Occasionally, GPS satellites are spread across the sky in configurations that prevent a receiver from calculating a good position fix. When that happens, the unit’s receiver autonomous integrity monitor (RAIM) will generate an alert to the pilot to use an alternative navigation source.
These events, called RAIM outages, or “holes,” can last from a few seconds to many minutes, because the satellites are always moving, essentially independently, thereby creating a constantly changing configuration as they cross from one horizon to the other. Think of RAIM holes as science fiction-like blobs, continually altering their shape and size, and frequently disappearing, as they move around the globe.
Data recorded over two-and-a-half years at an Airservices Australia monitoring site show outage periods lasting from seconds up to 49 minutes, with an outage occurring on average every 10 days and lasting an average of 19 minutes.
But since it knows each satellite’s orbit, the GPS system can calculate these outages in advance, and the resulting RAIM forecasts–available at FAA Flight Service Stations–are used by several U.S. operators to determine whether, for example, GPS will be fully available at their planned arrival time at destinations where GPS is to be relied upon for the approach. Generally, however, RAIM holes are infrequent enough not to constitute an en route flight hazard.
But Airservices Australia has a particular interest in them, because it has launched a nationwide “pseudo-radar” network through which, by 2006, ADS-B positions based on GPS will be used to separate traffic over the vast deserts and barren lands of its interior “outback.” At a recent FAA international symposium in Anchorage, Alaska, Greg Dunstone, Airservices’ ADS-B project manager, said the network would allow “radar like” separation standards over remote areas where conventional radar installations would be prohibitively costly, and where very widely spaced (and capacity consuming) procedural traffic separations are currently applied.
Radar screens at the ATC center covering the airspace surrounding Airservices’ ADS-B evaluation site at Bundaburg, on Australia’s east coast, show the forecast location and size of RAIM holes, allowing controllers to plan switchovers from pseudo-radar to procedural separation standards. Dunstone explained that flight safety is not affected, since the procedures are similar to those used during planned radar maintenance periods and are applied without difficulty throughout ADS-B coverage, which extends from the test site out to more than 250 nm at high altitudes.
What concerned Dunstone were unplanned RAIM outages. While these are very much less frequent, and typically caused by a problem with or failure of an individual satellite or by unusual ionospheric events, he said they would be akin to an unexpected radar failure and would cause a “scramble” at the ARTCC.
But there is a solution. To determine GPS integrity, the RAIM algorithm in current TSO C-129 units requires at least five satellites to be visible above the local horizon and in good relative “geometry” to each other. Then, since the GPS set needs four satellites to obtain its position, the receiver software periodically derives a series of fixes from all combinations of any four out of the five satellites in view. Normally, all combinations will produce the same position fix, thereby proving system integrity. Should the positions not coincide, the software will alert the operator to a GPS integrity failure, or hole, and direct a switch to an alternative form of navigation.
The solution to this lies in the new TSO C-145 and C-146 WAAS receivers, which don’t use RAIM but apply more advanced software to check fewer satellites, even when these are in poor relative geometry. As a result, FAA data has shown no RAIM-type outages for WAAS receivers–other than during severe ionospheric disturbances last year–since WAAS was commissioned 13 months ago. WAAS receivers are now slowly entering the marketplace, but industry officials suggest that their widespread use is not expected until 2006 and beyond.
On the other hand, there are circumstances under which neither RAIM nor WAAS can help. Over a 10-day period in June, the U.S. Coast Guard (now an agency of the Department of Homeland Security) ran a series of three- and four-hour GPS jamming tests centered on the Cherry Point, N.C., Tacan.
The relevant notam advised that GPS would be unreliable within a 350-nm line-of-sight-radius of the beacon at FL400; within a 300-nm radius at FL250; within a 230-nm radius at 10,000 feet msl; and within a 185-nm radius at 4,000 feet msl.
While the tests were doubtless essential from the standpoint of national security, it does underline the FAA’s dilemma in planning the gradual decommissioning of terrestrial navaids, since the Department of Defense–and now also, presumably, the Department of Homeland Security–has the right to jam or otherwise interfere with GPS signals whenever and wherever it feels necessary.
Perhaps the loran community does have a point about its system’s merit as a virtually unjammable GPS backup. An extract obtained by AIN from a soon-to-be-released FAA evaluation of loran as a GPS backup says: “The evaluation shows that the modernized loran system…could be used to mitigate the operational effects of disruption of GPS services.”