VELOX-CI: Advanced Application of GPS for Radio Occultation and Satellite Attitude Determination

Transcription

1 VELOX-CI: Advanced Application of GPS for Radio Occultation and Satellite Attitude Determination Yung-Fu Tsai, Guo Xiong Lee and Kay Soon Low Satellite Research Centre (SaRC) School of Electrical and Electronic Engineering 30/09/

2 Satellite

3 VELOX-CI Mission objectives Radio occultation for climate study GPS based attitude determination Star tracker algorithm PDR CDR FRR EQM FM APR 2013 AUG 2014 Sep 2015 PDR : Preliminary Design Review CDR : Critical Design Review FRR : Flight Readiness Review 5 3

4 Radio Occultation Principle GPS measurement in atmosphere Doppler shift Ray bending angle Profile of index of refraction (function of atmospheric density) Temperature & humidity profile, characteristics of the ionosphere LEO Clear LOS GPS LEO With Bending GPS LEO GPS Earth 4 4

5 GPS RO Activities in Space CHAMP (CHAllenging Minisatellite Payload) Since 2000~2009 Near polar orbit Earth's gravity and magnetic field study / atmospheric limb sounding GRACE (Gravity Recovery And Climate Experiment) Since 2002 Near circular polar orbit Gravity field variations study / GPS occultation FORMOSAT-3/COSMIC Since 2006 (6 satellite constellation) Inclination 72 deg circular orbit GPS occultation experiment / tiny ionosphere photometer Metop, TerraSAR-X, KOMPSat-5, COSMIC-2, etc. CubeSat-Sized CANX-2: First radio occultation experiment using a CubeSat University of Calgary and Toronto Launched in April 2008 PSSC-2: First successful occultation profiles on a CubeSat United States Air Force Launched in July 2011 Spire Lemur2, MIT MicroMAS, etc. GRACE CHAMP FORMOSAT-3/ COSMIC 5 5

6 Near Equatorial Orbit GPS RO Experiment Satellite orbit at 550 km altitude, 9 degree inclination Only 600 profiles 450 profiles per day per could day could be obtained be obtained (under house-keeping mode) Distributed uniformly around tropical region 80 North Latitude Latitude (deg) (deg) East Longitude (deg) (deg) 6 6

7 Ground-based Collaboration International GNSS Service (IGS Network) Collect high quality GNSS data in support of Earth science research, multidisciplinary applications, and education. 7 7

8 Ground-based Ionospheric Map TEC F TEC F vertical slant R cose sec arcsin e R e h I 8 8

9 VELOX-CI RO Mission Planning 9 9

10 GPS-Based Attitude Determination Advantages Measures the absolute attitude No accumulation error Adding functionality to existing equipment Slight cost and weight increase Disadvantages Requires direct view of GPS satellites Lower update rate (1-20 Hz maximum) Coarse accuracy (0.1-5º RMS error) Two baseline vectors Attitude Determination based on: Carrier Phase / Pseudorange measurements Interferometery Principle/ Double Difference measurements 10 10

11 GPS Based AD Experiment Reference baseline vector from Ant. 1 to Ant. 2 = [ ] (Spirent Simulator) Baseline length = m True Pitch & Roll = 0 deg, True Yaw = deg Calculated Results: Baseline length = m, error = m Yaw mean error = 1.18 deg 64 Yaw (deg) Pitch (deg) epoch (s) 11 11

12 Challenges of GPS AD on VELOX-CI Common visible satellites are fast changing and insufficient at times for reliable ambiguity resolution leading to disruption of AD Short baseline on small NEO satellites limit the achievable accuracy Typical 1 Hz update rate of GPS receivers may not be high enough for applications To overcome the limitations, a study of GPS & 3-axis fibre optic gyro (FOG) integrated AD systems for short baseline on a earth-pointing NEO satellite was conducted 12 12

13 Hardware-in-the-loop Experiment Orbital elements of the simulated NEO Satellite Semi-major axis m Inclination Angle Right Ascension Eccentricity Mean Anomaly Argument of Perigee Specification of fibre optic gyro Error Gyro Drift Axis Long term bias ( /hr) Short term bias ( /hr) Angular random walk ( / hr) X Y Z

14 GPS/FOG Integrated AD One orbit result Accuracy of GPS based AD: Pitch (2-4 deg), Yaw (1-2 deg) Accuracy of GPS/ FOG AD: Pitch ( deg), Yaw ( deg) 14 14

15 Conclusions VELOX-CI is ready to launch It will be launched into NEO so that it has the advantage of a high revisit rate over Singapore and more radio occultation observability in tropical region. Although VELOX-CI RO experiment might not probe lower atmosphere, tropical ionosphere monitoring could provide a key value for scientific research. Through combining space based RO profiles and ground based observations, a study of tropical space weather monitoring could be conducted. The radio occultation payload can be used for experimental satellite attitude determination algorithm without major modification. Algorithm has been validated using hardware-in-the-loop experiment 15 15

16 Thank you! Acknowledgement Special Thank to OSTIn for funding support 16