Real-Time Attitude Determination of a Nanosatellite Using GPS Signal-to-Noise Ratio Observations

Transcription

1 Real-Tie Attitude Deterination of a Nanosatellite Using GPS Signal-to-Noise Ratio Oservations The University of Teas at Austin Shaun M. Stewart Greg N. Holt

2 FASTRAC Mission Foration Autonoy Spacecraft with Thrust, Relnav, Attitude, and Crosslink Multipurpose GPS Sensor Position / Velocity Relative Nav. Attitude Deterination

3 Roadap to a Spaceorne GPS Basic COTS hardware / software Modify hardware for space flight Modify software for space flight Positioning / Relative Navigation Verify On-Orit Perforance Single Antenna Attitude Deterination

4 Siulation Setup GPS Constellation Siulator Spirent STR476 No Purposeful Signal Degradation (Ionosphere, Multipath, etc.)

5 Benchark Testing Measureent Measureent Reference Reference Geoetry Free - Oscillator Errors Doinate Geoetry and Oscillator Free - Receiver and Channel Specific Errors

6 Benchark Testing Sutract (easureent)-(siulated) Analogous to receiver-receiver single difference Error fro truth representation Sutract results of previous operations Analogous to classic GPS doule difference Error easureent rather than aseline

7 FASTRAC Orion Engr. Unit E Satellite Relative Velocity Systeatic Carrier Phase Error Follows Trend of Satellite Relative Velocity Error Only Oservale with This Differencing Techniue

8 FASTRAC Orion Flight Unit F Based on Code Version DLR677H Second-Order FLL-aided PLL

9 Data Arc Selection Concurrent Visiility Si Standard Data Arcs Selected Variety of Relative Dynaics and Signal Levels Siilar Rise/Set Profiles Difference Rise/Set Profiles It works, ut can a COTS receiver do ore?

10 GPS Attitude The traditional ethod of differential GPS attitude deterination using carrier phase ay not e suitale for sall spacecraft due to the reuireent for resolution of carrier phase integer aiguity, the use of several canted antennas, and a large nuer of GPS RF channels. A single antenna GPS attitude deterination syste is desirale for sall nanosatellite issions such as FASTRAC where the use of COTS GPS hardware can significantly reduce cost and copleity.

11 Spacecraft Attitude Based on GPS Signal-to-Noise Ratio GPS Signal-to-Noise Ratio (SNR) easureents when coupled with an eternal vector sensor, such as a Three-Ais Magnetoeter (TAM), can e used to provide coarse attitude deterination accurate to within 5 -. SNR g * ( cos( α)) cos( α) Â Lˆ. Â Lˆ is the antenna oresight vector. is the noralized LOS vector to the GPS satellite. α is the incident angle of incoing GPS signals called the antenna off-oresight angle.

12 Quaternion Attitude Representation φ θ ψ ˆ ˆ ˆ k j i ) ( tan φ ( ) sin θ ) ( tan ψ

13 SNR Measureent Model A uaternion point rotation can e used to relate the noinal antenna oresight vector with the rotated ody-fied vector: Â rot ( )Â ( Â ) (Â ) k k k The SNR easureent odel is the cosine of the angle etween this rotated antenna oresight vector and the corresponding GPS Line of Sight vector: G Ârot Lˆ cos( α ) Y f (SNR)

14 Three-Ais Magnetoeter (TAM) Siulated Investigation Truth th order IGRF odel Measureents 6 th order IGRF 995 odel plus.g easureent noise. Spacecraft Ipleentation Honeywell HMC Three Ais Magnetic Sensor Atel ATega8 Microcontroller Interface Circuit

15 Magnetoeter Model A uaternion coordinate rotation can e used to relate the eternally fied agnetic field odel known in the eternal frae to the agnetoeter easureent ade in the ody frae. G rot ( ) ( ) ( ) Y TAM where is the odeled agnetic field vector. is the current estiate for the rotation. TAM is the easured agnetic field vector.

16 Quaternion Filter Euations ) ( ) ( ) ( G rot ) cos( Lˆ Â G rot α X z y * X H G ] * [5 ~ ~ ~ ~ Mag Mag GPS GPS H H H H H M M

17 Quaternion Filter Euations z y GPS L L L ~ H T NED k A ], [, ˆ for ~ Mag H ~ 4 4 Mag H ~ 4 4 Mag H

18 Testing Setup GPS Siulator Orion GPS Receiver True Spacecraft Attitude Synthesized RF Coparison Estiated Spacecraft Attitude EKF Algorith Siulated Magnetoeter Quaternion Accuracy

19 Testing Results Antenna Caliration

20 Testing Results Tuling Satellite Case (slow drift)

21 Testing Results

22 Testing Results Poor Initialization Case

23 Testing Results Poor Initialization Case

24 Final Integration (Not there yet) Rooftop Test w/ ground version of the code coupled with agnetoeter hardware in the loop. GPS antenna self caliration routine Magnetoeter Error Estiates Misalignent offset Bias caliration Final Code Burn of Flight Receivers AFRL Delivery 5 Launch 6

25 Questions?

26 Dynaics Quaternion Propagation: y z z y y z z y &

27 Dynaics Euler s Euations: ( ) [ ] ( ) [ ] ( ) [ ] z y y z z y z z y y z y z y T I I I T I I I T I I I & & &