SVN-23/PRN-23 INTEGRITY FAILURE OF 01 JANUARY 2004 Capt Heather Eastlack 2nd Space Operations Squadron Written by: K. Kovach 9 Mar 04

OUTLINE Note on Nomenclature Message to Remember The SVN-23/PRN-23 Integrity Failure The Range of User Impacts Recommended Preventative Summary

A NOTE ON NOMENCLATURE SVN-23 is the "Space Vehicle Number" The production line number –Permanently "painted on the satellite's inside" –Users have no independent way to see the SVN PRN-23 is the "PseudoRandom Noise code" (PRN) Number Identifies P(Y)-code & C/A-code broadcast by SV –Users can independently see the SV's PRN codes Also where SV shows up in the almanac data –Known as the "SV ID" number in ICD-GPS-200 SVN-23 was Broadcasting as PRN-23 on 01 Jan 04 A GPS SV's PRN number is ground selectable

PRN-23 Anomaly Timeline SVN-23 Suffered an Integrity Failure on 01 Jan 04 Operating on a Rubidium clock Clock failed at about 18:30Z (Holiday Afternoon) Clock failure caused a substantial frequency error –Varied over time, max sustained error was 371 Hz at L1 –371 Hz at L1 gives an 70.6 m/sec pseudorange rate error An integrity failure because no timely warning issued The Operational Control Segment (OCS) Set SVN-23 Unhealthy at about 21:18Z Accumulated pseudorange error was about 285,000 m No SATZAP procedure performed –SATZAP changes identity of the SV's PRN code –SVN-23 remained trackable as PRN-23

23

International GPS Service (IGS) 5-MINUTE DATA

IGS 5-MINUTE DATA Failure at ~1830. Loss of tracking. Set Unhealthy at ~2118.

Master Control Station (MCS) & IGS DATA

FAA DATA -- FIRST 10 MINUTES

SVN-23 RECOVERY SVN-23 Clock Switched Off at 23:05Z on 1 Jan 04 One Cesium clock remained on-board (marginal) SVN-23 Service Restored Set HEALTHY at 2253Z on 20 Jan 04 SVN-23 End-of-Life (PRN23 available for IIR-12) Set DEAD at 2030Z on 5 Feb 04 Decommissioned (unavailable to users) at 2200Z on 13 Feb 04

CONTEXT This is the First Major Integrity Failure Since 2001 Integrity failures are rare (!) For Aviation, Integrity Requirement is /hr Or 1 integrity failure per 1,142 years Obviously, SVs will Never meet the Aviation Integrity Requirement Alone Would need a 1,000-fold improvement Integrity Failure Rate for GPS SVs is  3 per year Specified in SPS Performance Standard Specified in PPS Performance Standard

GOOD NEWS IIR/IIR-M Satellites Don't Have This Failure Mode It has been fixed (!) Run two clocks concurrently –Atomic (Rubidium) –VCXO (Quartz) Monitor short-term difference between the clocks –If difference greater than a threshold (~5 m) Immediately switch to non-standard code IIR/IIR-M Satellites thus Provide Integrity 1,000-fold improvement for large errors