Introduction – The need for higher accuracy
GPS units have continued to improve in accuracy over the past decade. This is particularly due to the federal government discontinuing the practice of “selective availability”. The current state-of-the-art commercial GPS units are accurate up to 5 meters. For certain applications however, better accuracy (of less than 3 meters) is required, such as aircraft navigation and maritime navigation. The reduced accuracy of commercial units is due to a number of factors ranging from atmospheric conditions (including absorption by water), signal degradation, multipath distortion, technical limitations of the GPS device, and electromagnetic and radio frequency interference. WAAS provides ways of getting around these inaccuracies.
What is WAAS?
One approach towards improving accuracy is Differential GPS (or DGPS). The Differential GPS system (DGPS) system uses a number of fixed reference points on the earth to gauge the accuracy of a received GPS signal. DGPS units in the vicinity of one of the reference points then can adjust their calculations accordingly to improve the accuracy of their location estimate.
The limitation of the DGPS system is that it is largely dependent on the proximity of the given DGPS unit to a reference point. The farther away one is, the worse the error and the lower the accuracy of the position estimate. Moreover, DGPS systems are limited by the fact that the reference points or base stations themselves seem to be clustered in some places, and in others, such as over mountainous regions or huge forests there are hardly any. Thus, for the purposes of aircraft navigation or wide-area navigation of any kind, DGPS comes up short.
To alleviate this problem, the Federal Aviation Administration (FAA) developed a system to augment the GPS system called the Wide Area Augmentation System (WAAS). The WAAS is a somewhat more formalized approach than the DGPS which also relies on a number of fixed points on the earth. The WAAS system applies to North America, but there are similar systems being developed in other regions of the world.
Understanding WAAS – Wide Area Augmentation System
The reference points in the WAAS are called Wide-Area Reference Stations (WRS). As of 2007, there are 38 WRS in North America: 20 in the contiguous United States, 7 in Alaska, 1 in Hawaii, 1 in Puerto Rico, 5 in Mexico, and 4 in Canada. These WRS constantly monitor the signal from GPS satellites and calculate the error in the position estimate. This information is then routed to three Wide-Area Master Stations (WMS).
The WMS are in charge of calculating corrections for the WAAS. They do so in two sets. One set called the “fast” corrections is for rapidly changing errors and account for the GPS satellites instantaneous position and clock errors. The second set called the “slow” corrections account for long-term ephemeric as well as ionospheric delays. The correction information generated by the WMS is passed on to a number of geosynchronous communication satellites. These geosynchronous satellites then broadcast correction information as the WAAS signal.
At the user end, there is the user’s GPS unit which does a normal GPS position estimate. In a WAAS enabled unit there is also a WAAS receiver which receives the information from the geosynchronous satellites. The fast corrections can be immediately applied to the given GPS position estimate. The slow corrections are periodically (about every 2 minutes) applied to improve the accuracy of the position estimates. The ephemeris and ionospheric effects tend to change slowly and so this frequency of correction is considered satisfactory.
Apart from its gradual development in aviation, WAAS is slowly finding its way into commercial GPS units, improving their accuracy considerably. Especially in locations where DGPS may prove inadequate. Remember however that a WAAS unit requires continuous reception of not only the GPS signal but also the WAAS signal and are therefore more expensive (additional electronics) and also consume more power.