Air Force Seeks Comments on SVN49 Mitigations

Yesterday, a Federal Register Notice was published seeking comments on possible mitigation options for GPS SVN-49. See:

and background briefing materials at:

The comment period closes 28 May. The first of two telecons being held by the GPS Wing was yesterday, the same date as the FRN. The second will be on 30 April:

The U.S. Government is providing notice that it is actively considering several mitigation options prior to changing the health status of Global Positioning System (GPS) satellite IIR/20M (satellite vehicle number 49/SVN 49) from unhealthy to healthy. The potential mitigations are each designed to reduce the impact of the unique nature of the SVN 49 signal to a portion of the user segment. The mitigations are described in an Appendix posted in the public docket. The mitigations are intended only for use with the SVN 49 satellite and will not be implemented for any other GPS satellite. Responses from this public comment period will be considered during the final decision to choose which mitigation(s) to implement.

After successful implementation of the selected mitigation(s), U.S. Government leaders will determine the conditions necessary to set SVN 49 healthy. It is anticipated this timeline will occur over the next one to three years. Please review the posted Appendix and submit concerns or adverse impacts that may affect your user equipment to the Docket.

The GPS satellite SVN 49 has a unique signal that may result in degraded performance for some GPS user equipment. This satellite currently operates with the navigational message set in an unhealthy state to prevent any adverse impact to users. To minimize the adverse impacts, several mitigations are under consideration. Each mitigation is intended for a specific user group and can be used with or without the other mitigations. All mitigations are intended for use with SVN 49 only and no changes will be made regarding other GPS satellites. The mitigations are briefly described as:
• Set SVN 49 healthy with current 152m Antenna Phase Center (APC) and associated clock offsets.
• Set SVN 49 healthy with factory default APC and clock offset.
• Users switch to multipath-resistant receivers to minimize adverse impact from SVN 49 signal.
• Modify receiver software to use look-up table corrections to account for unique SVN 49 signal.
• Increase SVN 49 User Range Accuracy (URA) minimum value to 3 by  changing bits in the GPS data message that allow user equipment to de-weight or exclude SVN 49 signals.
• Remove data modulation from L2 P(Y)-code to mitigate impact to high precision users.
• Change L2C PRN code to a "unique sequence" to prevent L2C users from including SVN 49 in their solutions.
• Change SVN 49 from PRN 01 to PRN 32 to prevent updates to WAGE users.
• Use spare health code so future users could use SVN 49 despite unhealthy setting.

More information is presented in the Appendix posted in the public docket. Implementation of each mitigation is contingent upon several factors including cost, number of users impacted, schedule, and the needs of the U.S. Government. If you would like to submit a public comment concerning these proposed actions, please follow the instructions listed above.