Looking through the previous posts to this board, there appears to be a fair amount of both interest and confusion regarding WAAS use in Manitoba. Since this feature was one that interested me when I was shopping for a GPSr, I did a fair amount of research on it, both before and after I picked up the receiver (a Garmin Geko 201, for those keeping score).

The short version: WAAS corrected GPS is available in Winnipeg at the very least, and likely considerably farther north. My average reported accuracy with a WAAS fix is 4 metres, and I have had it go as low as 2. Getting a WAAS fix lock is easy once you've got your GPSr set up for it.

The long version. Note that much of this information is blatently stolen from http://www.gpsinformation.org/dale/dgps.htm and http://gpsinformation.net/exe/waas.html.

What is WAAS and how does it work? WAAS is a form of Differential GPS, where approximately 25 very accurately surveyed ground stations scattered throughout the US determine a number of corrective factors (ionospheric delays, clock errors, orbital wobbles, etc) for each satellite. This data is transmitted to two master groundstations, and then uplinked to the (currently) two WAAS satellites. This data is broadcast along with the normal GPS signal to your reciever, which then applies the correction to each satellite it sees. Note that for the ionospheric correction (which is generally the largest error) you must be within around 500 miles (800 km) of one of the ground stations. There is one located in Minneapolis, at around 775 km.

The two existing master WAAS satellites are in geosyncronous orbits off the east and west coast of the US, over the equator. The satellite visible from Winnipeg is "Inmarsat 3F4 (AOR-W)", which is labelled as "35" by Garmin recievers. This satellite is stationed at 0 degrees North, 54 degrees West. This places it in the Winnipeg sky at an azimuth of 130 degrees (Roughly southeast) and an elevation of about 20 degrees over the horizon. The other is "Inmarsat 3F3 (POR)", Garmin 47, which is stationed at 0N, 178E and is well below Winnipeg's horizon.

How do I set my GPSr to receive WAAS? (This is going to be Garmin based, so if you've got something else, sorry about that....) If your GPSr is capable of receiving a WAAS signal, there will be a configuration option to do so. This is the easy part. You'll also need download the satellite almanac for the GPS. This can be a little more challenging.

When you've initially turned WAAS on, and powered up your receiver, you'll note that (in the 'advanced' sky view page) the last two slots for satellites will scan through satellites 33-51, two at a time, switching every 45 seconds. Once the reciever begins scanning for satellite 35, aim the antenna roughly at the satellite (Southeast, midway between the horizon and 45 degrees elevation). You should get a good, strong signal from 35. You'll need to hold or prop up the reciever so that it maintains this signal until the almanac for the satellite is downloaded, which could 5 to 10 minutes (or more, if the signal is interrupted). Once the signal bar goes black, and a "D" appears over it, you're done.

Once the almanac is downloaded, the GPSr will pick up satellite 35 as quickly as any regular satellite. As it downloads correction data, the signal bars for the other visible satellites will have "D"s appear over them, and the accuracy esitmate label will change from "Accuracy" to "Diff". Remember when taking readings that the satellite 35 will always be in the southwest.

That's it. Enjoy your WAAS enabled GPSr....


grnbrg
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grnbrg@grnbrg.org