Earlier on April 8th, Intelsat announced it had lost control of its Galaxy 15 satellite that hosts the WAAS SBAS transponder used by the FAA.
The Federal Aviation Administration (FAA) announced that one of the two WAAS GEO satellites will drift out of usable orbit within two to four weeks.
The FAA says it is monitoring the satellite, but that failure is imminent (by mid-May 2010).
When G-15 is out of usuable orbit, WAAS will be disrupted for users in northwest Alaska.
The rest of the WAAS service area — U.S., Canada, Mexico — will operate normally but will be reduced to a single point of failure with only one WAAS broadcasting Anik F1R satellite remaining (PRN #138/NMEA#51).
So it seems recommending that all users should be locked onto PRN 138.
For those users that have GPS receivers with the ability to track either one of the above PRNs, it would be best to remove PRN#135 (NMEA#48) as an option.
For the short term, 138 will be the only option. Longer term, there will have other options (PRN#133 today in test mode until the end of the year).
Note : there are two numerical designations for each WAAS satellite. PRN numbers identify the code transmitted by the satellite while the NMEA SV ID designation which is a proprietary format is simply the PRN minus 87. Garmin for example uses the NMEA number.
The impending failure of this satellite—which only affects WAAS signals—comes at a time when there are questions about the regular GPS constellation. The U.S. Government Accountability Office has said that many of the older satellites currently in use could reach the end of their operational life faster than they can be replenished, resulting in a drop below the number of satellites needed to meet some GPS users’ needs, as early as this year.
For more information, see the FAA’s “WAAS Intelsat GEO Outage Impacts” presentation.
AVweb raised the issue of satellite redundancy with AOPA President Craig Fuller (podcast)