User equipment incorrectly interpreting data from a satellite set “unhealthy” led to an apparent constellation outage for roughly 1,000 fleet vehicles across Australia in April. The problem was traced to the way a GPS/telecomm chip reacted to an extended navigation test aboard SVN-49, having to do with the recently launched IIF satellite, SVN-64.
Although SVN-49 was set unhealthy at the time, the integrator-equipped fleet vehicles across the continent experienced periods of several hours without satellite visibility, in unobscured environments.
The U.S. Air Force GPS Operations Center reported that in mid-May tests, “PRN 30 [was] broadcasting almanac datasets that do not reflect constellation changes that occurred since it was last uploaded with navigation message data. [. . . ] The utilization of these almanacs in a manner that regards the time of week, but neglects or mishandles the week number (effectively executing as if the current week number is the week number associated with these almanac parameters), will result in an increasing error in visibility determination and other almanac based estimations (elevation/azimuth, Doppler shift, SV clock offset from GPS time, etc) as the dataset’s actual week offset from the current week increases.”
The situation occurred once previously, in 2011 with Mercedes in Europe. The problem was traced to chips from the same manufacturer, installed by the car-maker’s integrator partner, also misinterpreting data from a satellite set unhealthy while broadcasting system test data.