DIFFERENTIAL GPS

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Technical details on how WAAS works

Similar to a beacon transmitter the WAAS system collects data from strategically placed and well characterized ground locations. However, unlike the standard DGPS a WAAS system cannot directly send corrections to the pseudorange data. This is because the unit has no idea where you are. Instead it attacks the problem by addressing the individual sources of error and sending corrections for each one of them. If you remember from the FAQ on error sources the biggest component of error is due to ionospheric delays followed by clock errors and ephemeris errors. In addition another significant error source was troposphere errors. To attack these error sources the WAAS system sends clock corrections, ephemeris corrections, and ionospheric corrections. It cannot compute tropospheric corrections due to the localized nature of this error but it does remove the tropospheric error component from the data it computes so that the local receiver can apply its own corrections based on an atmospheric model that is based on the current sky location of the SV. The ground stations do not send the data directly to a WAAS satellite for re-transmission but rather to a master ground station that analyzes the input and computes the full detailed error information. This fully correct data is then sent to the GEO satellite to be sent to the GPS receiver. Note that there is no correction done by the GEO satellite; the master ground station will even correct for errors produced by the GEO satellite itself. There are redundant master stations as well for system wide integrity.

Clock errors can change rapidly so this data is update every minute if required, ephemeris errors and ionosphere errors don't change nearly so fast so they are only updated every 2 minutes and can be generally be considered valid for up to 3 times that period of time. Even this time is very conservative in practice. Clock and ephemeris data is specific to a satellite but ionospheric errors are specific to your location therefore they must be sent separately. After receiving raw data from each of the ground station the master station divides the country into a grid and then builds ionospheric correction information on a per grid location basis from the data received from each reporting station. It is this grid location that is used by the GPS receiver to determine the applicable ionospheric corrections. In addition the master station determines the validity of the data it receives and can indicate invalid data within 6 seconds to the GPS receiver.

Of the forms of error correction supported by WAAS only the ionospheric data requires knowledge of the receiver position. Clock and ephemeris data is available for any receiver even if it is currently located outside the area covered by ionospheric correction data. In addition system integrity data can be used outside theis area. The designers of WAAS developed a grid system of correction data that permits a receiver to use the data it needs for this correction. Here is what the grid system looks like.

The master ground station computes a correction for ionospheric data for each of the points on the grid based on field data it gets from the other ground units. Of course it is very unlikely that there will be data for all of the points on the grid over the entire earth so the GPS receiver downloads a grid mask that is part of the almanac data that tells it where to expect corrections. The mask is divided into bands as shown on the drawing and each band contains 201 grid points (except the last one which has 200). A single bit is used to represent the availability of data on each grid point so the entire band can be contained in a single packet of data. Each geo satellite can have data for up to a maximum of three or four bands but may have less. The master station will generate correction data or each of the grid points tagged in the mask. A receiver will locate its position relative to 4 grid points and interpolate the data from those 4 points based on their relative distance. If only 3 points have data then the receiver will compute a triangle of the 3 points and if it is inside the triangle it will use the 3 points to interpolate its correction. Otherwise, the use of any of the grid points for correction is undefined. It seems however that most GPS implementations will use the data from even a single point if it is "close" to the current location. As can be seen from the map of the US phase I implementation this is required to provide WAAS for areas such as Alaska and Hawaii. It may be that multiple grid locations are generated from the single data input.

In addition to correction data the WAAS system places a high degree of importance to system integrity. Each ground station and the master station has independent sources of critical data and can determine if an SV is out of calibration. Bad data can be identified and relayed to the receiver within about 6 seconds. The geosynchronus satellites that retransmit the data can even be used as regular GPS satellites as part of the regular GPS solution since they also relay regular satellite ephemeris data for themselves. This data , like all of the other data, is generated at the master station and can be turned off independent from the regular WAAS corrections if the satellite drifts too far.

All of the data for a region is loaded into the WAAS SV, so only one is required to receive everything. A second provides redundacy.

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