Increasing the Accuracy of Orbital Position Information from NORAD SGP4 Using Intermittent GPS Readings

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1 Increasing the Accuracy of Orbital Position Information from NORAD SGP4 Using Intermittent GPS Readings Michael Greene Robert E. Zee Space Flight Laboratory University of Toronto 12 August rd Annual Small Satellite Conference Logan, Utah, USA

2 CanX-2 Canadian Advanced Nanospace experiment 2 34x10x10cm; 3.5kg Nanosatellite GPS Occultation: Investigate water vapor in the Troposphere & electron density in the Ionosphere Uses SGP4 for onboard orbital propagation GPS Occultation CanX-2

3 CanX-2 TLE versus CanX-2 GPS lock April 20, 2009 CanX-2 TLE vs GPS CanX-2 GPS error estimates

4 Motivation GPS receiver obtains meter-level accuracy. Can we use the GPS receiver on CanX-2 to improve our propagated PVT estimates? Limited availability: - GPS occultation experiment - Very power hungry Novatel OEM4-G2 GPS Receiver, courtesy of Novatel inc. SFL Ground Station and Mission control

5 Objective Develop methodology to use GPS readings in SGP4 Determine the accuracy of the SGP4 propagator using intermittent GPS measurements. Determine GPS duty cycle required to remain within a given error tolerance. Purpose: Increase accuracy of orbital position information Increase reliability of the error estimates Account for the infrequent basis on which the TLEs are issued

6 Methodology GPS For initial PVT acquisition (CanX-2) Convert osculating state vector into the mean orbital elements in TLE format suitable for SGP4 VEC2TLE - Ernandes (1994) Retrieve TLE from NORAD - for B* term from most recent epoch Input into the SGP4 propagator Comparison to CanX-2 GPS measurements & STK s HPOP

7 Results Initial GPS PVT: CanX-2 GPS Measurement 20 Apr :57: UTCG Position (m) - ECEF Component Value 1σ Error x y z Velocity (m/s) - ECEF Component Value 1σ Error x-dot y-dot z-dot VEC2TLE Computed mean orbital elements: Mean Orbital Elements Time: 20 Apr :57: UTCG Epoch [yyddd] Inclination (i) Right Ascension of the Ascending Node (Ω) Eccentricity (e) Argument of the Perigee (ω) Mean Anomaly (M) Mean Motion (n) rev/day B* [1/Earth Radii] x 10-5 NORAD

8 Results CanX-2 GPS lock Comparison 12 minutes Errors (RIC) Radial, In-track, Cross-track error

9 Results 12 hours HPOP Comparison

10 Conclusion In order to remain within 2km, the GPS should update PVT every 6.5 hours. Approximately once every 4 orbits (LEO) In order to remain within 1km, the GPS should update PVT every 4.5 hours. Approximately once every 3 orbits (LEO) Assumptions STK s HPOP has no error GPS error estimates are correct

11 Conclusion Advantages Attitude determination estimate Magnetometer/IGRF comparison Minimal Communications scheduling/tracking Minimal Payloads Autonomous Imaging Valuable On board processing Valuable Feasibility Minimal impact on the existing infrastructure However does require code-upload.

12 Questions?

13 Acknowledgements The authors would like to thank the following contributors: Ontario Graduate Scholarships (OGS) Dr. Susan Skone, University of Calgary Defence Research and Development Ottawa Canadian Space Agency (CSA) Natural Sciences and Engineering Research Council of Canada (NSERC) MDA Space Missions Ontario Centres of Excellence (OCE) Sinclair Interplanetary

14 References 1. Bandyopadhyay, P. Sharma, R.K., and Adimurthy, V. Space debris proximity analysis in powered and orbital phases during satellite launch. Advances in Space Research 24, , Kelso, T.S., "Validation of SGP4 and IS-GPS-200D Against GPS Precision Ephemerides," presented at the 17th AAS/AIAA Space Flight Mechanics Conference, Sedona, AZ, 2007 January Cho, C.H., Lee, B.S., Lee, J.S., Kim, J.H., Choi, K.H., NORAD TLE Type Orbit Determination of LEO Satellites using GPS Navigation Solutions. J. Astron. Space Sci. 19(3), August 27, NovAtel, OEM4 Family of Receivers User Manual Volume 2, Command and Log Reference, 31 July, Vallado, David A., Paul Crawford, Richard Hujsak, and T.S. Kelso, "Revisiting Spacetrack Report #3," presented at the AIAA/AAS Astrodynamics Specialist Conference, Keystone, CO, 2006 August Hoots, F. R., Roehrich, R. L., Spacetrack Report No. 3: Models for Propagation of NORAD Element Sets. December 1980 Compiled by: Kelso, T.S. 31 December Lee, B. S, NORAD TLE Conversion from Osculating Orbital Element. J. Astron. Space Sci. 19(4), November 18, Moraes, R.V., Kuga, H.K., Dampos, D.Y., Orbital Propagation for Brazilian Satellites using NORAD Models