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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
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 km
IAC-18-A6.7.8
69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

9. IAC-18-A6.7.8
69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

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 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
IAC-18-A6.7.8
69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

11. Other Cases Missed maneuvers Cross-tagging
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
IAC-18-A6.7.8
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
IAC-18-A6.7.8
69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5

13. Questions?
IAC-18-A6.7.8
69th International Astronautical Congress (IAC), Bremen, Germany, 2018 October 1-5