Digitally controlled beam former for a spacecraft

1. A digitally controlled beam former for a spacecraft having a multi-element antenna array and a control processor having N-outputs for each element of the antenna array, the beam former comprising:

N-paths for each element of the antenna array, each of the N-paths being connected to a separate one of the outputs of the control processor for controlling weightings applied to amplitude and phase signals of a respective N-path;

N-beam former channels, each one of which is connected to a separate one of the N-paths for each element of the antenna array, a nominal beam associated with each of the N-paths having a first beam position corresponding to a respective region on earth; and

calibration means for periodically calibrating each of the N-paths of each element of the spacecraft's antenna array using a reference beam having a second beam position corresponding to a specific region on earth, the calibration means being adapted to measure any offset of the second beam position from said specific region using an uplink at said specific region, the measured offset being used by the control processor to compensate for phase drift in the N-paths for each element of the antenna array.

2. A digitally controlled beam former as claimed in claim 1, wherein the calibration means is operative for sequentially selecting and calibrating each of the N-beam former channels while the other N-beam former channels are operational, the weightings of the amplitude and phase signals of a selected N-path being varied in dependence upon a difference between initial weightings and final weightings required for the reference beam.

3. A digitally controlled beam former as claimed in claim 2, wherein the antenna array is a receive array, and wherein each of the sequentially selected N-beam former channels is calibrated in response to receipt of a reference uplink signal from a ground transmitter at said specific region, the measured offset in both X and Y phases of the reference beam relative to the reference uplink signal being detected and applied to the control processor for causing the weightings to be varied in dependence upon the level of the measured offset.

4. A digitally controlled beam former as claimed in claim 3, wherein the reference uplink during a first stage of calibration is a spread spectrum uplink signal which is received by sweeping a wide receive beam in both X and Y co-ordinates by the receive array to establish a coarse boresight for nominal weightings, and wherein the same reference uplink during a second stage of calibration is received by sweeping a narrow beam in both X and Y co-ordinates by the receive array to obtain characteristic slopes and offsets for storage by the control processor.

5. A digitally controlled beam former as claimed in claim 4, wherein the narrow beam incorporates a coarse fixed offset corresponding to the offset in the X and Y phases for the coarse boresight.

6. A digitally controlled beam former as claimed in claim 2, wherein the antenna array is a transmit array, wherein a reference transmit beam is established to provide nominal coverage over said specific region, said reference beam being modulated by a recognition code, wherein the reference transmit beam is swept over the ground station by the application of control signals to the elements of the N-paths of the reference channel by the control processor, and wherein the ground station generates said uplink which is stored by, the control processor for effecting optimization of the weightings applied to the reference transmit beam and the sequential calibration of the other channels of the transmit array utilizing the uplink.

7. A digitally controlled beam former as claimed in claim 4, wherein the calibration means include correlation and detection means for the reference uplink signal.

8. A digitally controlled beam former as claimed in claim 1, wherein the spacecraft has an attitude and orbit control system (AOCS) including sensors for sensing the attitude of the spacecraft, wherein the beam former further includes means for switching operation of the AOCS for the spacecraft to the calibration means in the event of failure of the AOCS sensors, wherein X and Y co-ordinate data for the AOCS is provided by the control processor.

9. A spacecraft, comprising:

a digitally controlled beam former, said former having a multi-element antenna array and a control processor having N-outputs for each element of the antenna array, the beam former comprising:

N-paths for each element of the antenna array, each of the N-paths being connected to a separate one of the outputs of the control processor for controlling weightings applied to amplitude and phase signals of a respective N-path;

N-beam former channels, each one of which is connected to a separate one of the N-paths for each element of the antenna array, a nominal beam associated with each of the N-paths having a first beam position corresponding to a respective region on earth; and

calibration means for periodically calibrating each of the N-paths of each element of the spacecraft's antenna array using a reference beam having a second beam position corresponding to a specific region on earth, the calibration means being adapted to measure any offset of the second beam position from said specific region using an uplink at said specific region, the measured offset being used by the control processor to compensate for phase drift in the N-paths for each element of the antenna array.