The Need for Comparative SSA

1. The Need for Comparative SSA T.S. Kelso & Dan Oltrogge Center for Space Standards & Innovation 2018 Oct 5

2. Overview • Introduction • Background • Case Studies • Conclusions 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

3. Introduction • Segal’s Law: A man with a watch knows what time it is. A man with two watches is never sure. • Reality is quite a bit different • Example: Timekeeping at USNO, BIPM, GPS • Comparing data against similar independent data: • Minimizes uncertainty and associated risk • Identifies outliers to help eliminate problems affecting safety of flight 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

4. Background • Satellite operators are developing independent SSA for their systems • Motivated by perceived lack of quality & others’ unwillingness to share • Many believe they only need good data for their satellites • Reluctant to exchange orbital data • Motivated by national security or commercial competitiveness • Implicitly assumes independent systems are immune to unexpected errors • Space Data Center (SDC) • Ingests data from 30 operators & JSpOC for conjunction assessment • ~270 GEO & ~440 LEO/MEO satellites with ephemerides • Gets full TLEs & SP ephemerides for full public catalog from JSpOC • Routinely performs comparative assessments in daily operations • Shares data & results with affected parties to ensure better overall SSA 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

5. Case Study #1 • Initial validation of new user found problems with 2 satellites’ data • Comparison of ephemerides & TLEs differed by over 10,000 km • 3D analysis ultimately showed problem was due to incorrect reference frame used in operator’s JSpOC-formatted data (ITRF vs. J2000) • Problem had gone undetected for over six months in data exchange with JSpOC • Proper comparative analysis should have caught this problem immediately, as it did in the SDC 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

6. Case Study #2 • Problem detected comparing ephemerides used by JSpOC in CA screening using operator ephemerides provided by SDC • Using the same ephemeris, difference should be less than 10 m (due to EOP) • Situation arose where difference was > 100 km • Subsequent analysis showed an anomaly in the operator data, as the mismodeling of a maneuver • Resulting JSpOC conversion of data for CDM produced GEO orbit with perigee altitude below the Earth’s surface (-2,132 km) • Operator advised of anomalous data; JSpOC advised, as well • All SDA members advised of how to detect this type of situation 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

7. Case Study #3 • Problem detected in comparing SDC & JSpOC ephemerides (provided by operator) at TCA • Difference of ~12,000 km detected • Separate weekly comparative analysis showed no discrepancy • Comparison of piecewise ephemerides pointed to a problem with operator ephemerides • Five of eleven ephemerides were bad over 15 days • Time tag in JSpOC format incorrectly calculated the day of the year on some dates • Adjusting bad ephemerides by one full day corrected the problem • Weekly comparisons happened to use good data • Operator advised of results and took corrective action 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

8. Case Study #4 • Problem detected in weekly comparative analyses for GEO • Only occurred for SES and when TLE had epoch of 0000 UTC • Showed as TLE jumping away from SDC ephemeris and drifting • Occurred for more than a dozen satellites with differences of 50-100 km 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

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10. Case Study #4 • Problem detected in weekly comparative analyses for GEO • Only occurred for SES and when TLE had epoch of 0000 UTC • Showed as TLE jumping away from SDC ephemeris and drifting • Occurred for more than a dozen satellites with differences of 50-100 km • Ultimately determined the problem was caused by Millstone using SES data to generate TLEs, which were then added to the public catalog • Data was simply reformatted as TLE, even though it used osculating elements and True of Date reference system, instead of Brouwer mean elements and TEME • JSpOC was advised of the problem • SDC (via CelesTrak) created supplemental TLEs to address JSpOC need 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

11. Other Cases • Missed maneuvers • Natural consequence of not incorporating planned maneuvers • Supplemental TLEs provided by SDC to JSpOC to resolve issues • Cross-tagging • Can occur often in GEO clusters • Complicated by maneuvers and time of day, daylight, weather • Problems are immediately reported to JSpOC to ensure best-possible SSA is available to the entire community for safety of flight 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

12. Conclusions • These are just a few of the problems detected and reported • All systems fail at some point • Bad input data • Incorrectly formatted data or an unanticipated change in format • Faulty algorithms or the application of algorithms in situations outside of their expected use • Problems converting data from one format to another • Misunderstanding of coordinate systems or other metrics being used • Unnoticed failures in data transfer that may lead to data latency • Regular comparative analyses help identify issues for resolution 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

13. Questions? 69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5