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Validation of SGP4 and IS-GPS-200D Against GPS Precise Ephemerides T.S. Kelso 2007 January 29.

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Presentation on theme: "Validation of SGP4 and IS-GPS-200D Against GPS Precise Ephemerides T.S. Kelso 2007 January 29."— Presentation transcript:

1 Validation of SGP4 and IS-GPS-200D Against GPS Precise Ephemerides T.S. Kelso 2007 January 29

2 Pg 2 of 29 Overview Introduction Objectives Test & Truth Data Methodology & Results Conclusions Future Research

3 Pg 3 of 29 Introduction TLEs are the only source of full-catalog elements TLEs do not come with covariance data Several past attempts to estimate covariance –MAESTRO Used limited-access observations Same observations used to create TLEs –COVGEN Performed TLE consistency check with publicly available data Incorrectly assumed errors were unbiased and independent of propagation direction

4 Pg 4 of 29 Objectives Examine COVGEN approach –Test original assumptions –Use high-precision ephemerides (GPS) –Ensure all test data is publicly available

5 Pg 5 of 29 Test Data Used only operational GPS satellites Eliminated satellites with extended outages Selected period where remaining satellites were outage free Days of 2006 selected –Obtained all SEM almanacs for this period –Obtained all TLEs for selected satellites for this period –All data publicly available from CelesTrak

6 Pg 6 of 29 Test Data

7 Pg 7 of 29 Truth Data Used GPS Precise Ephemerides from NGA –ECEF position and velocity at 15-min intervals –Accurate to better than 25 cm Agreement with IGS data was 16.8 cm ±1.1 cm (1σ) IGS data advertised as accurate to < 5 cm –Data publicly available

8 Pg 8 of 29 Methodology: Almanac Comparison Compare SEM almanacs to precise ephemerides –Propagate IAW IS-GPS-200D to same time points as precise ephemerides –Precise ephemerides used as reference –RIC coordinates of almanac position error calculated –Collected RIC error as a function of propagation interval Interval limited to ±15 days from epoch (TOA)

9 Pg 9 of 29 Results: Almanac Comparison

10 Pg 10 of 29 Results: Almanac Comparison

11 Pg 11 of 29 Results: Almanac Comparison

12 Pg 12 of 29 Results: Almanac Comparison

13 Pg 13 of 29 Results: Almanac Comparison In-track error dominant Radial and cross-track errors not significantly biased In-track error showed a range of biases Errors symmetric to propagation direction Errors grow as a function of propagation interval

14 Pg 14 of 29 Methodology: TLE Comparison Compare TLEs to precise ephemerides –Propagate IAW SGP4 to same time points as precise ephemerides –Precise ephemerides used as reference –RIC coordinates of TLE position error calculated –Collected RIC error as a function of propagation interval Interval limited to ±15 days from TLE epoch

15 Pg 15 of 29 Results: TLE Comparison

16 Pg 16 of 29 Results: TLE Comparison

17 Pg 17 of 29 Results: TLE Comparison

18 Pg 18 of 29 Results: TLE Comparison

19 Pg 19 of 29 Results: TLE Comparison In-track error dominant Significant biases in in-track error Errors clearly not symmetric with respect to propagation direction Biases increase with propagation direction Variances often nearly static

20 Pg 20 of 29 Results: Almanac & TLE Comparison Error profiles significantly different –Maximum errors comparable over ±15 day interval –Minimum 1σ almanac error smaller than minimum 1σ TLE error –Minimum almanac error occurred at 0 propagation time –Minimum TLE error occurred prior to TLE epoch –Almanac errors only moderately biased –TLE errors significantly biased –Almanac errors symmetric –TLE errors asymmetric

21 Pg 21 of 29 Methodology: TLE Consistency Compare each pair of TLEs –TLE i propagated t j -t i and compared to TLE j at t j –TLE j propagated t i -t j and compared to TLE i at t i –RIC position difference calculated relative to reference –Collected RIC difference as a function of propagation interval Interval limited to ±15 days from reference TLE epoch

22 Pg 22 of 29 Results: TLE Consistency

23 Pg 23 of 29 Results: TLE Consistency

24 Pg 24 of 29 Results: TLE Consistency

25 Pg 25 of 29 Results: TLE Consistency

26 Pg 26 of 29 Results: TLE Consistency Good overall match to TLE comparison errors Artificial pinching at 0 propagation time Slight skewing due to minimum error not being at 0 propagation time

27 Pg 27 of 29 Conclusions Almanac and TLE prediction errors comparable –Error profiles differ significantly TLE consistency analysis does reasonably approximate true error characteristics Significant biases in TLE errors can lead to an overestimation in total error Removing bias could improve prediction Error characteristics differ significantly within orbit class

28 Pg 28 of 29 Future Research Use Kalman filter to: –Estimate and eliminate bias while calculating covariance –Regenerate improved TLE Allows use of improved data in existing software Provides covariance for uncertainty estimation Additionally, perform analysis for LEO and GEO satellites to confirm results of this study

29 Questions?


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