U.S. patent application number 09/870365 was filed with the patent office on 2002-12-05 for method for accurately tracking and communicating with a satellite from a mobile platform.
Invention is credited to Carson, Ronald Steven.
Application Number | 20020180634 09/870365 |
Document ID | / |
Family ID | 25355226 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020180634 |
Kind Code |
A1 |
Carson, Ronald Steven |
December 5, 2002 |
METHOD FOR ACCURATELY TRACKING AND COMMUNICATING WITH A SATELLITE
FROM A MOBILE PLATFORM
Abstract
A method for accurately tracking and communicating with a
satellite from a mobile platform, wherein the satellite has an
antenna which performs both transmit and receive functions from a
single antenna aperture. The method involves using an inertial
reference unit (IRU) of the mobile platform to initially acquire
the signal from the satellite. A sequential lobing process is then
used to more accurately center the antenna aperture relative to the
receive beam from the satellite. The antenna is then used to
transmit data or other information, and the antenna pointing is
maintained by an additional IRU local to the antenna with higher
accuracy and lower latency than the IRU of the mobile platform.
Periodically, transmissions from the antenna are inhibited and the
sequential lobing process is repeated to eliminate for any inertial
reference drift error. The method allows for more accurate antenna
pointing when a single antenna aperture is used for both transmit
and receive functions without significantly interfering with the
transmission of data or information from the antenna.
Inventors: |
Carson, Ronald Steven;
(Renton, WA) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
25355226 |
Appl. No.: |
09/870365 |
Filed: |
May 30, 2001 |
Current U.S.
Class: |
342/74 ; 342/77;
342/79 |
Current CPC
Class: |
G01S 3/20 20130101; H01Q
3/08 20130101; H04B 7/18508 20130101; H01Q 1/125 20130101 |
Class at
Publication: |
342/74 ; 342/77;
342/79 |
International
Class: |
G01S 013/72 |
Claims
What is claimed is:
1. A method for accurately pointing an antenna aperture performing
receive and transmit functions at a target and tracking said
target, wherein said antenna aperture is disposed on a vehicle
having an inertial reference system (IRU), said method comprising
the steps of: a) using said IRU to enable said antenna aperture to
initially acquire said target; b) using a sequential lobing process
while said antenna is performing a receive-only function to more
accurately center said antenna aperture relative to a receive beam
transmitted from said target; c) using information from an
additional IRU located to said antenna aperture to track said
target while said antenna aperture is transmitting; and d)
intermittently interrupting said transmitting to perform said
sequential lobing process of step b) to correct for inertial
reference drift error.
2. The method of claim 1, wherein step c) is performed for a longer
period of time than said sequential lobing process of step b).
3. A method for accurately pointing an antenna aperture performing
receive and transmit functions at a target and tracking said
target, wherein said antenna aperture is disposed on a mobile
platform having an inertial reference system (IRU), said method
comprising the steps of: a) using said IRU to enable said antenna
aperture to initially acquire said target; b) using said antenna
aperture in a receive-only mode to perform a sequential lobing
process with a receive beam received from said target to center
said antenna aperture on said target; and c) once step b) is
completed, using an antenna rate gyro to maintain said antenna
aperture pointed at said target; and d) periodically interrupting
use of said antenna rate gyro in step c) to perform said sequential
lobing process of step b).
4. The method of claim 3, wherein step c) comprises the step of
transmitting information from said antenna aperture while said
antenna rate gyro is being used to maintain said antenna aperture
pointed at said target.
5. A method for accurately pointing an antenna aperture performing
receive and transmit functions at a target and tracking said
target, wherein said antenna aperture is disposed on a mobile
platform having an inertial reference system (IRU) and said IRU has
been used to initially acquire said target, said method comprising
the steps of: a) using said antenna aperture in a receive-only mode
to perform a sequential lobing process with a receive beam received
from said target to center said antenna aperture on said target;
and b) when said sequential lobing process is completed, then using
an antenna rate gyro to supply information for maintaining said
antenna aperture pointed at said target while said antenna aperture
is used to transmit information to said target; and c) periodically
interrupting use of said antenna rate gyro in step b) to perform
said sequential lobing process of step b) to correct for inertial
reference drift error of said IRU, to thereby maintain said antenna
aperture pointed at said target.
6. The method of claim 5, wherein step c) is performed for a
shorter time period than step a).
7. A method for accurately pointing an antenna aperture performing
receive and transmit functions at a target and tracking said
target, wherein said antenna aperture is disposed on a mobile
platform having an inertial reference system (IRU), the method
comprising the steps of: alternately using said antenna aperture in
receive-only and transmit-plus-receive modes, wherein a sequential
lobing process is performed while said antenna aperture is
operating in said receive-only mode to point said antenna aperture
at said target; and wherein a gyro local to said antenna aperture
is used to maintain said antenna aperture pointed at said target
when said antenna aperture is in said transmit-plus-receive
mode.
8. The method of claim 7, further comprising the step of using said
IRU to initially acquire said target before performing said
sequential lobing process.
9. The method of claim 7, wherein said sequential lobing process is
performed within a time frame substantially shorter than a time
period during which said antenna aperture is used in said
transmit-plus-receive mode.
10. A method for accurately pointing an antenna aperture performing
receive and transmit functions at a target and tracking said
target, wherein said antenna aperture is disposed on a mobile
platform having an inertial reference system (IRU) and said IRU has
been used to initially acquire said target, said method comprising
the steps of: a) using said antenna aperture in a receive-only mode
to perform a sequential lobing process with a receive beam received
from said target to center said antenna aperture on said target;
and b) when said sequential lobing process is completed, then using
an antenna rate gyro to supply information for maintaining said
antenna aperture pointed at said target while said antenna aperture
is used in a transmit-plus-receive mode to transmit information to
said target; and c) periodically interrupting transmissions from
said antenna aperture to re-perform said sequential lobing process
of step a), to thereby maintain said antenna aperture pointed at
said target.
11. The method of claim 10, wherein said sequential lobing process
is performed within a time frame substantially shorter than a time
period during which said antenna aperture is in said
transmit-plus-receive mode.
12. The method of claim 11, wherein said sequential lobing process
is performed in step a) in less than about one second.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to satellite communication
systems, and more particularly to a system for use with a mobile
platform having an antenna aperture performing receive and transmit
functions, and to a method for causing the antenna aperture to
accurately track a target satellite while the mobile platform is
moving.
BACKGROUND OF THE INVENTION
[0002] High precision tracking of communication satellites from
mobile platforms such as aircraft, ships and land vehicles is
required for both optimizing data rate (i.e., peak energy from/to
target) and for preventing interference with satellites orbiting
adjacent to a target satellite. Various methods for tracking have
been used including "dead-reckoning" or open-loop, where a
calculation is made as to the correct pointing angles based on
known satellite and platform positions and platform attitude. Other
methods, such as "closed loop" tracking methods, make use of some
form of feedback control by signal optimization. These methods work
adequately for received signals but not adequately for transmitted
signals from the mobile platform to satellites over distances which
create latency in the feedback control loop with respect to the
required bandwidth. Such a situation would be represented by, for
example, geostationary satellites communicating with mobile
platforms having high attitude accelerations, such as aircraft and
land vehicles.
[0003] For centering an antenna aperture with respect to a received
antenna beam from a satellite, one well known technique is
sequential lobing. Sequential lobing involves steering the antenna
aperture deliberately a known distance away from a peak received
signal in each of four directions around the peak signal. This is
illustrated in FIG. 1. Measurements of the received power or
decoded signal are used to calculate where the actual peak of the
beam or signal is located, as indicated in FIG. 1b, and the beam is
recentered for the next cycle. A receive-only antenna can use this
approach quite effectively to maintain pointing accuracy. However,
the deficiency with this approach is that a transmitting antenna
slaved to such a receive beam will execute the same step-tracking,
inherently adding inaccuracy as the beam is deliberately stepped
away from the known center.
[0004] A second technique of antenna pointing relies on
increasingly accurate methods of open-loop calculation to maintain
accurate pointing. Even if arbitrarily accurate tracking during
movement of the mobile platform is possible, the major drawback
with this approach is in initially establishing an accurate
starting point (i.e., initially determining an accurate estimate of
the position of the target satellite relative to the mobile
platform).
[0005] Prior systems have separated the receive and transmit
functions into discrete apertures so that sequential lobing can be
performed with the receive beam, and then the transmit beam is
computationally slaved to the correct position. This is illustrated
in FIG. 2.
[0006] For antennas in which transmit and receive functions are
performed in the same physical antenna aperture, several error
sources are eliminated, but the technique of sequential lobing
cannot be used when the transmit beam pointing accuracy requirement
is smaller than the step-size of the misalignments used during the
sequential lobing process. Such a situation would still result in
an error (i.e., misalignment) in the pointing angle of the transmit
antenna relative to the target satellite. This is shown in FIG.
3.
[0007] Therefore, there is a need for a method of accurately
pointing an antenna which performs both receive and transmit
functions, and which is located on a moving platform, at a target
satellite. More specifically, there is needed a method which allows
an antenna aperture performing both transmit and receive functions
to initially acquire a signal from a target satellite, and
thereafter to accurately determine the beam center of a receive
beam transmitted by the satellite, to use an inertial reference
unit (IRU) of the mobile platform for tracking the satellite, and
to periodically "fine adjust" the pointing of the antenna aperture
without significantly impeding the transmission of data from the
mobile platform to the satellite.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to a method for more
accurately tracking and communicating with a satellite from a
mobile platform. The method of the present invention is
particularly adapted for use with antenna apertures which are
required to receive and transmit information to and from a target
satellite while the mobile platform which the antenna aperture is
mounted on is moving.
[0009] The method of the present invention involves initially
requiring the satellite to use the mobile platform's inertial
reference unit (IRU) and information stored in an antenna
controller associated with the antenna aperture as to the
approximate location of the target satellite. Once the signal from
the satellite is acquired, a conventional sequential lobing process
is performed to more accurately center the antenna aperture
relative to the receive beam received by the aperture. Once the
sequential lobing process is completed, the antenna can then be
used to transmit data or other information from the mobile platform
to the satellite. While transmitting, a second inertial reference
unit in the form of a "rate gyro" local to the antenna is used
instead of the IRU of the aircraft to maintain the antenna aperture
pointed at the target satellite.
[0010] Periodically the antenna controller interrupts the
transmission of data or other information from the antenna to the
target satellite and again uses the receive capability of the
antenna to perform the sequential lobing process. Once this process
is completed, transmissions from the antenna are again enabled,
thus allowing the antenna to be used to transmit information to the
satellite. This process of alternately transmitting from the
antenna aperture and inhibiting transmissions while performing the
sequential lobing process is repeated continuously to eliminate
inertial reference drift error which could otherwise eventually
accumulate to an unacceptable level.
[0011] The above-described method does not significantly interfere
with the transmission of data or other information from the antenna
because the sequential lobing process can be performed in a matter
of milliseconds.
[0012] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention will become more fully understood from
the detailed description and the accompanying drawings,
wherein:
[0014] FIGS. 1a and 1b illustrates a sequential lobing process by
which an antenna aperture can be pointed at a beam center of a
received beam from a satellite;
[0015] FIG. 2 illustrates separate receive and antenna apertures
disposed on a mobile platform, with the transmit antenna aperture
slaved to the pointing direction of the receive antenna aperture,
and the inherent misalignment between the two antenna apertures
because of the physical separation thereof on the mobile
platform;
[0016] FIG. 3 is a simplified diagram of an antenna aperture which
performs transmit and receive functions, and illustrating the
degree of error present when the pointing accuracy requirement
during transmit is smaller than the step-size used in the
sequential lobing process performed with a receive beam;
[0017] FIG. 4 is a simplified view of a mobile platform having an
antenna aperture performing receive and transmit functions in
communication with a target satellite;
[0018] FIG. 5 is a flowchart illustrating the steps performed in
accordance with a preferred method of the present invention for
more accurately acquiring and tracking a target satellite with an
antenna aperture performing both transmit and receive functions
located on a mobile platform;
[0019] FIG. 6 is a simplified block diagram of a system in
accordance with a preferred embodiment of the present invention;
and
[0020] FIG. 7 is a graph illustrating how the method of the present
invention continuously corrects for inertial drift error by
periodically performing sequential lobing with the antenna.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The following description of the preferred embodiment(s) is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0022] Referring to FIG. 4, there is shown a mobile platform 10
having an antenna aperture 12 in communication with a target
satellite 14. The antenna aperture 12 is adapted to perform both
receive and transmit functions so as to be able to receive signals
from the satellite 14, as well as to transmit data and other
information to the satellite. The satellite 14 typically carries
one or more transponders 16 for transponding the signals to a
ground station (not shown). It will also be appreciated that the
mobile platform 10 includes an inertial reference unit (IRU), an
antenna controller and a receive/transmit antenna subsystem, as
indicated in FIGS. 2 and 3, for controlling pointing of the antenna
aperture 12.
[0023] It will also be appreciated that while the mobile platform
is illustrated as a fixed wing aircraft in FIG. 4, that the method
of the present invention is applicable to any mobile platform such
as a ship or a land vehicle.
[0024] Referring now to FIG. 5, the method of the present invention
will be described. The method involves initially using the IRU of
the aircraft 10 to acquire a signal from the satellite 14, as
indicated at step 18. This signal is then optimized using the
sequential lobing process described in connection with FIG. 1, as
indicated at step 20. It will be appreciated that while the
sequential lobing process is being performed, the antenna aperture
12 will be operating in a receive-only mode. Step 20 essentially
causes the antenna controller of the aircraft 10 to center the
antenna aperture 12 relative to the receive beam which it receives
from the satellite 14.
[0025] Once the sequential lobing process is completed, the antenna
12 can be used to begin transmitting data, as indicated at step 22.
As the attitude and location of the aircraft 10 change, another
inertial reference unit in the form of an antenna rate gyro
subsystem local to the antenna 12 is used to track the satellite 14
by re-pointing the antenna 12 as needed to maintain the antenna
pointed at the satellite 14, as indicated at step 24.
[0026] Periodically the transmissions from the antenna 12 are
inhibited, as indicated at step 26, and a sequential lobing process
is again performed, as indicated at step 28. Step 28 essentially
more accurately centers the antenna 12 on the receive beam to
eliminate any inertial reference drift error that may have occurred
since the initial sequential lobing process was performed at step
20.
[0027] Once the sequential lobing process performed at step 28 is
complete, tracking of the satellite 14 using the antenna rate gyro
of the aircraft 10 is resumed, as indicated at step 30. This
process is repeated, as indicated by feedback line 32, continuously
during flight of the aircraft 10. Typically, steps 26-30 may be
performed in a manner of milliseconds while step 24 may be
performed for a period of seconds to minutes. Thus, those time
intervals during which sequential lobing is periodically performed
are very brief relative to the time frame which the antenna rate
gyro of the aircraft 10 is used to track the satellite 14. As a
result, interruptions to the data transmissions from the antenna 12
are extremely brief and do not significantly impede the use of the
antenna 12 to transmit information and/or data to the satellite
14.
[0028] FIG. 6 is a simplified block diagram of a system 34 in
accordance with a preferred embodiment of the present invention. A
motor and sensor subsystem 36 is used to monitor and control
antenna positioning. A rate gyro subsystem 38 is used to provide
inertial reference information used to aim the antenna aperture
40.
[0029] FIG. 7 is a graph illustrating the pointing error versus
time, and how the periodic, repeated sequential lobing serves to
"re-center" the antenna 12. Portions 42 of waveform 44 indicate
gradually increasing inertial drift error. Spikes 46 indicate those
very short time periods (typically in milliseconds) during which
the transmit function of the antenna 12 is inhibited while
sequential lobing is performed to re-center the antenna relative to
the receive beam.
[0030] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *