On September 8, 2009, the US Coast Guard Navigation Center issued a notice reporting a pending GPS service anomaly for September 11, the anomaly being a noticeable increase in dilution of precision (DOP) over an extended period in various parts of the world.  The announcement reported the anomaly would last about 21 minutes over southern Nebraska, Kansas, Oklahoma, southwestern Arkansas, eastern Texas, and western NAVCEN NoticeLouisiana.  There would be similar effects in Asia and New Zealand.  The DOP increases are on the order of 8 and higher, with values reaching to 83.  Values below 6 are considered nominal.  Increases in DOP such as these being reported mean GPS users will not be able to get meaningful positioning accuracy for the duration of the anomaly.

According to the daily operational advisory published by the US Coast Guard, SVN52/PRN31 occupies slot A2, one of the “primary” slots of the 24 designated in the Standard Positioning Service Performance Standard.  Furthermore, the satellite is alone in this slot.  Removal of this satellite by setting it unhealthy will cause this slot to be unavailable during the outage, causing the DOP to rise.  If there had been another satellite nearby, the effect would not be nearly as great, since the other satellite would continue to provide the GPS signal while SVN52/PRN31 was offline.

Reporting of this expected service impact by the US Coast Guard is good news for GPS users, and is consistent with the commitments made by the US government in
September 11, 2009 Predicted Max HDOP Performance (All in View)

September 11, 2009 Predicted Max HDOP Performance (All in View)

the Federal Radionavigation Plan, section 5.1.1 where it states in part that “DoD will provide a 48-hour advanced notice of changes in the constellation operational status that affect the service being provided to GPS SPS users in peacetime”.

The FRP adds however, “in the case of a scheduled event affecting service being provided to GPS users, the US Government will issue an appropriate Notice Advisory to Navstar Users (NANU) at least 48 hours prior to the event”.   In this case, a notice was issued, but not in the form of a NANU.

Advanced Research performed an assessment of the All in View case, and found that HDOP exceeded 8 for only 8 minutes vs. the 21 minutes reported by the USCG which used the Best 4 case and reached a maximum of 70 vs. the 83 also reported for by the Best 4 case.   To see a video of the service anomaly for the North American region, run the Quicktime movie in the picture below (each step is one minute).

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We have a few observations and recommendations to make on this episode:

  • Although the FRP states that an appropriate NANU would be issued, notice was not provided in a numbered NANU, but in an unnumbered general message to users.  The US Government may wish to review its policy and reconcile the commitments made with the process that is followed.
  • In reporting impacts to users, the US Coast Guard should report impacts on All in View receivers.  Today, there are essentially no civilian users that operate equipment using the Best 4 algorithm.  For the US Government to report system performance based on the Best 4 receiver is irrelevant to users.  This recommendation not only pertains to this anomaly, but to the daily notices provided on the Navigation Center web site.

Typically GPS users do not see these service disruptions, since the GPS operators adjust the times of their satellite maintenance activities to occur when users will not be affected.  Since the satellite maintenance is being carried forward anyway, we can assume that in this case, there was no available time other than the time selected to conduct the maintenance.

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