Beacon-assisted spacecraft attitude control systems and methods

• US 6,288,671 B1
• Filed: 04/25/2000
• Issued: 09/11/2001
• Est. Priority Date: 04/25/2000
• Status: Expired due to Term

• Current Assignee: Hughes Electronics Corporation (Rtx Corporation)
• Original Assignee: Hughes Electronics Corporation (Rtx Corporation)
• Inventors: Chiang, Richard Y., Ketkar, Umesh S., Li, Rongsheng, Wu, Yeong-Wei Andy
• Primary Examiner(s): PHAN, DAO LINDA
• Application Number:
  US09/557,378
• Time in Patent Office: 504 Days
• Field of Search: 342/352, 342/354, 342/355, 342/357.11, 342/462, 701/4, 701/13, 244/164, 244/165, 244/171
• US Class Current: 342/357.37
• CPC Class Codes: G01S 19/38 Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
• CPC Class Codes: G01S 3/30 derived directly from separate directional systems
• CPC Class Codes: G01S 5/0247 Determining attitude

Abstract

Control systems and methods are provided which enhance acquisition range, transient response and offset pointing accuracy in beacon-assisted orientation of a spacecraft to a desired spacecraft attitude A_dsrd. The methods receive a beacon signal with antennas that are arranged in a fixed angular relationship to thereby form an attitude error signal S_err whose magnitude is a measure of the attitude difference between a current spacecraft attitude A_crnt and the desired spacecraft attitude A_dsrd. A sum power P_Σ that is the sum of the signal powers from the antennas is sensed and a signal modifier S_mod is generated that increases with decreases in the sum power P_Σ. The attitude error signal S_err is then modified with a term (1+S_mod) to form an enhanced attitude error signal S_errenhd. In one embodiment, the signal modifier S_mod is n|P_max−P_Σ|^m wherein n and m are positive numbers and P_max is the sum power P_Σ when the attitude error is zero.

Claims

1. A method of orientating a spacecraft to a desired spacecraft attitude A_dsrd relative to a beacon that transmits a beacon signal, the method comprising the steps of:

providing said spacecraft with at least three antennas A - - - N that have respective boresights arranged in a fixed angular relationship;

from said antennas, receiving said beacon signal with signal powers P_A - - - P_N that are each a measure of the angular distance between the boresight of a respective one of said antennas A - - - N and said beacon;

combining said signal powers P_A - - - P_N in accordance with said angular relationship to generate at least one attitude error signal S_err whose magnitude is a measure of the attitude difference between a current spacecraft attitude A_crnt and said desired spacecraft attitude A_dsrd;

generating a signal modifier S_mod that increases with decreases in a sum power P_Σ

that is the sum of said signal powers P_A - - - P_N;

modifying said attitude error signal S_err with a term (1+S_mod) to form an enhanced attitude error signal S_errenhd;

and

orienting said spacecraft to reduce said enhanced attitude error signal S_errenhd.

2. The method of claim 1, wherein said providing step includes the step of forming each of said antennas with a horn that is positioned to receive said beacon signal after reflection from a reflector.

3. The method of claim 1, further including the step of limiting said antennas to three antennas A, B and C.

4. The method of claim 1, wherein said combining step includes the steps of:

adding signal powers of antennas whose boresights are substantially on the same side of said beacon; and

differencing signal powers of antennas whose boresights are substantially on different sides of said beacon.

5. The method of claim 4, further including the steps of:

determining said same side and said different sides along different first and second axes x and y through said beacon; and

thereby causing said attitude error signal S_err to comprise first and second attitude error signals S_errx and S_erry along respective said first and second orthogonal axes x and y.

6. The method of claim 1, wherein said generating step includes the step of choosing said signal modifier to be n|P_max−

P_Σ

|^m wherein n and m are positive numbers and maximum power P_max is said sum power P_Σ

when said attitude error signal A_err=0.

7. The method of claim 6, further including the steps of:

selecting a threshold level P_thld of said sum power P_Σ

that is less than said maximum power P_max;

setting n and m to first and second selected values when said sum power P_Σ

is less than said threshold level P_thld; and

setting n and m to third and fourth selected values when said sum power P_Σ

exceeds said threshold level P_thld.

8. The method of claim 1, further including the step of combining said attitude error signal S_err with an offset signal S_off to thereby cause said desired spacecraft attitude A_dsrd to include a spatial offset from said beacon.

9. The method of claim 8, further including the step of subtracting a power modifier from said signal powers P_A - - - P_N to facilitate realization of said spatial offset.

10. The method of claim 1, wherein said spacecraft has a selected member and said antennas are arranged about said member to thereby direct said member towards said beacon.

11. An attitude control system that orients a spacecraft to a desired spacecraft attitude A_dsrd relative to a beacon that transmits a beacon signal, the system comprising:

at least three antennas A - - - N that have respective boresights arranged with a fixed angular relationship, said antennas thus arranged to receive said beacon power with signal powers P_A - - - P_N that are each a measure of the angular distance between the boresight of a respective one of said antennas A - - - N and said beacon;

a downconverter that. downconverts received signals of said antennas; and

a control processor that responds to said downconverter and;

a) combines said signal powers P_A - - - P_N in accordance with said angular relationship to generate at least one attitude error signal S_err whose magnitude is a measure of the attitude difference between a current spacecraft attitude A_crnt and said desired spacecraft attitude A_dsrd;

b) generates a signal modifier S_mod that increases with decreases in a sum power P_Σ

that is the sum of said signal powers P_A - - - P_N;

c) modifies said attitude error signal S_err with a term (1+S_mod) to form an enhanced attitude error signal A_errenhd; and

d) couples said enhanced attitude error signal A_errenhd to said torque-generation system.

12. The system of claim 11, wherein said antennas are limited to three antennas A, B and C.

13. The system of claim 11, wherein said control processor forms said attitude error signal S_err as n|P_max−

P_Σ

|^m wherein n and m are positive numbers and maximum power P_max is said sum power P_Σ

when said attitude error signal A_err=0.

14. The system of claim 13, wherein said control processor:

selects a threshold level P_thld of said sum power P_Σ

that is less than said maximum power P_max;

sets n and m to first and second selected values when said sum power P_Σ

is less than said threshold level P_thld; and

sets n and m to third and fourth selected values when said sum power P_Σ

exceeds said threshold level P_thld.

15. The system of claim 11, further including the step of combining said attitude error signal S_err with an offset signal S_off to thereby cause said desired spacecraft attitude A_dsrd to include a spatial offset from said beacon.

16. A spacecraft having an attitude control system that orients a spacecraft to a desired spacecraft attitude A_dsrd relative to a beacon that transmits a beacon signal, the spacecraft comprising:

a spacecraft body;

at least three antennas A - - - N carried on said body and arranged with a fixed angular relationship, said antennas thus arranged to receive said beacon power with signal powers P_A - - - P_N that are each a measure of the angular distance between the boresight of a respective one of said antennas A - - - N and said beacon;

a downconverter carried in said body to downconvert said signal powers;

a power measurement system carried in said body to generate signal amplitudes P_A - - - P_N in response to downconverted signal powers P_A - - - P_N from said downconverter;

a control processor that responds to said downconverter and;

a) combines said signal amplitudes S_A - - - S_N in accordance with said angular relationship to generate at least one attitude error signal S_err whose magnitude is a measure of the attitude difference between a current spacecraft attitude A_crnt and said desired spacecraft attitude A_dsrd;

b) generates a signal modifier S_mod that increases with decreases in a sum power P_Σ

that is the sum of said signal powers P_A - - - P_N;

c) modifies said attitude error signal S_err with a term (1+S_mod) to form an enhanced attitude error signal A_errenhd; and

d) couples said enhanced attitude error signal A_errenhd to said torque-generation system.

17. The spacecraft of claim 16, wherein said antennas are limited to three antennas A, B and C.

18. The spacecraft of claim 16, wherein said control processor forms said attitude error signal S_err as n|P_max−

P_Σ

|^m wherein n and m are positive numbers and maximum power P_max is said sum power P_Σ

when said attitude error signal A_err=0.

19. The spacecraft of claim 18, wherein said control processor:

selects a threshold level P_thld of said sum power P_Σ

that is less than said maximum power P_max;

sets n and m to first and second selected values when said sum power P_Σ

is less than said threshold level P_thld; and

sets n and m to third and fourth selected values when said sum power P_Σ

exceeds said threshold level P_thld.

20. The spacecraft of claim 19, further including the step of combining said attitude error signal S_err with an offset signal S_off to thereby cause said desired spacecraft attitude A_dsrd to include a spatial offset from said beacon.