WAAS satellite failure cuts signal to one

April 13, 2010

One of two satellites providing Wide Area Augmentation System (WAAS) coverage is experiencing technical issues and will soon discontinue its broadcast.

The FAA does not anticipate any immediate impact to service, but the remaining satellite will be the only one broadcasting the WAAS signal in space; WAAS users may experience temporary service outages due to lack of redundant geostationary satellite (GEO) signals if a switch between the primary and backup GEO uplink stations occurs.

WAAS enables pilots to fly IFR without reliance on ground-based navigation aids; LPV approaches rely on WAAS signals. Over the next few weeks, the Intelsat Galaxy XV GEO will drift out of its current orbit position, ultimately requiring its WAAS signal to be discontinued, the FAA said. The failure of the GEO will directly affect 16 airports in Northwest Alaska, but none of those airports have published LPV approaches. Users in that area can continue to fly the existing LNAV procedures. However, users must confirm that GPS receiver autonomous integrity monitoring (RAIM) with fault detection and exclusion (FDE) will be available for the flight during planning, the FAA said.

For the rest of the national airspace system, a single GEO will provide WAAS signals—what is referred to as single-string—until an additional satellite comes on line, expected in December.

“AOPA is working with the FAA’s navigation services division to ensure that the impacts of this outage are addressed, and that information is relayed in a timely manner,” said Heidi Williams, AOPA senior director of airspace and modernization.

Because GEO signals will be single string, there may be service interruptions if the GEO uplink stations switch from primary to backup. These switchovers are rare events, but if one occurs it may take up to 5 minutes to fully restore LPV service.

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.