Wide Area Differential GPS (WADGPS)

The Wide Area Differential GPS (WADGPS) Laboratory is a subgroup of the GPS Laboratory at Stanford University and is closely affiliated with the Local Area Augmentation System Lab. Our research focuses on GPS navigation aids for aviation in all phases of flight. We're located on campus, in the Dept. of Aeronautics/Astronautics, so stop by Durand 452 for a visit.

What is Wide Area Augmentation?

Late in 2003, the FAA will declare the Wide Area Augmentation System (WAAS) operational. This system will monitor the GPS constellation to provide differential corrections and, more importantly, a certified level of integrity. The corrections will improve the accuracy of the system from ten or more meters to just one or two. It is the integrity, however, that will open the doors for widespread aviation use. Although GPS does have a strong track record, it has not yet been generally approved for the most demanding of aviation applications. GPS was not designed for these applications and lacks the necessary real-time monitoring. Previously, aviation has had to use RAIM, a technique exploiting redundant satellites, to provide sufficient integrity. This technique requires good geometry and wide margins. As such it is suitable for en route flying and non-precision approach. However, precision approach, which brings airplanes within a few hundred feet of the ground, has more stringent needs. WAAS will enable aircraft to conduct varying levels of precision approach. Additionally the instrumentation required to use WAAS is substantially less expensive than today's suite of equipment. Although WAAS is not yet certified, there is an operational test signal that is available for use and there already are several commercial WAAS capable receivers. Because it incorporates nearly all the elements of the final monitoring algorithms, this experimental signal has a higher level of integrity than your typical differential system. It is finding widespread use in agricultural and other applications. More and more non-aviation users are discovering the benefits of this free signal. It offers the great advantage that the differential corrections come in the same antenna that the GPS signals do. There are no additional antennas to install, no local reference stations to set up, and no additional communication channels to maintain. Just turn it on and use it anywhere in the United States. Similar systems are also being developed in Europe and Japan. Over time the level of service is expected to improve. In 2003, the first level of service, LNAV/VNAV, will be offered for aviation instrument approach use. In later phases better availability and lower decision heights will become possible.

Current Research Areas

Ionospheric Estimation
Signal Integrity and Availability
Antenna Design
Backup Navigation Systems
Evil Waveforms
Air Traffic Control Applications

Highlights

Juneau, AK flight testing (1-15 August 1998)
Creating the WAAS Correction Message
WAAS/NSTB Performance (triangle charts) (scripts available)
GPS Clock/SA anomaly

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