim 1 which includes
means responsive to said quality-representing signals or values
for identifying the transponder transmitting the strongest signal from a
given satellite,
means for energizing said motor to move the antenna along a
predetermined optimizing path with said receiver tuned to the center
frequency of the transponder identified as transmitting the strongest
signal, and
means responsive to said quality-representing signals or values
produced during the antenna movement along said optimizing path for
determining the optimum antenna position for said satellite.
9. The TVRO earth station of claim 1 which includes means for
storing the values of said quality-representing signals for the
transponder transmitting the strongest signal for a given satellite, and
means for comparing these stored values with
corresponding values and substituting the new values for the
corresponding stored values whenever the new values are superior to the
stored values whereby said stored values always represent the best
values obtained as of any given time.
10. The TVRO earth station of claim 1 wherein said searching path encompasses an azimuth range of at least 2.degree..
11. The TVRO earth station of claim 1 wherein said
quality-representing signals include a signal representing information
about the signal-to-signal ratio of the signals detected by said
receiver.
12. The TVRO earth station of claim 11 wherein said information
representing the signal-to-noise ratio is the noise figure of the
signals detected by said receiver.
13. The TVRO earth station of claim 12 wherein said
quality-representing signals include a signal representing the signal
strength within a narrow bandwidth at each of said center frequencies.
14. A TVRO earth station having a satellite seeking system comprising
at least one controllable motor for adjusting the position of an
antenna for receiving signals from a satellite having multiple
transponders transmitting signals at prescribed nominal center
frequencies and with different polarizations,
control means for energizing said motor to move said antenna along a predetermined satellite-searching path,
a receiver for receiving the incoming signals from said antenna
and successively tuning to said center frequencies at each of a
succession of intervals along said searching path,
means responsive to the signals detected by said receiver for
producing a first signal representing the strength of the incoming
signals within a narrow bandwidth at each of said center frequencies,
and a second signal representing the noise
figure associated with the incoming signals when the receiver is tuned
to each of said center frequencies, and
means responsive to said first and second signals for
determining the best position of said antenna for receiving signals from
a satellite, said best position corresponding to the position of the
antenna and the polarization angle at which
incoming signals are received with a minimum associated noise figure.
15. A method of seeking satellites using an antenna of an earth
station for satellite communication systems with a plurality of
geo-synchronous orbiting satellites broadcasting on a plurality of
channels,
said antenna being provided with remotely controllable
positioning means for controlling and referencing the position of the
antenna along the directions of both azimuth and elevation, the earth
station being provided with a receiver system
including means to ascertain the incoming signal strength and means to
ascertain an associated noise figure,
said method comprising the steps of:
orienting the antenna in the general direction of known
satellites and searching at a low resolution level, along a predefined
first search pattern within a predefined first search area, for the
presence of any discernible video signals from
broadcasting satellite on any of the plurality of broadcast channels and
at any of a plurality of predefined incoming signal polarization
angles,
searching at a high resolution level, if said low resolution
level detects the presence of video signals, to search along a
predefined second search pattern within a predefined second search area,
for the best possible position to receive signals
broadcast from said satellite, said best position corresponding to the
position of the antenna and the polarization angle at which incoming
signals are received with a minimum associated noise figure, and
successively repositioning said first search area, if said low
resolution level of searching does not detect the presence of any video
signals, at non-overlapping positions and continuing the satellite
search with the low resolution level until
the presence of some video signals in detected.
16. The method of claim 15 wherein the searching at said low
resolution level is performed at incremental positions of said antenna
along said first search pattern within said first search area, and
seeking a satellite at each incremental
position by:
scanning through said plurality of receivable channels to detect
the presence of video signals, measuring the noise figure of received
signals at each of said channels being scanned at each of said plurality
of polarization angles, and
determining the channel and polarization angle producing the lowest
noise figure.
17. The method of claim 15 wherein the searching at said high resolution level includes the steps of:
scanning through said plurality of receivable channels,
measuring the noise figure of received signals for each of said channels
being scanned at each of a plurality of polarization angles in order to
determine the strongest channel and the best
polarization angle, without changing the position of the antenna,
measuring the noise figure of received signals at incremental
positions of said antenna along said second search pattern within said
second search area, and determining the optimum position of said antenna
which corresponds to the lowest measured
noise figure, and
positioning said antenna to said determined optimum antenna position.
18. The method of claim 15 wherein said second search area for
the searching at said high resolution level is defined by a square with
sides two degrees in length in both the azimuth and elevation
directions.
19. The method of claim 15 wherein said first search area for
the searching at said low resolution level is a rectangular area defined
by sides of eight degrees and six degrees in length in the azimuth and
elevation directions, respectively.
20. A method of orienting an earth station antenna for
receiving telecommunication signals from a plurality of geo-synchronous
orbiting satellites broadcasting on a plurality of channels,
said earth station being provided with a receiver system
including means for ascertaining the incoming signal strengths and means
for ascertaining an associated noise figure at a plurality of
polarization angles,
said method comprising the steps of:
orienting the antenna in the general direction of said
broadcasting satellites, successively altering the position of said
antenna and the polarization angle of incoming signals along incremented
positions along predefined search patterns and in
predefined search areas, and measuring incoming signal strength and
related noise figure at each of said incremented positions on the basis
of a predefined search procedure to determined the best position for
reception of signals from a given satellite,
said best position corresponding to the position of the antenna and the
polarization angle at which incoming signals are received with a minimum
associated noise figure.
21. The method of claim 20 wherein said predefined search procedure includes first, second and third search levels,
said first search level comprising a low resolution search,
along a predefined first search pattern within a predefined first search
area, for the presence of discernable video signals from said
broadcasting satellites on any of said plurality of
broadcast channels and at any of a plurality of polarization angles,
said second search level being performed if said first search
level detects the presence of video signals, and comprising a high
resolution search, along a predefined second search pattern within a
predefined second search area, for the position
of said antenna at which said noise figure of incoming signals has the
lowest value, and
said third level being performed if said first search level does
not detect the presence of video signals, and successively
repositioning said first search area at non-overlapping positions.
22. The method of claim 21 wherein said first search level is
performed at incremental positions of said antenna along said first
search pattern within said first search area, with the seek procedure at
each incremental positions including the
steps of:
scanning through said plurality of receivable channels to detect
the presence of video signals, measuring the noise figure of received
signals at each of said channels being scanned at a plurality of
polarization angles, and determining the
channel and polarization angle having the lowest noise figure.
23. The method of claim 20 wherein said second search level includes the steps of
scanning through said plurality of receivable channels,
measuring the noise figure of received signals for each of said channels
being scanned at a plurality of polarization angles in order to
determine the strongest channel and the best
polarization angle, without changing in the position of the antenna,
measuring the noise figure of received signals at incremental
positions of said antenna along said second search pattern within said
second search area, and determining the optimum position of said antenna
which corresponds to the lowest measured
noise figure, and
setting said antenna to said determined optimum antenna position.
Description
BACKGROUND OF THE INVENTION
This invention generally relates to communication systems such
as TVRO's for the reception of audio and/or video transmission signals
broadcast from a plurality of orbiting earth satellites. More
particularly, the invention relates to a
earth-station antennas and techniques for accurately positioning them
for the reception of signals broadcast on one or more channels by
geosynchronous orbiting satellites, for reproduction on TVRO's or
similar systems.
In satellite communication systems, a transmitting earth station
generates a modulated carrier in the form of electromagnetic fields up
to a satellite, forming an "uplink". The incident electromagnetic waves
are collected by the satellite,
processed electronically to reformat the modulated carrier in some way,
and retransmitted to receiving earth stations, forming "downlinks." The
earth stations in these systems basically consist of a transmitting
and/or receiving power station functioning
in conjunction with an antenna subsystem and form strategic parts of the
satellite communication system.
In earth stations, particularly the receive-only type such as
TVRO's, the antenna and the way in which its orientation is controlled
plays a very important role especially with the rapidly increasing
number of orbiting satellites being positioned
in today's communication satellite systems. Antennas for receive-only
earth stations, such as conventional TVRO systems, have to be extremely
directional and must be capable of being oriented with increasing
accuracy in order to track and differentiate
among signals from satellites that are spaced increasingly closer
together. Misorientations of the order of even fractions of a degree
can mean the difference between perfect reception of a required channel
and total loss of reception altogether. This
makes manual positioning of earth-station antennas extremely bothersome
and inaccurate.
The increased positional accuracy also has to be complemented
with simplicity and convenience in locating orbiting satellites;
especially so because of the rapidly increasing number of private
individuals or consumers using TVRO systems to
receive television transmissions directly from orbiting satellites. The
projection of TVRO systems or similar compact earth station terminals
into the consumer electronics market has raised the need for an
efficient satellite-seeking technique for
antennas used with such systems, which is together simple, fast,
accurate and, in particular, lends itself easily to automation so that
the end user can conveniently control the antenna sub-system to receive
the channel of his choice from any
commercially broadcasting orbiting satellite.
SUMMARY OF THE INVENTION
It is the general object of this invention to provide a method
for the seeking of orbiting satellites with increased accuracy using
earth station antennas.
It is a related object of this invention to provide such a
method in a form that is significantly faster than conventional manual
or mechanical satellite seeking techniques for antennas.
A further object of this invention is to provide such a
satellite-seeking method in a form which can be conveniently automated
in order to make the whole process of looking for a satellite, orienting
the antenna for good reception on all
channels, and reorienting to another satellite automatic.
Other objects and advantages of the invention will be apparent
from the following detailed description and the accompanying drawings.
In accordance with the present invention, a TVRO receiving
system is provided with a satellite seeking system comprising at least
one controllable motor for adjusting the position of an antenna for
receiving signals from a satellite having
multiple transponders transmitting signals at prescribed nominal center
frequencies and with different polarizations, control means for
energizing the motor to move the antenna along a predetermined
satellite-searching path, a receiver for receiving the
incoming signals from the antenna and successively tuning to the center
frequencies at each of a succession of intervals along the searching
path, means responsive to the signals detected by the receiver for
producing a signal or value representing the
quality of the detected signals at each of the nominal center
frequencies at each of the successive intervals along the searching
path, and means responsive to the quality-representing signal or value
for identifying the locations along the searching
path at which the antenna receives signals from a satellite.
The quality-representing signal or value preferably includes
information representing the signal-to-noise ratio of the signals
detected by the receiver, such as the noise level of those signals, and
may also include information representing the
signal strength within a narrow bandwidth at each of the center
frequencies.
One particular embodiment of the invention uses three different
levels of seeking with different degrees of resolution. The highest
level, called the Level 1 seek, has the highest resolution and is used
by the antenna to search within a
predefined small patch of the "sky" for the best reception of one of the
channels (typically 24 in current satellite communication systems)
receivable from a particular satellite, once the antenna points in the
expected vicinity of the satellite. The
search is made in alternating azimuthal and elevational increments of
the antenna position starting from a point approximately centered on the
predefined patch, and the detected signal at the demodulator stage of
the TVRO is monitored for the lowest
noise as the patch is scanned to determine the position of best
reception.
The succeeding level is the Level 2 seek which is basically a
repeat of the Level 1 seek with the difference that the predefined patch
is comparatively larger than in Level 1, and the seek here is done for
each of the 24 channels receivable from
a given satellite as well as for different polarization angles in each
channel. Level 2 also ensures that there is no overlapping in
succeeding patches that are scanned. In Level 2 the indication of the
presence of a satellite is the reception of video
signals on any channel, and a Level 2 seek is stopped whenever the
operator sees what he considers to be a video image; otherwise the level
2 seek is continued until the entire predefined patch is scanned.
The lowest level is the Level 3 seek which functions to provide
non-overlapping physical movements in order to avoid repetitious area
seeking. Level 3 is basically used to move level 2 around in a
predefined pattern whenever a satellite is
initially being searched for. Level 3 is called upon to initiate a new
Level 2 seek when a Level 2 search does not turn up a receivable
satellite.
The above technique provides simple, convenient, accurate and easily automated satellite seeking as described below in detail.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and other objects and advantages thereof, may best
be understood by referring to the following detailed description in
conjunction with the accompanying drawings, in which:
FIG. 1 is a simplified block diagram of a conventional TVRO earth terminal showing the basic sections comprising the TVRO;
FIG. 2 is a block diagram of a preferred tuner system for use in the tuner block of FIG. 1;
FIG. 3 is a block diagram of a preferred demodulator for use in the demodulator block of FIG. 1;
FIG. 4 is a simplified block diagram of a TVRO earth station
terminal including the antenna positioning system with which this
invention may be conveniently used;
FIG. 5 is a flow chart of the steps involved in the overall search procedure according to the system of this invention;
FIG. 6 is a diagram of a preferred search pattern for use with the Level 1 seek according to this invention;
FIG. 7 is a flow chart of the sort procedure used as part of the
Level 1 seek at each incremental position of the satellite antenna
along the search pattern of FIG. 6;
FIG. 8 is a flow chart of the scan procedure used as part of the Level 2 seek according to the system of this invention;
FIG. 9 is a flow chart of the main stage of the Level 1 seek according to the present invention;
FIG. 10 is a diagram of a preferred search pattern for use with the Level 2 seek according to this invention;
FIG. 11 is a flow chart of the Level 2 seek procedure describing
how scanning for satellite signals is conducted along the predefined
search pattern of FIG. 10;
FIG. 12 is a flow chart of the Level 3 seek according to the present invention;
FIG. 13 is a diagram showing a preferred way of repositioning the Level 2 search pattern as part of the Level 3 seek; and
FIG. 14 is a schematic diagram of a preferred noise detector for use in the system of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the invention will be described in connection with
certain preferred embodiments, it will be understood that it is not
intended to limit the invention to those particular embodiments. On the
contrary, it is intended to cover all
alternatives, modifications and equivalent arrangements as may be
included within the spirit and scope of the invention as defined by the
appended claims.
Referring now to the drawings, in FIG. 1 there is shown a
functional block diagram of a TVRO earth station for the reception of
satellite signals. The system includes an antenna 11, which is
typically a paraboloidal dish equipped with a low
noise block (LNB) converter and related accessories and positioning
mechanisms, for capturing signals transmitted from orbiting satellites;
and a receiver system including a tuner 12, a demodulator 13, a video
processing and amplification section 14, and
an audio tuner 15.
The antenna 11 receives signals transmitted from the satellite
in the 4-GHz frequency band (3.7 to 4.2 GHz); and this entire block of
frequencies is converted to a 1st IF frequency range of 950 to 1450 MHz
by the block converter located at the
antenna site. The 1st IF signals are then sent via coaxial cable to the
tuner 12 which selects a particular channel for viewing and converts
the signals in that particular channel to a 2nd IF frequency range. The
2nd IF frequency range is preferably
high enough to permit the 2nd IF VCO frequencies to be above the 1st IF
block of frequencies, to prevent the VCO from interfering with the
desired signals. For a 1st IF frequency range of 950 to 1450 MHz, this
means that the center frequency of the
second IF frequency range must be at least 500 MHz. A particularly
preferred 2nd IF center frequency in the system of the present invention
is 612 MHz.
In the demodulator 13, the 2nd IF signal is passed through an
amplifier and a filter and on to a conventional video detector which
demodulates the frequency-modulated signal to the baseband of the
original video signal (e.g., 0 to 10 MHz),
producing a composite video signal output. The filter preferably has a
pass band that is only about 22 MHz wide; a pass band of this width
passes the essential video and audio information while rejecting
unwanted noise received by the antenna on the
edges of the selected channel.
The output of the demodulator comprises the baseband signals
which range from DC to about 8.5 MHz; this includes video information
from about 15 KHz to 4.2 MHz, and subcarriers from about 4.5 to 8.5 MHz.
FIG. 2 shows a simplified block diagram of a suitable tuner 12
for use in the TVRO system of FIG. 1. This tuner 12 includes a passband
filter 19 having a passband that is 500 MHz wide (to pass signals in
the 1st IF range of 950 to 1450 MHz).
From the filter 19, the 1st IF signals are passed through a preamplifier
20 to a superheterodyne circuit including a voltage-controlled
oscillator (VCO) 21 receiving a controlling input voltage on line 22,
and a mixer 23 for combining the output of the
VCO 21 with the 1st IF output of the amplifier 20. This converts the
1st IF signals to a desired 2nd IF frequency range. The resulting 2nd
IF signals are passed through a pair of amplifiers 24 and 25 and then on
to the demodulator 13.
By adjusting the controlling input voltage supplied to the VCO
21 via line 22, different channels (frequency bands) in the 1st IF
signals are centered on the center frequency of the 2nd IF output of the
mixer 23. Each channel typically contains
at least a video carrier signal, a color subcarrier signal, and an audio
signal at different prescribed frequencies. These carrier and
subcarrier signals for all the channels are transmitted simultaneously
from the satellite to the earth station antenna
11 and then over a cable to the tuner 12.
The following "Table I" is a list of the center frequencies for
24 transponders on a single satellite. Table I also lists the
corresponding center frequencies in the output from the block converter
(identified in Table I as the 1st IF center
frequencies) and the output frequencies required from the VCO 21 in
order to tune the receiver to each individual transponder. It will be
noted that the difference between the 1st IF center frequency and the
corresponding VCO output frequency for each
transponder is 612 MHz, which means that the center frequency of the 2nd
IF output from the mixer 23 is 612 MHz for every transponder. That is,
the VCO output frequencies listed in Table I will cause the 612-MHz
output frequency of the mixer 23 to be
centered on the corresponding 1st IF center frequency. For example, a
VCO output frequency of 2042 MHz will cause the 612-MHz output frequency
of the mixer to be centered on the 1430-MHz 1st IF center frequency of
transponder No. 1. A preferred system
for controlling the input voltage to the VCO 21 to produce the desired
output frequencies listed above is described in Ma et al. copending U.S.
patent application Ser. No. 792,784, filed 10-30-85, for "TVRO Earth
Station Receiver for Reducing
Interference and Improving Picture Quality."
TABLE I ______________________________________ Trans-
Transponder ponder 1st IF VCO 2nd IF Number Center Center Output
Center ("Channel") Freq. Freq. Freq. Freq.
______________________________________ 3720 MHz 1430 MHz 2042 MHz 612
MHz
2 3740 1410 2022 612 3 3760 1390 2002 612 4 3780 1370 1982 612 5 3800
1350 1962 612 6 3820 1330 1942 612 7 3840 1310 1922 612 8 3860 1290
1902 612 9 3880 1270 1882 612 10 3900 1250 1862 612 11 3920 1230 1842
612 12 3940 1210 1822 612 13 3960
1190 1802 612 14 3980 1170 1782 612 15 4000 1150 1762 612 16 4020
1130 1742 612 17 4040 1110 1722 612 18 4060 1090 1702 612 19 4080
1070 1682 612 20 4100 1050 1662 612 21 4120 1030 1642 612 22 4140
1010 1622 612 23 4160 990 1602 612 24 4180
970 1582 612 ______________________________________
FIG. 3 is a block diagram of a demodulator 13 for receiving the
2nd IF output of the tuner 12 in the TVRO system of FIG. 1. This
demodulator circuit includes a pair of conventional IF amplifiers 30 and
31 for receiving the 2nd IF signal from the
final amplifier 25 in the tuner 12. Both of these amplifiers 30 and 31
receive an automatic gain control (AGC) signal from an input terminal
32. From the amplifier 31, the 2nd IF signal is passed through a filter
33 and on to a conventional video
detector 34 which demodulates the frequency-modulated signal to the
baseband of the original video signal (e.g., 0 to 10 MHz), producing a
composite video output signal. The 2nd IF filter 33 preferably has a
pass band that is only about 22 MHz wide; a
pass band of this width passes the essential video and audio information
while rejecting unwanted noise received by the antenna on the edges of
the selected channel.
The AGC feedback loop includes an IF amplifier 36 which
amplifies the output of the filter 33 and supplies it to an AGC detector
37. The output of this detector 37 is passed through an AGC amplifier
38, which produces a signal strength meter
drive signal at a terminal 39. This signal strength meter is usually
located on the front panel of the TVRO receiver.
The illustrative demodulator also includes an IF amplifier 40
which receives the same input supplied to the video detector 34,
amplifies it, and passes it through a narrow passband filter 41. The
output of the filter 41 is passed through a
detector in the form of a diode 42. The signal passed by the diode 42
is smoothed by an amplifier 43 to produce a DC output voltage that can
be used to detect the presence of a signal near the center frequency of
the particular satellite channel to
which the receiver is tuned.
The output of the demodulator illustrated in FIG. 3 comprises
the baseband signals which range from DC to about 8.5 MHz; this includes
video information from about 15 KHz to 4.2 MHz, and subcarriers from
about 4.5 to 8.5 MHz. The video
information in these baseband signals is passed through the video
processing and amplification section 14 before being displayed on a
video monitor or television set, and the audio signals are passed
through the audio tuner 15 and then on to one or more
speakers which convert the signals to audible sound.
FIG. 4 is a representation of a typical TVRO earth station 200
including the antenna positioning system, with which the method of the
invention may be used to advantage. As shown, the earth station 200
basically consists of a paraboloidal
reception antenna 201 for capturing the satellite television signals,
broadcast in the form of a modulated carrier, and focusing them onto a
feed horn 202; a low-loss coaxial cable 203 for transferring the
received signals from the antenna to a TVRO
receiver 204 which processes the modulated signals into a displayable
format and performs various other control functions; and a conventional
audio/video monitor 205 for reproducing the originally broadcast
transmission.
FIG. 4 also shows a common way of mounting the reception antenna
201 which allows easy movement along both the azimuthal and the
elevational directions. Specifically, the antenna 201 is mounted
through a swivel mechanism 206 to a support rod
207. The extent of swivel motion or azimuthal placement of the antenna
is controlled by an electric motor 208 which is connected by a suitable
linkage to the swivel mechanism. The support rod 207 is mounted on its
end remote from the ground, through a
thrust bearing or hinge joint 209, to a vertical member 210 usually of
fixed height. On its end closer to the ground, the support axle 207 is
mounted through another thrust bearing or hinge joint 211 to a second
vertical member 212. This member 212 is
of controllable height, with an electric motor 213 mounted so as to be
capable of adjusting the height of the member 212 and hence the degree
of slant or elevation of the antenna 201.
The above type of mounting, generally referred to as a "polar
mount", has the advantage that if the support rod is aligned along a
true North-by-South line and the elevation adjusted for a heading which
is truly southerly, no further adjustments
in elevation are required in order to track the complete belt of
geo-stationary orbit satellites. The provision of the two motors for
easily controlling variations in azimuth as well as elevation makes the
positioning system versatile and especially
applicable to the satellite seeking method according to the system of
this invention.
The extent of the revolutions of the two motors 208 and 213 is
measured by special motor pulse extraction circuits within a motor
control console 214, to which the motors are connected via supply and
sense lines 215 and 216, respectively. The
pulse extraction circuits use the commutation pulses of the motors as a
reference to provide an accurate measurement of the number of
revolutions undergone by the motors in a given direction and hence the
relative change in the position of the satellite
antenna. A detailed description of such a circuit is presented in
co-pending Ma et al. U.S. patent application Ser. No. 771,667, filed
Sept. 3, 1985, for "Motor Pulse Extraction System". The information
relating to the revolutions of the motors 208
and 213 provides an accurate record of the azimuthal and elevational
changes, respectively, which the antenna 201 undergoes. This data is
fed to a microprocessor in the TVRO receiver 204 and is processed to be
used as a part of the satellite seek
procedure to be described below.
Referring now to FIG. 5, there is shown a flowchart 300 of the
overall search procedure executed by a software program controlling a
conventional microprocessor in the receiver 204. The first step 301 is
where system initialization takes place
and includes the referencing of all system variables involved in the
satellite seeking system. Of importance here are the parameters
relating the motor controls to the current position of the satellite
dish. Also falling within the scope of the system
initialization step 301 is the initial setting up of the satellite dish
so that it is oriented in the general direction of the geo-stationary
satellite orbit belt. This can be accomplished by the use of currently
available computer charts that provide
the location of every geo-stationary satellite that is within line of
sight of given geographic coordinates.
For example, for a geographic location directly above the north
pole, all North American domestic relay television satellites are
located within the geo-synchronous orbit belt from 70 degrees west to
140 degrees west. Using such information, the
satellite earth station antenna can be positioned so that it is
approximately oriented toward a known satellite location.
After the above arrangements have been completed, step 302 is
accessed, which in combination with steps 303 and 304 constitutes the
Level 3 seek which is explained in detail below with reference to FIGS.
12 and 13.
Step 302 uses the Level 2 seek procedure (described below in
connection with FIG. 11) to search within a predefined area for video
signals corresponding to any of the satellite channels. At step 303, a
check is made to determine whether any
video signals have been detected by the Level 2 seek procedure. If the
answer at step 303 is negative, step 304 is reached where the search
area for the Level 2 seek is redefined to an adjacent non-overlapping
location before reverting to step 302 where
a Level 2 seek is reiterated.
If the answer at step 303 is in the affirmative, that is, some
trace of discernable video has been found by the Level 2 seek procedure,
step 305 is accessed, which involves a high resolution Level 1 seek in
order to determine the precise position
of the antenna dish for optimum reception of signals from the satellite
in question.
Each of the three levels of seek represented in FIG. 5, referred
to hereinafter as "L1", "L2" and "L3", are described in detail below
using their respective flow charts and search patterns.
FIG. 6 shows the search pattern for the L1 seek procedure, which is the procedure providing the highest degree of resolution.
The initial part of the L1 seek consists of keeping the antenna
at its current position and measuring the noise figure of the received
signals. It can be safely assumed that the antenna, during an L1 seek,
is oriented in the direction of a
receivable satellite because the L1 seek is called in for fine tuning
the antenna position only after an L2 seek has located signals from a
receivable satellite. Subsequently, all the available channels are
scanned, without changing the antenna
position, and the system determines which channel, and which
polarization angle within that channel, provide optimum reception.
After the optimum channel and polarization angle are found, the
L1 seek conducts a search within a predefined area for the antenna
position that provides the best reception of this channel. That
position then represents the best orientation of
the satellite antenna for the reception of all channels from the
satellite in consideration. As shown in FIG. 6, the search area is
defined by a square ABDC having sides 2.degree. long in both azimuth
and elevation, with the initial antenna position in
the center of the square. The search is started by moving the antenna
to point A at the upper left corner of the square, and then along the
path shown by the arrows in incremental steps of half a degree in either
the azimuthal or the elevational
direction. At each new incremental position, a measurement is made for
the noise figure related to the channel being scanned. A comparison is
made at each step to determine the lowest of the measured noise figures.
Each time a comparison is made, the
higher noise figure is discarded and the lower noise figure and the
satellite position corresponding to it are stored. In this way, when
the search reaches the end of the search pattern, i.e., at point D, the
current stored value of the noise figure and
corresponding antenna position represent the lowest noise figure and the
best position of the satellite antenna for the reception of the
selected channel and hence the satellite under question.
FIG. 7 is a flowchart of the "sort" procedure 450 used by the
Level 1 seek at each incremental position of the satellite dish along
the search pattern of FIG. 6. This procedure 450 begins at step 451
which reads the current antenna position as
represented by the current azimuth value AZ.sub.c and the elevation
value EL.sub.c. At the next step 452, the current value N.sub.c of the
noise figure of the incoming signal is read and stored.
At the succeeding step 453, a comparison is made between the
current noise figure value N.sub.c and the previously recorded value
N.sub.o, and step 454 then determines whether N.sub.o is greater than
N.sub.c. If the answer at step 454 is
affirmative, i.e., the previous noise figure is greater than that of the
measurement, the present noise figure value N.sub.c is substituted for
N.sub.o at step 455. At the next step 456, the present azimuthal and
elevational position values AZ.sub.c,
EL.sub.c are also substituted for the previously stored azimuthal and
elevational positions AZ.sub.o and EL.sub.o. If the answer at step 454
is negative, i.e., the comparison of step 453 shows that the previously
stored noise figure value N.sub.o is
less than the value N.sub.c just measured, steps 455 and 456 are
bypassed so that there is no change in the stored values N.sub.o,
AZ.sub.o and EL.sub.o
FIG. 8 shows a flow chart 470 for the initial stage of the Level
1 seek. As described above this "scan 2" procedure involves the
selection of the strongest receivable channel and the best mode of
polarization for this channel, with the antenna
aimed in the direction in which it was aimed when the Level 1 seek was
called for.
The initial steps 471 and 472 initialize the loop variables
SAV.sub.c, C.sub.c, Co, No and SACC.sub.c which respectively represent
the current average signal strength, the current channel, the best
channel, the noise figure of the best channel,
and the current accumulated signal strength of all channels.
At step 473, the current channel value C.sub.c is read, and at
step 474 the current polarization value P.sub.c is set to 1. The value
P.sub.c is then used at step 475 to set the polarizer to a predetermined
polarization angle. With the TVRO
system now tuned to a known channel and set at a known polarization
angle, the current noise figure value N.sub.c and signal strength value
S.sub.c are read at step 476 (an exemplary system for producing the
noise figure values will be described below).
Step 477 then updates the value SACC.sub.c by adding the current signal
strength value S.sub.c to the previous value SACC.sub.p, so that the
stored value SACC.sub.c always represents the accumulated signal
strength of all the channels measured up to any
given time.
To determine whether the current signal strength is above or
below the average signal strength, step 408 determines whether S.sub.c
is less than SAV.sub.c. If the answer is affirmative, the system
advances directly to step 481 where the current
polarization value P.sub.c is incremented by one. A negative answer at
step 478 advances the system to step 479 which determines whether the
current noise figure N.sub.c is greater than the lowest previously
measured noise figure value N.sub.o. If the
answer is affirmative, the system again proceeds directly to step 481. A
negative answer at step 479 advances the system to step 480 where the
current values N.sub.c, S.sub.c and C.sub.c are all substituted for the
previously stored values N.sub.o,
S.sub.o and C.sub.o, and then advances to step 481.
Following the incrementing of the value P.sub.c at step 481,
step 482 determines whether or not the polarization value is greater
than four. This particular system is designed to test only four
polarization angles in each channel, but of course
this number could be varied to increase or decrease the sensitivity of
the system to different polarization angles. An affirmative answer at
step 482 indicates that the desired number of polarization angles have
been tested in the current channel, and
thus the channel value C.sub.p is decremented by one at step 483. Step
484 determines when the current channel value C.sub.c reaches 0, which
is an indication that all channels have been selected. It will be
recalled that the value C.sub.c was
initialized at 24, which means that 24 channels must be tested before an
affirmative answer is produced at step 484. Of course, with satellites
having a greater or lesser number of transponders, the initialized
value of C.sub.c can be changed
accordingly.
A negative answer at step 482 returns the system to step 475 so
that steps 476 through 482 are repeated for the same channel but with a
different polarization angle. A negative response at step 484 returns
the system to step 473, thereby causing
steps 474 through 484 to be repeated for a new channel, and for the
desired number of different polarization angles within that channel.
After all channels of a given satellite have been tested, as
indicated by an affirmative answer at step 484, the average signal
strength of all the channels is computed at step 485 as a value
SAV.sub.c, which is the value SACC.sub.c (representing
the accumulated single strength of all twenty-four channels) divided by
twenty-four. Step 486 then determines whether the stored value S.sub.o,
representing the signal strength of the best of all the channels, is
less than the average signal strength
value SAV.sub.c. If an affirmative answer is obtained at step 486, the
system returns to step 471 and repeats the entire procedure. A negative
response at step 486 advances the system to step 487 where the current
values C.sub.c and P.sub.c are set
equal to the stored values C.sub.o and P.sub.o representing the best
channel and the best polarization angle for that channel.
FIG. 9 is a flow chart 400 of the main stage of the Level 1
seek. Prior to the beginning of this stage, the procedure of FIG. 7 has
been used to identify the strongest channel receivable from the
particular satellite at which the antenna is
pointed.
In the main stage of the Level 1 seek, the satellite antenna is
moved along the search pattern defined by FIG. 6 in order to accurately
locate the position which provides the best reception of the particular
channel identified by the procedure of
FIG. 7. This position will then provide the optimum orientation of the
antenna for receiving all channels from this particular satellite.
In FIG. 9 the first step 401 moves the antenna to the upper left
corner of the square to be searched and initialzes a pair of
incremental counters .DELTA.AZ and .DELTA.EL which track the stepwise
changes in the position of the satellite antenna
in azimuth and elevation, respectively. A loop counter N is also
initialized.
At step 402, the "sort 1" procedure is called into the program.
This is the procedure of FIG. 7 and includes the comparison of noise
figures of the received signal at the current antenna position and the
previous antenna position, and retention
of the lower noise figure and corresponding antenna position for further
comparison.
At step 403 a half degree increment is added to the azimuthal
increment counter. This represents a physical movement in the position
of the satellite antenna of 0.5 degrees along the azimuth. More
specifically, the antenna is now aimed toward a
point E which is half a degree to the right of the starting point A in
FIG. 6, with no change in elevation. At step 404, a check is made to
determine whether the azimuthal limit of the Level 1 search pattern (see
FIG. 6) has been reached. This limit
corresponds to a value of the azimuth incremental operator .DELTA.AZ
equal to 2.degree.. If the answer at step 404 is negative, i.e., the
satellite has yet to reach the azimuthal limit B of the search pattern,
the program reverts to step 402 to continue
scanning at half-degree intervals until the end point B is reached, at
which time the answer at step 404 becomes affirmative.
At step 405 the loop counter N is incremented by one, followed
by a check to see if the counter has reached a value of 3, whose
significance is explained below. For the first pass through the main
loop, the answer at step 406 will be negative,
which advances the system to step 407 where the elevation incremental
operator .DELTA.EL is decremented by half a degree. This corresponds to
a physical movement in the position of the satellite antenna of half a
degree in elevation. More specifically,
the antenna is now oriented toward a point F which is half a degree
lower in elevation than the earlier point B.
At step 407 the "sort 1" procedure described above is called
again to evaluate the quality of the signal reception at the current
antenna position (point F). Step 409 then decrements the operator
.DELTA.AZ by half a degree which represents a
physical movement in antenna position of half a degree in azimuth. More
specifically, the antenna is now aimed toward a point G which is half a
degree displaced from the earlier point F along the decreasing
direction of azimuth. At step 410 the "sort
1" procedure is again called into operation to evaluate the signal
quality at point G. Then step 411 determines whether the azimuthal limit
of the search pattern has been reached. This azimuthal limit is the
end point H, which is reached when the value
of the azimuth incremental operator .DELTA.AZ is equal to zero. If the
answer at step 411 is negative, indicating that the antenna has not yet
reached the azimuthal end point H, the program reverts to step 409 to
continue scanning at half degree
intervals until the end point H is reached. When step 411 yields an
affirmative answer, the program accesses step 412.
At step 412 the elevation incremental operator is decremented
again by half a degree, which as described above corresponds to a
physical movement in the antenna position of half a degree in elevation
so that the antenna is aimed toward a point
displaced by half a degree in elevation from point H. The program then
reverts to step 402 to reiterate the seek procedure. During this second
pass through the main loop the satellite scans along a path traced out
by points
I.fwdarw.J.fwdarw.K.fwdarw.L.fwdarw.C. The loop counter reaches a value
of 2 during this second pass, the answer at step 406 is still negative,
and the antenna continues scanning as in the first pass to finally end
up at point C at the end of the second
pass.
The third pass of the program begins with the "sort 1" procedure
(step 402) at point C and continues at half degree intervals until the
azimuth incremental operator .DELTA.AS has reached a value of 2.0 (steps
402, 203, 406), i.e., the azimuth
limit or end point D of the Level 1 search pattern is reached. During
this third pass, the incrementing of the loop counter at step 405
results in a value of 3, step 406 yields an affirmative answer, and step
413 is accessed.
It must be noted that the "sort" 1 procedure, as described above
with reference to FIG. 7, performs comparisons to detect and store the
lowest noise figure and the corresponding antenna position. Hence, at
the end of step 406, the currently
stored value of the antenna position, which corresponds to the lowest
measured noise figure, represents the optimal position of the satellite
antenna for receiving the channel selected by the procedure of FIG. 8.
At step 413, the antenna position is
shifted to this optimal position and an indication is given at step 414
to show that the desired satellite has been accurately located. At this
point the located satellite can be identified on the basis of received
program content and named. The coded
name is stored along with the optimal antanna position for automatic
repositioning of the antenna in the future.
FIG. 10 shows the search pattern for the Level 2 seek procedure.
As noted above the Level 2 seek has less resolution than the Level 1
seek and uses a larger search pattern, as defined in FIG. 10 by the
rectangular area XYZW with sides of 8
degrees and 6 degrees along the azimuth and the elevation, respectively.
The search in this case is started at the midpoint M of the side XW of
the search pattern, and continued along the path shown by the arrows in
incremental steps of one degree in
the elevational position and two degrees in the azimuthal position of
the antenna dish.
At each new incremental position, all the 24 possible channels
from a satellite receivable within the search area are scanned rapidly,
at all polarization angles. A comparison is made at each step to
determine the lowest noise figure from all
the channels and all polarization angles. At the end of the comparison
the channel with the lowest noise figure is latched onto until
comparisons for the next incremental position of the antenna can be
made. The basic goal of the Level 2 seek is to
scan the search pattern for any discernible video indicating the
presence of a satellite. Further optimization of the antenna position
is then carried out by the Level 1 seek procedure described above.
The presence of any video signal on any particular channel and
at any particular polarization angle can be ascertained in many ways.
The simplest way is to let a human operator interface with the receiver
system during the Level 2 seek and
manually push a given control button whenever he sees a semblance of an
image on the receiver monitor. An automatic but more complex way is to
use a built-in artificial intelligence type of pattern recognition
system which recognizes the presence of a
video image on the receiver monitor screen. In either case, whenever
the presence of a video image is sensed, the Level 2 seek can be
interrupted to perform the high-resolution Level 1 seek.
If the Level 2 search pattern is completed without detecting any
video signals, the Level 2 seek can be continued in an adjacent
non-overlapping search pattern. This is facilitated by the choice of a
symmetrical path for the Level 2 search
pattern. For example, in FIG. 10, the search pattern starts at the
mid-point M of the side XW and ends at the midpoint V of the side YZ.
The next Level 2 seek can hence be conducted directly from point V
without any overlapping of search patterns, and
without leaving a gap between successive search patterns.
Since the purpose of the Level 2 seek is just to detect the
presence of a satellite within a predefined area, without actually
locating it accurately, the comparison of noise figures is performed at
wider intervals than in the Level 1 seek. For
instance, the increments in the elevational direction are one degree and
increments in the azimuthal direction are two degrees each. The choice
of the two- degree azimuthal increments is dictated by the FCC
regulation stipulating a minimum spacing of
two earth degrees between orbiting satellites for communications
systems. If the antenna increments its azimuthal position more than two
degrees at a time, there is a risk of missing a satellite altogether.
By using wider increments than the Level 1
seek, the Level 2 seek provides an extremely rapid means of scanning
through all channels at all desired polarization angles to detect the
presence of a satellite.
FIG. 11 is a flowchart 500 of the steps followed by the Level 2
seek procedure in scanning for satellite signals along the predefined
search pattern of FIG. 10. The first step 501 of the procedure
initializes system variables such as the azimuth
incremental counter .DELTA.AZ and the elevation incremental counter
.DELTA.EL, which control the stepwise changes in the position of the
satellite antenna in azimuth and elevation, respectively. Loop counters
N and M are also set to zero at this step.
The search procedure starts at point M of the search pattern (FIG. 10),
and at step 502 the "scan 2" procedure of FIG. 8 is called into the
program to determine the channel and polarization angle that produce the
lowest noise level.
At step 503, the elevation incremental counter .DELTA.EL is
decremented by one degree. This represents a physical movement in the
satellite position of one degree in elevation. More specifically, the
antenna is now aimed toward a point N which
is displaced from point M by one degree in elevation, without any change
in azimuth.
At step 504 the counter N is incremented, and then step 505
determines whether the elevational limit of the Level 2 search pattern
(FIG. 10) has been reached. This limit corresponds to a value of the
loop counter N equal to 4 since the elevation
side of the search pattern is 6.degree. in length and the 1.degree.
incremental search is started at the midpoint of the side. If the
answer at step 505 is negative, the program returns to step 502 and the
scanning is continued at incremental steps of
one degree until the point W is reached. At point W the answer at step
505 becomes affirmative, which advances the system to step 506 where the
azimuth incremental operator .DELTA.AZ is incremented by 2.degree..
This represents a physical movement in
the position of the antenna of 2.degree. along the azimuth. More
specifically, the antenna is now aimed toward point O which is displaced
by 2.degree. in azimuth from the previous point W, without any change
in elevation. Further, at step 506, the
loop counter N is initialized to zero in order to conveniently use it
for further searching, as described below.
At step 507 the "sort 2" procedure is called again. At step 508
the elevation incremental operator .DELTA.EL is incremented by one
degree, which produces a change of 1.degree. in the elevational
position of the antenna. The counter N is then
incremented at step 509, and step 510 determines whether the upper
elevational limit of the Level 2 search pattern has been reached. This
limit corresponds to a loop counter value of 6, since the elevation side
of the search pattern is 6.degree. in
length. If the answer at step 510 is negative, the program returns to
step 507 and the scanning procedure is continued in incremental steps of
one degree until the point P is reached. At this point, step 510
yields an affirmative response, which
advances the system to step 511 where the azimuth incremental operator
is incremented again by 2.degree.. This effectively repositions the
antenna so that it is aimed toward point Q which is displaced by
2.degree. in azimuth from the previous point P.
At step 512 the "scan 2" procedure is again recalled into the
program, and the loop counter N is reset to zero. The elevation
incremental operator .DELTA.EL is then decremented by one degree at step
513, resulting in a change of one degree in
the elevational position of the antenna. The loop counter N is then
incremented at step 514, and step 515 determines whether the search has
reached the midpoint R of the elevation side QS of the search pattern.
This limit corresponds to a loop counter
value of N=3. If the answer at step 515 is negative, the program
reverts to step 512 and the 1.degree. incremental search is continued
until the midpoint R is reached. At this point, the answer at step 515
is affirmative and step 516 is reached.
The search pattern of FIG. 10 can be symmetrically split into
two segments. The first one, as tracked by the program so far,
comprises the path traced by points M, W, O, P, Q, and R. The second
segment is identical to the first, except for a
displacement in azimuth, and comprises the path traced by the points R,
S, T, U, Y and V. Hence, to scan along the second segment the program
described so far can be repeated using the point R as the starting
point.
Accordingly, at step 516, the loop counter N is initialized to
zero and the loop counter M is incremented to mark the end of scanning
of the first segment. Step 517 determines whether the second segment
has also been scanned as indicated by a
counter value of M=2. If the answer at step 517 is negative, the
program returns to step 502 and continues the incremental search along
the second segment until the end point V of the search pattern is
reached. At this point the answer at step 517 is
in the affirmative and step 518 marks the end of the Level 2 seek.
It will be noted that the choice of search pattern is important
for the proper functioning of the Level 2 seek procedure. It must be
chosen in such a way that the succeeding Level 2 seek may be implemented
immediately at the end of the previous
one without allowing any overlapping or skipping of the search area.
For example, at the end of a Level 2 seek, according to the search
pattern of FIG. 10, the antenna is oriented toward point V, and the
succeeding Level 2 seek can be started at point V
without any overlapping or skipping of the search area.
FIG. 12 is a flow chart of the Level 3 seek procedure according
to the system of this invention. As mentioned above, the Level 3 seek
procedure involves the positioning of the satellite antenna in order to
perform Level 2 seeks, according to the
Level 2 search pattern, at adjacent non-overlapping positions until a
receivable satellite signal is detected.
Accordingly at step 601, the current physical position of the
antenna is recorded in terms of azimuth and elevation readings AZ.sub.c
and EL.sub.c. At step 602 a Level 2 seek is performed at the current
antenna position. Step 603 then
determines whether a video flag is set, indicating the detection of a
video transmission by the Level 2 seek. If the answer is affirmative,
the program reaches step 604 where either come to a halt until a Level 1
seek is specifically called for or it
may proceed automatically with a Level 1 seek centered at the antenna
position where the video transmission was detected. If the answer at
step 603 is negative, i.e., the Level 2 seek has produced no discernible
video signals at the current antenna
position, the Level 2 seek is repositioned at an adjacent but
non-overlapping location at step 603, and the program returns to step
601 to continue with the satellite search procedure.
FIG. 13 shows one possible way of implementing the Level 3
search. In the absence of any detected video signals after completing
any Level 2 seek, the Level 2 seek search area or patch is moved from
its current patch to an adjacent and
non-overlapping patch 1. This repositioning is continued along a
spiraling path as defined in part by patches 2 through 12. Hence,
beginning with the initial position at which the Level 3 seek is
started, the satellite antenna is made to track the sky
along a predefined, gradually expanding and non-overlapping spiral path
until a Level 2 seek detects the presence of video signals. At this
point a Level 1 seek may be called to zero in on the satellite
broadcasting the video signals, or until the
physical constraints on the motion of the antenna are reached.
It will be noted that all positioning and referencing of the
antenna as part of the overall satellite seek procedure are based on the
extent of revolution of the two positioning motors as referenced by the
pulse count at the motor control block
of FIG. 4. At the start of the search procedure, the pulse counters for
the two motor pulse extraction systems are initialized so that all
further movement of the antenna may be referenced conveniently. All
subsequent changes in the azimuthal and
elevational readings are tracked and recorded by the microprocessor
within the TVRO receiver system.
Whenever an optimum position for a particular satellite is
found, it is stored, in terms of the number of pulses that the motors
are displaced from the reference position. Thus, the antenna may be
conveniently and automatically repositioned to
be oriented directly toward the same satellite whenever needed. In case
there is any displacement from the optimum position (due to mechanical
error or any other problem) during repositioning of the antenna towards a
satellite whose position has been
discovered and recorded earlier, the basic search procedure according to
the system described above can be undergone again in order to redefine
the optimum position of the antenna for the satellite in question.
By following the procedure outline above, the earth station
antenna can be used to successively seek all satellites broadcasting
commerically from the geo-synchronous orbit belt, and to record the
optimum antenna positions for the respective
satellites in terms of the displacement of the positioning motors. Once
such a database of satellite antenna positions is set up, locating a
satellite or shifting from one satellite to the other automatically is a
simple matter of recalling the
appropriate antenna position from the database in memory and then
controlling the positioning system to properly orient the antenna. If
needed, fine tuning of the antenna position can be performed as
mentioned above, and the new antenna position can be
used to update the earlier position recorded within the database.
FIG. 14 shows the details of a preferred noise detector for
furnishing the microprocessor with the noise figure values referred to
above. In this particular detector the video baseband signal from the
demodulator 13 is initially fed through a
bandpass filter 60 which preferably has a pass band that is about 500
KHz wide centered at about 23 MHz, which is well above the video
information in the baseband signal. The 23-MHz center frequency also
avoids interference from 27-MHz CB signals,
21-MHz and 24.5-MHz ham radio signals, and harmonics of the 4-MHz output
of the crystal oscillator in the tuner 12.
The output of the bandpass filter 60 is passed through a
conventional RF amplifier 61 to a detector in the form of a diode 62.
This diode 62 rectifies the AC output from the amplifier 61, and the
resulting signal is smoothed by passing it
through a DC amplifier 63 and a low pass filter 64. It is the smooth DC
output of the filter 64 that is applied to the microprocessor via an
analog-to-digital converter; the magnitude of this DC signal will vary
in direct proportion to the noise level
in the video baseband output from the demodulator.
The polarization angle referred to above is adjusted by a
microprocessor output signal which is passed through a digital-to-analog
converter to produce a DC voltage for application to a conventional
polarizer. TVRO systems normally include
polarizers which can adjust the relative alignment of the polarization
of the incoming signals and the orientation of the antenna. One type of
polarizer mechanically rotates the small probe that is included in the
feed horn of most earth station
antennas, by means of a small servomotor which is powered by either the
indoor receiver or an antenna positioner. A second type of polarizer
adjusts the polarization of the incoming signal electronically, by
changing the voltage applied to a coil wound
around an electromagnetic ferrite core located at the throat of the
feedhorn.
As is well known, the ferrite-core polarizer essentially acts as
a controlling phase shifter and has a feed horn arrangement for
accepting the incoming satellite signals and then passing them through
the ferrite core. When a voltage is applied
across the coil, an electromagnetic field of corresponding strength is
set up around the ferrite core. This field interacts with the
electromagnetic fields propagating through the core and rotates the
plane of polarization of the received signals to a
predetermined angle corresponding to the magnitude of the DC voltage
applied to the coil.
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