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CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to co-pending U.S. patent applications
Ser. Nos. 263,849 entitled Satellite Cellular Telephone and Data
Communication System, 402,743 entitled Power Management System For A
Worldwide Multiple Satellite Communications System, U.S. Pat. No.
5,017,925 entitled Multiple Beam Deployable Space Antenna System, Ser. No.
415,815 entitled A Method To Optimize Cell-To-Cell Handoffs In A Satellite
Cellular System and U.S. Pat. No. 5,095,538 entitled Calibrated Method and
Device for Narrow Band Doppler Compensation.
BACKGROUND OF THE INVENTION
The subject invention generally pertains to Telemetry, Tracking and Control
(TT&C) of satellites and particularly of satellites employed in global
mobile communication systems employing cellular technology.
Present spacecraft or satellites for satellite constellation systems each
employ a TT&C transponder that is separate from the user voice/data
communication system for such satellites. These TT&C transponders
generally provide for "control" commands to be sent to the spacecraft from
a fixed ground station. "Telemetry" and "tracking" information is also
communicated from the spacecraft to the ground station over the TT&C
transponder. Thus, such communication requires a two-way transponder link
between each satellite and the ground station.
Telemetry data coming from the satellite informs a network operator about
the health and status of the satellite. For example, telemetry data may
include the amount of remaining hydrazine fuel for propelling rockets so
that the useful life of the satellite may be estimated. Moreover, critical
voltage and current magnitudes are monitored and provided as telemetry
data which enables the operator to determine whether or not the circuitry
of the satellite is operating properly. Tracking information includes
ephemeris data which allows the location of the satellite to be
determined. More specifically, a present satellite system utilizes the
TT&C transponder on board the satellite to send a tone down to the base
station to provide the range and the range rate of the satellite. The
altitude and angle of orbit of the satellite can be computed from this
information by the ground station operator. The tone may be modulated to
provide a higher degree of accuracy in determining the range and range
rate. The ground station provides "Control" commands in response to the
tracking or telemetry data to the satellite which may be utilized to
adjust the orbit of the satellite by energizing a selected jet of the
satellite, for instance. Moreover, other independent control commands can
be provided to reprogram the operation of the satellite to control other
functions of the satellite.
The TT&C information is generally encrypted to avoid undesirable
interference from the signals of other operators. Prior art systems
generally only allow exchange of TT&C information with a satellite when
the satellite is in line of sight with the fixed ground station. Also,
prior art TT&C links are between a particular fixed ground station and its
satellite and generally provide no TT&C communication link with other
satellites, for instance.
TT&C transponder links, that are separate from the user voice/data
channels, are presently employed on hundreds of satellites. Separate
transponders are generally used because the information handled by them is
generally of a different nature from the information in the user
communication channels. More specifically, TT&C information may be of a
predominantly digital form whereas the voice/data communication of some
prior art satellite systems is of an analog form which requires all of the
available bandwidth for the voice/data user communication channel.
Moreover, the data rate for TT&C signals is generally much lower than for
user data.
Unfortunately, utilization of the foregoing systems having separate
transponders for TT&C data transfer results in several problems. Such
prior art systems are not capable of mobile TT&C operation. Even in
satellite constellations where user voice/data channels are interlinked
between various satellites, generally the non-interlinking of TT&C
transponders prevents such mobile TT&C operation. Mobile TT&C operations
are advantageous for trouble shooting or for situations when the system
operator is required to be at any one of various locations. Also, each
satellite has only one TT&C transponder which tends to be expensive
because it is vital that such transponder reliably enables the associated
ground station to retain control over the satellite. Moreover, these
transponders utilize electrical power obtained from the onboard power
generating system which usually employs solar cells and batteries.
Moreover, the use of separate TT&C transponders undesirably increases the
weight of the prior art satellite systems and adds to the expense of the
manufacture, testing and delivery of such satellites into orbit.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a TT&C
system which employs a voice/data channel for communication of TT&C data
and hence, doesn't require a transponder that is separate from the user
data/voice communication channel equipment.
Another object is to provide a TT&C system which is suitable for satellites
being utilized in global, mobile, cellular communication functions.
In one form of the invention, a control system is included in a satellite
communication system having at least one satellite with transceivers
providing a plurality of communication channels for establishing
communication among a plurality of users. The control system includes a
satellite control subsystem on board each satellite and a ground station.
The satellite subsystem controls the functions of the satellite. One of
the user communication channels is coupled to the ground station and with
the satellite control subsystem for establishing TT&C communication so
that commands can be transmitted to the satellite control subsystem which
responds by controlling a selected function of the satellite. The control
system also includes a sensor subsystem on board the satellite for sensing
predetermined conditions on the satellite and providing telemetry data
through the user communication channel to the ground station. Moreover,
the control system can further include a position receiver on board the
satellite for monitoring and providing the ephemeris data of the
satellite. The ephemeris data is coupled through the user communication
channel so that the ephemeris data is sent from the satellite to the
ground station. Also, the ephemeris data can be coupled to the satellite
control subsystem to provide automatic on board control of the course of
the satellite.
The above and other objects, features, and advantages of the present
invention will be better understood from the following detailed
description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cell pattern provided by one satellite of a
multi-satellite cellular communication system;
FIG. 2 indicates cross linking between a ground control station and a
plurality of a satellites; and
FIG. 3 is a block diagram of the electronic system for a ground control
station and a satellite.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, a satellite 10 is illustrated which includes a
plurality of user data transmitter/receiver combinations hereinafter each
called a "transceiver". Satellite 10 also includes solar receptors 12,
transmitting antennas 14, and receiving antennas 16. The transmitters of
the transceivers each utilizes a separate transmitting antenna 14 to
simultaneously project a plurality of moving cells forming pattern 18 onto
a portion of the surface of the earth. A satellite cellular system
suitable for use with the subject invention is described in a patent
application entitled "SATELLITE CELLULAR TELEPHONE AND DATA COMMUNICATIONS
SYSTEM", Ser. No. 263,849 and which is assigned to the assignee of the
subject application and which has common inventors to the subject
application. The subject matter of the referenced application is
incorporated herein by reference to the extent that it is not
contradictory to the subject application.
Each individual cell, such as cell 20 of pattern 18, also includes the
airspace above the ground and can be characterized as a conical cell. The
system operator of ground station 22, even though mobile, is generally
regarded as being at a fixed place on the ground relative to the rapidly
moving satellite 10, which can travel at a speed of 17,000 miles per hour.
The cells are always moving because satellite 10 continuously moves. This
is in contrast to terrestrial mobile cellular systems wherein the cells
are generally thought of as being fixed and the mobile user moves through
the cells. As a cell moves "over" the user, the cellular switching system
must "hand-off" the user's communication to an adjacent cell. If the
satellites all move in the same direction and have substantially parallel,
low earth, polar orbits, then the adjacent cell pattern and/or adjacent
cell can be predicted by the cellular switching system with a high degree
of accuracy. "Hand-off" can be accomplished in the manner disclosed in the
aforementioned patent application. Amplitude information or alternately
bit error rate information can be utilized to effectuate the "hand-off".
Each satellite pattern of the cellular system may use a plurality of
four-cell clusters. One cluster includes cells 24, 26, 20 and 28 wherein
the cells operate at frequencies having magnitudes respectively designated
by A, B, C, and D. Nine such clusters are illustrated in FIG. 1 to provide
pattern 18. The reuse of frequencies A, B, C and D divides the amount of
spectrum otherwise required for communication with pattern 18 by
approximately nine. One of the transceivers of satellite 10, for example,
may use an uplink frequency of 1.5 gigahertz (GHz) to 1.52 GHz and a
downlink frequency of 1.6 to 1.62 GHz. Each cell pattern 18 may be
arranged to be 250 nautical miles in diameter and it can take 610 seconds
for a satellite cellular system to process a complete cell pattern. The
cellular frequency spectrum can be arranged as proposed by standards
published by the Electronic Industries Association (EIA) for terrestrial
cellular system coding. Digital techniques are employed by the user
channels for communicating voice and/or data information from one user to
another.
In accordance with the described embodiment, control station 22 located in
"A" frequency cell 24 will communicate TT&C information with satellite 10
utilizing one of the cellular user voice/data communication channels
instead of a separate TT&C transponder. Each of these cellular user
channels represents one data/voice line identified by a route or telephone
number. Typically, these channels originate and terminate on the surface
of the earth. However, when used as a TT&C link termination of the channel
and the recipient of a "call" can be satellite 10, for instance. Each
satellite in a constellation is assigned a unique address (i.e. a
telephone number). Ground station 22 can communicate directly with any
satellite coming within its line of sight range by signaling the
satellite's address. Similarly, ground station 22 also has a unique
address.
If satellite 10 is moving in the direction of arrow 30 such that cell 26
will next move over operator 22, then "A" cell 24 will "hand-off" to "B"
cell 26 which will later "hand-off" to "D" cell 32, for instance. If cell
26 becomes inoperative , then TT&C communication will be only temporarily
interrupted rather than possibly completely destroyed as is the case with
prior art systems having only one TT&C transponder per satellite. Hence,
the cellular system of FIG. 1 provides a high degree of reliability for
TT&C exchange because of the redundancy of the transceivers providing each
of the cells.
Referring to FIG. 2, a ground station 50 can communicate TT&C information
with a satellite 52 while in its line of sight through user channel 51.
Satellite 52 receives and sends TT&C from station 50 along with
multiplexed user data channels such as from user 53 over channel 55. The
cellular switching network recognizes the satellite identifier or address
for satellite 52 in the same manner as the network recognizes
terrestrial-based destinations. Also, if it is desired to pass TT&C data
to another satellite 54, which is not in the line of sight with station
50, then such data can be sent to satellite 52 and then transferred over
link 56 to satellite 54. Similarly, special provision can be made for
broadcasting overall network updates and TT&C data to and from every
satellite in the network.
If a satellite 58 needs to communicate its health and status sensor data to
ground control station 50, it originates a call and passes the data
through link 60 using the unique number for satellite 52. The TT&C
information is next downlinked through channel 51 to control station 50.
Typically, satellites such as 52, 54 and 58 are polled for TT&C data,
however, serious events affecting the health of any given satellite is
originated and communicated by that satellite through other satellites, if
necessary, to the control station. Thus the system of the invention allows
constant communication of TT&C data to and from control station 50 even
though control station 50 is not in the line of sight with the
communicating satellite.
FIG. 3 shows block diagrams of ground a station 100 and a satellite 102.
Ground station 100 can be either a fixed permanent station or a mobile
user employing a computer having a modem for communicating through a
standard telephone, for instance. Encoder 103 provides "addressed" signals
to transmitter 105. Transceiver link 104 transmits signals from
transmitter 105 of control station 100 to antenna subsystem 106 of
satellite 102. Receiver 108 of satellite 102 is connected between antenna
subsystem 106 and demodulator/demultiplexer system 110.
Router 112 is connected between the output of system 110 and the input of
multiplexer/modulator 114. Router 112 also processes the addresses of all
incoming data and sends appropriately addressed data to other satellites,
for instance, through multiplexer/modulator 114 which is also connected to
cross-link transceiver subsystem 116. Router 112 encodes the appropriate
addresses onto signals having destinations other than satellite 102.
Router 112 sorts out any messages for satellite 102 which are identified
by their address code. Global Positioning Satellite (GPS) receiver 118 is
connected to router 112 through conductor 120 and to satellite subsystem
122 through conductor 124. Router 112 is connected to satellite subsystem
122 through conductor 126 and to sensor subsystem 128 through conductor
130. Satellite control subsystem 122 decrypts command messages from router
112 for satellite 102 and cause appropriate action to be taken. Sensor
subsystem 128 provides telemetry data to router 112.
Global Positioning System (GPS) receiver 118 receives information from
existing GPS satellites in a known manner and determines the exact
location of satellite 102 in space Orbital space vectors are derived from
this information. GPS receiver 118 also determines the position of
satellite 102 relative to the GPS constellation. This information is
compared with the desired position information stored in router 112. Error
signals are generated by GPS receiver 118 which are sent to the satellite
control subsystem 122 for automatic course correction. The error signal is
used within control subsystem 122 to control small rockets which effect
the "course keeping" function. Hence, satellite 102 utilizes GPS
information to control its own course rather than only obtaining course
control from station 100. This on board control allows the position of
satellite 102 to be set and controlled within a few meters.
GPS receiver 118 also provides the space vectors to router 112 and sensor
subsystem 128 provides other telemetry information through conductor 130
to router 112 which composes messages that are coupled through conductor
132 to multiplexer/modulator 114 and through conductor 134, transmitter
136 and conductor 138 for transmission by antenna subsystem 106. These
messages are then transmitted through link 140 to receiver 108 of ground
station 100. Alternatively, when it is desired to correspond with a
different control station through another satellite link, then the
messages compiled by router 112 are sent through transceiver cross-link
subsystem 116. Thus, each satellite can "know" its position as well as the
position of its neighbors in the constellation. The ground operator also
has constant access to this ephemeris information.
Hence, unlike prior art systems which don't include GPS receivers, the
tracking or ephemeris data for satellite 102 is calculated on board
satellite 102. Satellite 102 doesn't have to have constant tracking
updates from ground station 100. Tracking control information is, however,
provided by ground station 100 when desirable to do so. The GPS signal is
a digital signal which is compatible with the digital cellular
communication links or channels employed for ground based user-to-user
communication. The foregoing method and system of providing tracking
information is different from prior art methods and systems such as tone
actuated satellite tracking systems presently employed through the
separate TT&C transponders. The on board acquisition of the digital GPS
signal format allows embedment of the tracking information in the channels
normally used for voice and/or data communication.
The previously described embodiment of the invention has many advantages
over prior art systems utilizing a separate TT&C transponder in each
satellite. Specifically, if the transponder of the prior art system fails,
then the satellite may become useless. Alternatively, since ground station
22 of FIG. 1, for instance, can use any of the transceivers associated
with satellite 10, even though one of these transceivers might fail, there
are still 35 others by which station 22 can maintain TT&C communication
with satellite 10. Furthermore, as shown in FIG. 2, even if all of the
downlinks of a particular satellite, e.g. 58, might fail, ground station
50 can communicate to that satellite through a cross link, e.g. 60,
through another satellite, e.g. 52. Thus the system of the invention
provides reliable TT&C communication.
Also, the TT&C system of the described embodiment can be in constant
communication with a particular satellite through cross links rather than
waiting for line of sight opportunities as with some prior art TT&C
systems. These prior art TT&C systems require that the ground station be
fixed whereas the system of the present invention can utilize mobile
ground operating stations. The mobile ground station has a unique address
or telephone number assigned to it and the location of the ground station
can be tracked in the same manner that subscribers are tracked by the
satellite cellular constellation.
In addition, by embedding the TT&C information in a voice/data channel of a
cellular communication system, the expense, size, power and weight
requirements of satellite for the described embodiment are minimized.
Moreover, the tracking system of the described embodiment utilizes a GPS
receiver on board the satellite to provide on board tracking and tracking
control rather than solely relying on ground control of tracking. This
digital tracking information is readily embedded in a digital cellular
user channel.
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