Satellite on-board switching system with satellite-to-satellite link

• US 4,004,098 A
• Filed: 07/10/1975
• Issued: 01/18/1977
• Est. Priority Date: 12/06/1973
• Status: Expired due to Term

• Current Assignee: Joy Mm Delaware Incorporated (Mitsui & Company Limited)
• Original Assignee: Communications Satellite Corporation (Reorganized ISA SA)
• Inventors: Shimasaki, Nobuhiko
• Primary Examiner(s): Stewart, David L.
• Application Number:
  US05/594,881
• Time in Patent Office: 558 Days
• Field of Search: 179/15 BS, 325/4, 178/69.5 R, 178/53
• US Class Current: 370/325
• CPC Class Codes: H04B 7/2045 SS-FDMA, FDMA satellite switching

Abstract

A Space Division Multiple Access (SDMA) Communications satellite system which utilizes a pair of communications satellite relays having a communications link between them. Each satellite includes a plurality of receive spotbeam antennas which are selectively connected to a plurality of transmit spotbeam antennas by an on-board switching matrix under control of an on-board oscillator. Each spotbeam antenna communicates with a different geographical zone which includes one or more earth stations. Each station may communicate with all other stations in the system including those within the field of view of the spotbeam antennas aboard the other satellite. In order to maintain synchronization between the satellites, a designated reference station transmits a reference synchronization burst of four sequential unique words to the satellite within its field of view. The reference burst is related to the other satellite and back again to the first satellite where it is relayed back to the reference station. Relative phase drift of the satellites'"'"' respective oscillators is detected at the reference station as a failure to detect the first or fourth unique word. Improper propagation delay due to distance variation between the satellites is detected at the reference station as a failure to detect the second or third unique word during a predetermined time period. Oscillator drift is corrected by changing the frequency and/or phase of the oscillator in one of the satellites through telemetry control circuitry at a control facility which may be the reference station. Improper propagation delay is corrected by adding or subtracting delay in the satellite to satellite communications path by the use of variable delay pads aboard one of the satellites which are remotely controlled by telemetry signals from the control facility.

Claims

1. A space division multiple access communications satellite system comprising:

a. a first satellite and a second satellite each satellite having on-board switching matrix means under control of a precision oscillator for selectively interconnecting a plurality of communications receivers each receiving incoming signals from a separate spotbeam antenna each in view of a separate geographical zone according to a predefined sequence;

b. means for establishing a communications link between said satellites;

c. a reference station including means for transmitting a burst of four unique words to said first satellite; and

d. means for relaying the unique word burst from said first satellite over said communications link to said second satellite, back to said first satellite and back to said reference station.

2. A communications satellite system as claimed in claim 1 wherein said reference station further includes;

a. a first detection means for detecting the first and fourth unique words in said unique word burst; and

b. means responsive to said first detecting means for producing oscillator control signals for use by said first satellite when said first and fourth unique words are not properly detected.

3. A communications satellite system as claimed in claim 2 wherein the precision oscillator aboard said first satellite may be adjusted in frequency and phase in response to said oscillator control signals in order to compensate for relative drift between the precision oscillator aboard said first satellite and the precision oscillator aboard said second satellite.

4. A communications satellite system as claimed in claim 3 wherein said first satellite includes adjustable delay pads in said communcations link the delay of which may be increased or decreased in response to control signals from the ground to said first satellite in order to vary the propagation delay in said communications link.

5. A communications satellite system as claimed in claim 4 wherein said reference station further includes;

a. second detecting means for detecting the second and third unique words of said unique word burst during a predetermined time interval; and

b. means responsive to said second detecting means for producing delay pad control signals for use by said first satellite when said second and third unique words are not properly detected.

6. A communcations satellite system as claimed in claim 5 further comprising:

a. means in said reference station for transmitting a two unique word burst to said first satellite;

b. means on-board said first satellite for causing said two unique word burst to be retransmitted to said reference station;

c. means in said reference station for detecting said two unique word burst;

d. means in said reference station responsive to said last named detecting means for synchronizing the transmission of said reference station with the switching matrix on-board said first satellite; and

e. wherein said predetermined time interval begins a predetermined time after said two unique word burst has been properly detected at said reference station.

7. A communications satellite system as claimed in claim 1 wherein each geographical zone includes a plurality of earth stations located therein and time division multiple access means for causing the earth stations within that geographical zone to sequentially access the spotbeam antenna in view of that geographical zone in a time divided manner.

8. In a space division multiple access communications satellite system employing a first and second satellite each having an on-board switching matrix means under control of a precision oscillator and each satellite having separate spotbeam antennae in view of separate geographic zones and each satellite being communicatively link one to another and to a terrestrial reference station, a method of detecting a state of synchronization between said switching matrices comprising the steps of

a. transmitting to said first satellite from said reference station a burst of four unique words;

b. relaying the unique word burst from said first satellite to said second satellite, back to said first satellite and back to said reference station;

c. detecting at said reference a preselected pair of unique words in the unique word burst;

d. transmitting control signals to said oscillator upon failure to properly detect said preselected unique word pair; and

e. adjusting said oscillator in accordance with said control signals.