WAAS-Enabled GPS System

by: Jeff Sanders

Webmaster comments in bold red

The Global Positioning System continues to evolve and improve.  Today's GPS units are getting better and better - smaller, more capable hardware, better software support, Internet interfaces and more advanced antenna - and for less money too. As a result, GPS accuracy has gone from 25 meters 10 years ago to three meters today.  It's interesting to note that much of the accuracy of today's systems comes from ground stations and not the satellites themselves, although there have been improvements there, too.  Additionally, today's units pick up signals faster and hold them better under less than ideal conditions, such as under thick tree cover.

This article deals with the ground aspect of GPS.  It's a little on the technical side but it's good information for anyone who wants to know how the whole systems works. Now we just need something that will automatically sign the geocache log when we forget our pencil.

WAAS (Wide Area Augmentation System) was developed by the Federal Aviation Administration to augment the Global Positioning System to improve its accuracy, integrity, and availability. WAAS was originally intended to enable aircraft to rely on GPS for all phases of flight, including the precision approach to airports within its coverage area. The WAAS system typically provides better than 1.0 meters laterally and 1.5 meters vertically throughout most of the contiguous United States and large parts of Canada and Alaska. This accuracy is capable of providing aircraft with the precision needed for safe approaches and in-flight navigation for all weather conditions. Integrity of the WAAS information is no more than 3 seconds of bad data per year allowing the system to be considered safe by the FAA for instrument flight rules.

Although originally developed for aviation, WAAS is not just limited to the aviation industry. Any GPS receiver that is capable of receiving the WAAS signal will be able to benefit from its correction data, making the GPS positioning more accurate. In fact a WAAS-enabled GPS receiver can even give you directions right down to the lane your car is traveling in (as long as the mapping program supports "lane assist" directions). Because the Wide Area Augmentation System is quickly becoming standard in the GPS industry, most new GPS receivers today are WAAS-enabled. Just like with the conventional GPS, the WAAS system doesn't come with any extra cost or fees to use. All that is required is that the GPS receiver be WAAS-enabled so it can receive and decode the data then be able to apply corrections to its position. Currently the WAAS service is limited to the U.S.A., Canada, Alaska and Hawaii. Although independent from WAAS, Europe and Asia are working on their own supplemental GPS correction systems. Europe has the "Euro Geostationary Navigation Overlay Service" (EGNOS) and Japan is working on their "Multi-Functional Satellite Augmentation System" (MSAS).

How the WAAS System Works

WAAS uses a network of approximately 25 ground based Wide-area Reference Stations (WRS) in North America and Hawaii, to measure small variations in GPS satellite signals in the western hemisphere. These precisely surveyed ground stations monitor and collect information on the GPS signals and send their data to the three Wide-area Master Stations (WMS). The WMS's generate two different sets of corrections: fast and slow. The fast corrections are for errors that are changing rapidly and are a primary concern to the GPS satellites instantaneous positions and clock errors. These corrections are user position independent, which means they can be applied instantly by any receiver in the WAAS broadcasting area. The slow corrections are for long-term ephemeric and clock error estimates and ionospheric delay information.

Once these corrections are generated, the Master Stations sends them to two pairs of Ground Uplink Stations (GUS) that transmit the correction messages to a series of geostationary satellites that broadcast their correction data back to earth. Then WAAS-enabled GPS receivers use this information to make corrections to the original GPS signal, giving WAAS-enabled GPS receiver a more accurate position. GPS receiver's use the information broadcast from each GPS satellite to determine their location and the current time. Depending on the GPS device, a GPS receiver only needs to receive a signal from 3-4 satellites (out of the 31 satellites currently transmitting a signal for civilian users) to be able to calculate its position. In addition to the GPS signal, a WAAS-enabled GPS receiver can also receive the geostationary WAAS satellite signal.

The two different types of correction messages from the WAAS system (fast and slow) are used by the GPS receiver in different ways. The fast type of correction data includes the corrected satellite position and clock data to determine its current location using normal GPS calculations. Once an approximate position fix is obtained the GPS receiver begins to use the slow corrections to improve its accuracy. Slow correction data includes the ionospheric delay. When the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received an ionospheric delay value for that location, it corrects for the error that the ionosphere created. Unlike the fast data, the slow data doesn't need to be updated frequently because the ionosphere conditions don't change rapidly. While the slow data can be updated every minute if necessary, they are only updated every two minutes and are considered valid for up to six minutes.

Limitations of the WAAS System

1. The WAAS system is currently only available to United States and large parts of Canada and Alaska but there plans to expand the system to other countries and continents.

2. Because the WAAS broadcasting satellites are geostationary causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal.

3. In order to calculate an ionospheric grid point's delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated.

4. Aircraft conducting WAAS approaches must possess certified GPS receivers.

 About The Author: Jeff Sanders, Garmin GPS Navigation Systems - http://www.GpsFrontier.com