Ground-based beamforming for satellite communications systems

1. A satellite communications network having a ground-based beamforming (GBBF) system, said network comprising:

a satellite, said satellite comprising a satellite communications payload, said satellite communications payload comprising a satellite return path and a satellite forward path, a satellite tracking master reference oscillator, a calibration network, a plurality of couplers, a satellite generated payload beacon signal, a satellite generated payload pilot signal, and a plurality of multipart amplifiers coupled to a plurality of hybrid matrixes;

at least one gateway, communicatively coupled with the satellite via a feeder link;

a plurality of user terminals, each communicatively coupled with the satellite by a user link; and

a plurality of pointing beacon stations adapted to transmit and receive signals to and from the satellite;

wherein the GBBF system:

comprises a plurality of hybrid matrix outputs corresponding to a plurality of feed elements, a gateway generated pilot signal, a master reference oscillator, and a gateway tracking master reference oscillator;

measures a first set of amplitude and phase errors of a plurality of return path signals traveling from the user terminals via the satellite to the at least one gateway by:

generating an encoded return calibration signal having a known amplitude and phase that is received by the satellite communications payload and applied to each of a plurality of feed elements in a feed array;

receiving, from the calibration network over the satellite return path and a return downlink between the satellite and the gateway, a plurality of tagged signals, each of the plurality of tagged signals comprising a combination of user communications traffic and the encoded return calibration signal; and

determining a difference between the known amplitude and phase of the encoded return calibration signal and an amplitude and a phase of each of the plurality of tagged signals, the difference being representative of said first set of amplitude and phase errors;

corrects said first set of amplitude and phase errors by generating, on the ground, a first set of corrective beam forming coefficients, and applying said first set of corrective beam forming coefficients to said return path signals to minimize the amplitude and phase errors of the plurality of return path signals;

measures a second set of amplitude and phase errors of a plurality of forward path signals traveling from the at least one gateway via the satellite to the user terminals; and

corrects said second set of amplitude and phase errors by generating, on the ground, a second set of corrective beam forming coefficients, and applying said second set of corrective beam forming coefficients to said forward path signals.

2. The satellite communications network of claim 1 wherein the forward path signals are frequency division multiplexed.

3. The satellite communications network of claim 1 wherein the return path signals are frequency division multiplexed.

4. The satellite communications network of claim 1 wherein the GBBF system forms beams using a plurality of feed elements in a forward downlink from the satellite to the user terminals and in a return uplink from the user terminals to the satellite.

5. The satellite communications network of claim 1 wherein the GBBF system controls power of a forward uplink from the at least one gateway to the satellite.

6. The satellite communications network of claim 1 wherein the GBBF system minimizes errors associated with Doppler frequency shifts.

7. The satellite communications network of claim 1 wherein the GBBF system corrects for satellite pointing errors.

8. The satellite communications network of claim 1 , wherein the encoded return calibration signal is applied to each of the plurality of feed elements in the feed array through the calibration network.

9. The satellite communications network of claim 1 , wherein the encoded return calibration signal is applied to each of the plurality of feed elements in the feed array via a calibration horn.

10. The satellite communications network of claim 1 , wherein the encoded return calibration signal is encoded by means of a Walsh function and pseudo random number scrambling.

11. The satellite communications network of claim 1 , wherein the encoded return calibration signal is generated on-board the satellite.

12. The satellite communications network of claim 1 wherein the GBBF system measures and corrects amplitude and phase errors of the plurality of forward path signals by:

generating an encoded forward calibration signal having a known amplitude and phase and a plurality of tagged signals, each of the plurality of tagged signals comprising a combination of user communications traffic and the encoded forward calibration signal;

transmitting the plurality of tagged signals over a forward uplink to the communications payload, which passes the plurality of tagged signals through the satellite forward path and through the calibration network, which passes the plurality of tagged signals to a forward downlink, and passes amplitude and phase characteristics of the plurality of tagged signals through the satellite return path together with an inserted payload beacon signal having a known amplitude and phase;

receiving over a return downlink amplitude and phase characteristics of each of the plurality of tagged signals and a received payload beacon signal;

determining a difference between the known amplitude and phase of the encoded forward calibration signal and amplitude and phase characteristics of each of the plurality of tagged signals, which difference is representative of an error associated with a total path consisting of the forward uplink, the satellite forward path, the satellite return path and the return downlink;

determining a difference between the known amplitude and phase of the inserted payload beacon signal and an amplitude and phase of the received payload beacon signal, which difference is representative of an error associated with a complete return path, the complete return path consisting of the satellite return path and the return downlink;

subtracting the difference representative of the error associated with the complete return path from the difference representative of the error associated with the total path to obtain an error representative of a complete forward path, the complete forward path consisting of the forward uplink and the satellite forward path; and

generating and applying corrective beamforming coefficients and hybrid matrix correction factors to minimize amplitude and phase errors of the plurality of forward path signals.

13. The satellite communications network of claim 1 wherein the GBBF system controls power of a forward uplink by:

transmitting the gateway generated pilot signal over the forward uplink to the satellite communications payload, which passes the gateway generated pilot signal through the satellite forward path, passes the gateway generated pilot signal together with the satellite generated payload pilot signal over the satellite return path to a feeder link transmitter, and transmits a transponded gateway generated pilot signal and the satellite generated payload pilot signal to the at least one gateway over a return downlink;

receiving the transponded gateway generated pilot signal and a received payload pilot signal from the return downlink;

determining a propagation effect associated with the forward uplink by comparing a signal level of the received payload pilot signal to a signal level of the transponded gateway generated pilot signal; and

providing a control signal that adjusts a power level of the forward uplink to compensate for the propagation effect.

14. The satellite communications network of claim 1 wherein Doppler frequency shift errors are minimized by:

locking the gateway generated pilot signal to the master reference oscillator; transmitting the gateway generated pilot signal over a forward uplink to the satellite communications payload;

applying the gateway generated pilot signal to the satellite tracking master reference oscillator;

locking the satellite tracking master reference oscillator to the satellite generated payload pilot signal;

transmitting the satellite generated payload pilot signal over a return downlink to the at least one gateway;

receiving the satellite generated payload pilot signal at the at least one gateway; and

locking the gateway tracking master reference oscillator to the satellite generated payload pilot signal.

15. The satellite communications network of claim 1 wherein satellite pointing errors are corrected by:

receiving pointing beacon signals generated by the plurality of pointing beacon stations operating at known locations over a return uplink;

transmitting the pointing beacon signals over a return downlink; and

calculating and compensating for errors between measured beam pointing direction and desired beam pointing direction.

16. A method for ground-based beamforming (GBBF) of a communications payload on a satellite, said payload comprising a calibration network, a satellite return path, said method comprising:

measuring a first set of amplitude and phase errors of a plurality of a plurality of return path signals traveling from user terminals via the communications satellite payload to at least one gateway by:

generating an encoded return calibration signal having a known amplitude and phase that is received by the satellite communications payload and applied to each of a plurality of feed elements in a feed array;

receiving from the calibration network over the satellite return path and a return downlink between the satellite and the gateway a plurality of tagged signals, each of the plurality of tagged signals comprising a combination of user communications traffic and the encoded return calibration signal; and

determining a difference between the known amplitude and phase of the encoded return calibration signal and an amplitude and a phase of each of the plurality of tagged signals, the difference being representative of said first set of amplitude and phase errors;

correcting said first set of amplitude and phase errors by generating, on the ground, a first set of corrective beam forming coefficients, and applying said first set of corrective beam forming coefficients to said return path signals to minimize the amplitude and phase errors of the plurality of return path signals;

measuring a second set of amplitude and phase errors of a plurality of forward path signals traveling from the at least one gateway via the satellite to the user terminals; and

correcting said second set of amplitude and phase errors by generating, on the ground, a second set of corrective beam forming coefficients, and applying said second set of corrective beam forming coefficients to said forward path signals.

17. The method of claim 16 further comprising:

controlling a plurality of feed elements to form desired beam shapes in a forward downlink from the satellite to the user terminals and in a return uplink from the user terminals to the satellite.

18. The method of claim 17 , wherein, at least one of the forward and return path signals are frequency division multiplexed.