Determining a correction to be applied to a main beam of an antenna system for a moving vehicle

1. An antenna system for a moving vehicle, the antenna system comprising:

a main antenna to generate a main beam;

a measurement antenna to generate a measurement beam; and

control circuitry to perform an adjustment process by:

rotating the main beam to an initial bearing angle;

rotating the measurement beam independently of the main beam to receive signals at positions to either side of the initial bearing angle;

comparing at least one metric measured for the signals received by the measurement antenna at the positions to either side of the initial bearing angle to generate a comparison output; and

determining, based on the comparison output, a correction to be applied to the initial bearing angle of the main beam.

2. The antenna system of claim 1 , wherein:

the antenna system is configured to communicate with a further antenna system using the main beam; and

the control circuitry is configured to calculate the initial bearing angle in dependence on an estimated direction of the further antenna system relative to the antenna system.

3. The antenna system of claim 2 , wherein

the estimated direction is based on a recorded position of the further antenna system and at least one of:

an estimated position of the antenna system; and

an estimated orientation of the antenna system.

4. The antenna system of claim 3 , wherein

the control circuitry is configured to calculate the estimated position of the antenna system based on at least one of:

satellite positioning data indicative of a location of the moving vehicle; and

information indicating a position of the antenna system relative to the moving vehicle.

5. The antenna system of claim 3 , wherein

the control circuitry is configured to calculate the estimated orientation of the antenna system based on at least one of:

a direction reading indicative of a rotation of the moving vehicle in at least one plane; and

information indicating an orientation of the antenna system relative to the moving vehicle.

6. The antenna system of claim 3 , comprising

storage circuitry to store, for each of a plurality of further antenna systems, a corresponding recorded position,

wherein the control circuitry is configured to receive, from the storage circuitry, the recorded position of the further antenna system prior to calculating the initial bearing angle.

7. The antenna system of claim 2 , wherein

the antenna system is configured to communicate with the further antenna system using both the main beam and the measurement beam.

8. The antenna system of claim 1 , wherein:

the control circuitry is configured to perform a plurality of iterations of the adjustment process;

in each iteration, the control circuitry is configured to determine a corrected bearing angle by applying the correction to the initial bearing angle specified for that iteration; and

for each iteration after the first iteration, the initial bearing angle comprises the corrected bearing angle determined in the preceding iteration.

9. The antenna system of claim 1 , wherein

the at least one metric comprises a measure of a signal strength of the signals received by the measurement antenna at the positions to either side of the initial bearing angle.

10. The antenna system of claim 9 , wherein

the measure of the signal strength comprises a measure of Reference Signals Received Power (RSRP).

11. The antenna system of claim 9 , wherein

the measure of the signal strength comprises a filtered signal strength of the signals received by the measurement antenna at the positions to either side of the initial bearing angle.

12. The antenna system of claim 9 , wherein:

the control circuitry is configured to compare the measure of the signal strength of the signals received by the measurement antenna at the positions to either side of the initial bearing angle to determine which of the positions provides a greater signal strength; and

the control circuitry is configured to calculate the correction to, when the correction is applied to the initial bearing angle of the main beam, steer the main beam towards a position providing the greater signal strength.

13. The antenna system of claim 12 , wherein

the control circuitry is configured to apply the correction by rotating the main beam from the initial bearing angle by a predetermined amount, in a direction indicated by the correction.

14. The antenna system of claim 1 , wherein

the control circuitry comprises proportional-integral-derivative (PID) control circuitry to calculate the correction.

15. The antenna system of claim 14 , wherein:

the PID control circuitry is configured to calculate the correction on the basis of an error defined as a difference between values of the at least one metric measured for the signals received by the measurement antenna at the positions to either side of the initial bearing angle; and

the PID control circuitry is configured to calculate the correction by summing a proportional term (P) proportional to a current value of an error, an integral term (I) proportional to an integral of past values of the error over time, and a derivative term (D) proportional to a current rate of change of the error.

16. The antenna system of claim 1 , wherein:

the control circuitry is configured to compare the correction to an upper threshold value and a lower threshold value;

when the correction has a value greater than the upper threshold value, the control circuitry is configured to update the correction to the upper threshold value; and

when the correction has a value less than the lower threshold value, the control circuitry is configured to update the correction to the lower threshold value.

17. The antenna system of claim 1 , wherein:

each of the main antenna and the measurement antenna comprises an antenna array, and beamforming circuitry to control the antenna array to generate a beam having a peak antenna array gain in a peak direction;

the control circuitry is configured to rotate the main beam by adjusting the peak direction of the main beam, wherein the main beam is rotated to the initial bearing angle when the peak direction is at the initial bearing angle; and

the control circuitry is configured to rotate the measurement beam to a measurement angle by adjusting the peak direction of the measurement beam to be at the measurement angle.

18. The antenna system of claim 17 , wherein

the control circuitry comprises at least one of:

antenna rotation circuitry to mechanically steer the main beam by rotating the main antenna, and/or to mechanically steer the measurement beam by rotating the measurement antenna; and

beam steering circuitry to electronically steer the main beam by adjusting the shape of the main beam, and/or to electronically steer the measurement beam by adjusting the shape of the measurement beam.

19. The antenna system of claim 1 , wherein:

the control circuitry is configured to rotate the measurement beam to a measurement angle; and

the positions to either side of the initial bearing angle comprise a pair of measurement angles to either side of the initial bearing angle.

20. The antenna system of claim 19 , comprising

storage circuitry to store a look-up table (LUT) indicating corresponding measurement angles for each of a plurality of bearing angles of the main beam,

wherein the control circuitry is configured to determine, based on the LUT, the pair of measurement angles corresponding to the initial bearing angle, and to rotate the measurement beam to each of the pair of measurement angles to receive the signals at the positions to either side of the initial bearing angle.

21. The antenna system of claim 20 , wherein:

the measurement beam is configured to receive communication signals from a further antenna system;

an antenna array gain of the measurement beam is dependent on the measurement angle and a direction of the further antenna system; and

the storage circuitry is configured to store, in the look-up table, the corresponding measurement angles for the plurality of bearing angles of the main beam such that, when the measurement beam is rotated to each of the pair of measurement angles corresponding to the initial bearing angle, gain curves for each of the pair of measurement angles intersect when the direction of the further antenna system is the initial bearing angle,

wherein each of the gain curves comprises a graph representing the antenna array gain of the measurement beam as a function of the direction of the further antenna system.

22. The antenna system of claim 21 , wherein:

the storage circuitry is configured to store, in the look-up table, the corresponding measurement angles for the plurality of bearing angles of the main beam such that when the measurement beam is rotated to each of the corresponding measurement angles corresponding to a given bearing angle of the main beam, the gain curves for each of the pair of measurement angles intersect at a point which is a predetermined amount below a peak antenna array gain of the main beam when rotated to the given bearing angle; and

the predetermined amount is the same for each of the bearing angles.

23. The antenna system of claim 1 , wherein

the moving vehicle comprises an aircraft.

24. A method of operating an antenna system for a moving vehicle, the method comprising:

generating a main beam;

generating a measurement beam; and

performing an adjustment process by:

rotating the main beam to an initial bearing angle;

rotating the measurement beam independently of the main beam to receive signals at positions to either side of the initial bearing angle;

comparing at least one metric measured for the signals received by a measurement antenna at the positions to either side of the initial bearing angle to generate a comparison output; and

determining, based on the comparison output, a correction to be applied to the initial bearing angle of the main beam.