Using predicted movement to maintain optical-communication lock with nearby balloon

1. A balloon, comprising:

an optical-communication component, wherein the optical-communication component has a pointing axis, and wherein the optical-communication component is operable to communicate with a correspondent balloon via a free-space optical link;

a radio frequency (RF) communication system, wherein the RF communication system is operable to communicate with the correspondent balloon via an RF link;

a pointing mechanism configured to adjust the pointing axis; and

a controller, wherein the controller is configured to (i) determine a predicted relative location of the correspondent balloon based on data received from the correspondent balloon via the RF link and (ii) control the pointing mechanism to adjust the pointing axis based on the predicted relative location, to maintain the free-space optical link with the correspondent balloon.

2. The balloon of claim 1 , wherein the balloon is a high-altitude balloon in a high-altitude balloon mesh network.

3. The balloon of claim 1 , wherein the optical-communication component comprises an optical receiver configured to receive free-space optical signals.

4. The balloon of claim 1 , wherein the optical-communication component comprises an optical transmitter configured to transmit free-space optical signals.

5. The balloon of claim 1 , wherein the optical-communication component comprises an optical transceiver configured to transmit and receive free-space optical signals.

6. The balloon of claim 1 , wherein the controller is configured to determine the predicted relative location of the correspondent balloon based on a Kalman filter method.

7. The balloon of claim 6 , wherein the predicted relative location of the correspondent balloon is determined using a last known location of the correspondent balloon as an input to the Kalman filter method.

8. The balloon of claim 6 , wherein the predicted relative location of the correspondent balloon is determined using a last known location of the correspondent balloon and a last known motion vector of the correspondent balloon as inputs to the Kalman filter method.

9. The balloon of claim 1 , wherein the controller is configured to determine the predicted relative location of the correspondent balloon based on a linear-quadratic estimation method.

10. A method, comprising:

determining a location of a first balloon, wherein the first balloon comprises an optical-communication component that is configured to communicate with a second balloon via a free-space optical link;

receiving data from the second balloon via a radio frequency (RF) link;

determining a predicted location of the second balloon relative to the location of the first balloon based on the data received from the second balloon via the RF link; and

controlling a pointing mechanism to adjust a pointing axis of the optical-communication component in the first balloon based on the predicted location, to maintain the free-space optical link with the second balloon.

11. The method of claim 10 , wherein determining the predicted location of the second balloon comprises using a Kalman filter method.

12. The method of claim 11 , wherein determining the predicted location of the second balloon comprises using a last known location of the second balloon as an input to the Kalman filter method.

13. The method of claim 11 , wherein determining the predicted location of the second balloon comprises using a last known location of the second balloon and a last known motion vector of the second balloon as inputs to the Kalman filter method.

14. The method of claim 10 , wherein determining the predicted location of the second balloon comprises using a linear-quadratic estimation method.

15. The method of claim 10 , wherein the optical-communication component comprises an optical receiver configured to receive free-space optical signals.

16. The method of claim 10 , wherein the optical-communication component comprises an optical transmitter configured to transmit free-space optical signals.

17. The method of claim 10 , wherein the optical-communication component comprises an optical transceiver configured to transmit and receive free-space optical signals.

18. A non-transitory computer readable medium having stored therein instructions executable by a computing device to cause the computing device to perform functions comprising:

determining a location of a first balloon, wherein the first balloon comprises an optical communication component that is configured to communicate with a second balloon via a free-space optical link;

receiving data from the second balloon via a radio frequency (RF) link;

determining a predicted location of the second balloon relative to the location of the first balloon based on the data received from the second balloon via the RF link; and

controlling a pointing mechanism to adjust a pointing axis of an optical-communication component in the first balloon based on the predicted location, to maintain the free-space optical link with the second balloon.

19. The non-transitory computer readable medium of claim 18 , wherein determining the predicted location of the second balloon comprises using a Kalman filter method.

20. The non-transitory computer readable medium of claim 19 , wherein determining the predicted location of the second balloon comprises using a last known location of the second balloon and a last known motion vector of the second balloon as inputs to the Kalman filter method.