DIFFERENTIAL GPS

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Loading the Almanac

The latest released products from Garmin, Magellan, and others include WAAS capability (called EGNOS in europe and MSAS in Japan). For Garmin this includes the etrex: Venture, Legend, and Vista models as well as the GPSMap76, GPS76, and Garmin V. They have also updated some other units with this capability such as the aviation units like the GPS 295 and the Street Pilot 2610. Almost all new units Garmin have been release with WAAS support. However, unlike the standard GPS almanac that is preloaded into each Garmin receiver, the WAAS almanac is not loaded into the receiver when you get it. Each person is required to get the almanac for themselves and this is required before it can be used in WAAS mode. Some folks are able to get an almanac fairly quickly while others struggle for days and are still not able to get a successful load. Here is the technique that will result in obtaining the almanac in the minimum amount of time.

1. Study the map of WAAS GEO satellites and determine which ones you are going to be able to see from your location. From the west coast of the US you can access number 47 and number 35 while from the east coast you can only see number 35. (Also the EGNOS 33 of course.)
2. Find a place with a clear sky view in the direction of the WAAS GEO satellites you are interested in. If necessary set a waypoint to their approximate location so that you can use the GPS itself to provide a bearing to the satellite. For the USA GEOS 35 is at Lat 0 and Lon 54 West while 47 is at Lat 0 and Lon 178 East. For the European EGNOS system satellite 33 is at Lat 0 Lon 15.5 West and 44 is at Lon 65.5 East. (A new third Satellite called ESA Artemus is at Lon 21.5 East and is expected to come online for Navigation use in the October 2003 and is not shown on the above map.)
3. Be sure that your GPS is set to normal mode (not battery save) for this procedure. The nature of WAAS corrections precludes its use in Garmin battery save mode.
4. Enable WAAS on your GPS and notice that it takes the last two locations on the satellite page to display WAAS Geo activity. It will cycle through all of the 19 possible satellite locations 2 at a time and then repeat until two candidate satellites are found. It stays at one setting for about 45 to 50 seconds and then selects a new pair of SV's to look for. When it doesn't have a loaded almanac it will show the SV's on top of the N indicator on the satellite page.
5. When it reaches the ones you know it should see you need to make sure that the SV shows some receive strength. It it doesn't then reposition the unit in an attempt to find either a location or a direction that will cause the signal bar to appear. You only have 45 seconds so you need to try several positions to get that signal bar. Point directly at the SV, change the angle slightly (up to 90 degrees) and tilt the antenna in the direction of the SV, leave it in each trial position for a few seconds and if not successful then try another setting. You may need to move a little. The tree you thought was out of the way may be blocking the signal.
6. Once the satellite strength bar appears you are home free. It will not move off of that SV any longer although the second position will continue to hunt for another SV unless it too gets a signal bar. You can reposition slightly to help the second one lock on as well but be careful that you don't lose the first one.
7. Hold the unit until the full Almanac gets loaded. This will be indicated by the satellite(s) assuming the correct position on the page and will usually turn solid (but this is not a requirement). The figure on the right shows two GEO SV's in position from a location in California. When the satellite is in position and collecting data it will also begin working by starting to display small D's on the other satellite bars to indicate that they are in differential mode. It can take up to 5 minutes to load the almanac data and if it misses any of the data you could wait another 5 minutes for it to come around again. If it grabs an SV that does not have land data for your area it could take quite a while for it to figure this out, but it will eventually supply some amount of differential correction.
8. If the unit seems to ignore a satellite you know it should see and then switches and moves on to other satellites without generating a signal bar then you may need to re-evaluate your location and find a better spot with a few in the direction of the satellite. You will need to wait until it gets back to your satellite location before trying again. If you are searching for a low number you may speed up the search by resetting the search to begin at the beginning by turning off the unit or disabling and reenabling WAAS mode. Note that during this early phase of WAAS/EGNOS/MSAS it is likely that the system may be in test mode and not be supplying consistent data. Therefore you may see delays in lock and outages like the signal suddenly being reset, however in the US these outages are rapidly becoming a thing of the past since the system has settled down and is considered to be nearly operational.
9. Once you have one or two SV's locked you are ready to use WAAS mode. Congratulations. The unit knows whether it is in a location where there are one or two satellites available so if you collect the almanac and the unit determines only one satellite is available it will give the second channel back to the GPS for normal GPS satellite use.

Well, now that you have the almanac data, you might be interested in what you just loaded. For a GPS the almanac data includes a coarse position data for all possible GPS SV's. Similarly the GEO almanac includes coarse position data for all possible 19 GEO satellites including the fact of their existence. The almanac will be updated as new satellites are launched but will otherwise remain static since these birds don't move much. The UTC time will also be indicated showing when the data was collected, and an ionospheric grid mask (described under how WAAS works). While locked the unit will also collect ionospheric correction data, ephemeris correction data, clock correction data, integrity data and everything else since everything repeats at least once in 5 minutes.

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