DIFFERENTIAL GPS

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How WAAS gets used by the Garmin receiver

The information in this section is strictly conjecture based on study and observation. Garmin considers the inner-workings of their GPS receivers to be trade secrets so this information is not published. Note that WAAS use requires the GPS be in normal mode. Battery-save mode does not lend itself to the intermittent processing capability of battery-save mode. Generally the SV's for WAAS won't even show up in battery-save mode but it is possible to get the unit into battery-save and have it still attempt WAAS processing on some receivers but the results are unpredictable and generally not usable.

It is possible to leave the WAAS enabled all the time. The only negatives are that the unit will expend some energy attempting to lock onto the WAAS satellites and the two channels will not be available for standard GPS devices. Even a momentary break in terrain will cause the GPS to lock onto a GEO-SV if it can see one. WAAS data is sent in packets that are one second long (250 bits) and a lock can occur at any one second interval. Once locked, as shown by a grey bar on the screen) it will take some period for the GPS receiver to download enough data to be useful. How long depends on what data the GEO is sending at that moment. Often differential corrections can begin in 10 to 12 seconds from lock based on a need to download correction data for some of the SV's. The indication of correction data being present in the appearance of "D's" appearing on the regular satellite strenght bars showing which SV is receiving corrections. At this point, even if the GEO is lost again, differential corrections will continue to be applied for about 2 minutes. The WAAS specification doesn't cover the case where a GEO satellite is drifting in and out of sync with the GPS receiver since, for airplane use, a clear view of the sky is assumed. I believe Garmin is extra conservative is dropping the differential corrections after only 2 minutes. While the GEO-SV is in view the receiver will download corrections for additional SV's and the current ephemeris data for the GEO satellites themselves as well as correction data for the GEO satellites. Once ephemeris data is loaded the satellite bar will turn dark which means that it can also be used as part of the computed solution. Note that the GEO satellite can download correction data without, itself, being part of the GPS solution. A WAGPS satellite may or may not ever show a dark bar depending on the current ablitity of that SV to be used as part of GPS lock.

It seems that Garmin will favor differentially corrected satellites, when at least four exist, to the exclusion of regular satellites. If the four are in a poor geometric relationship the epe number, and possibly the accuracy of the solution, can be worse that it was with a regular solution. Luckily the GEO will generally download corrections for all of the satellites above a mask angle of 5 degrees so this anomaly is usually short lived. However, if the WAAS lock in on a geo satellite that is outside the coverage area for that particular SV the results can be differential corrections based on ephemeris and clocks on SV's that are all located in one corner of the sky which, because of poor geometry, can result in worse solution. In this case the only recourse is to turn off WAAS. All of the latest firmware releases from Garmin correct this potential problem by checking the coverage area before a lock is permitted on the WAAS satellites.

If a GEO drops behind a hill the GPSr will lose its information just like any other GPS satellite. When the vehicle moves far enough the GEO may be seen again and will recover with its ephemeris data still current and re-enter the solution but it seems that ephemeris data on a GEO is shorter lived that on a normal GPS where it is good for hours. I observed a loss of geo satellite dark bar status after only about 2 minutes of it being out of sight. It could be that ephemeris data on a GEO is short lived since it tends to wobble a bit and is not truly in a circular orbit having much more eccentricity that a normal gps satellite or it could be that the GEO had been reset due to some ongoing test mode. More data will be required to determine which.

If you receive a satellite but do not have any ionospheric data for your area the Garmin receivers with the latest firmware do detect this condition. They will not lock on a WAAS SV outside its coverage area. I think they should lock if no other WAAS capable SV covers the area and use the corrections for clock and ephemeris available while applying the internal ionospheric correction algorithm in this case but they don't seem to be this sophisticated.

Other GPS receivers may or may not detect this condition. In some cases they may receive data and use it for corrections but not support the ability to use the GEO-SV as a GPS satellite. While many receivers claim support for WAAS these days the implementation may not be the same in all receiver designs. For example, receivers using the SiRF chipset will only devote one channel to WAAS reception.

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