Survey Perspectives – June 2007

June 27, 2007  - By


It’s been about a year since I’ve touched on WAAS (Wide Area Augmentation System) with any depth, and a lot of things are happening on that front so I’d thought I’d give an update. Also, look for a column on EGNOS (Europe’s version of WAAS) in the next month or two.

WAAS is finally going to settle down, in terms of the WAAS broadcasting satellites; the final one is scheduled to become operational (for aviation users) in mid-July. Although it should never have been a headache for ground users like us (many of us have been using the satellite for mapping for more than a year), it was — mainly because several manufacturers of mapping receivers insisted on allowing their receivers to use only WAAS in aviation mode, which didn’t make any sense.

Anyway, two significant events will happen next month. One is mentioned above, which means that users in the eastern/northeastern US and eastern Canada who are using mapping receivers that require WAAS to be operational for aviation users will now be able to use WAAS more reliably.

The other event happening next month is that the two legacy WAAS broadcasting satellites, POR (PRN 122) and AOR-W (PRN 134), will stop broadcasting WAAS information. This has been the FAA’s plan all along. It will leave two WAAS broadcasting satellites (PRN 135 and PRN 138) that provide dual coverage throughout the US. Alas, WAAS satellite visibility will never be as good as it was for northeastern US and northeastern Canadian users when AOR-W was at 54W longitude.

After the two legacy WAAS satellites stop broadcasting, the coverage footprint will look like this:

Legacy WAAS Satelite

This will put the nearly 18-month WAAS satellite reconfiguration confusion to bed.

Another major milestone for WAAS later this summer is an upgrade that will add four new reference stations in Canada and five in Mexico. Also, new reference stations in Alaska will be added to the Iono grid.

What this means is that WAAS coverage will expand to the north (Canada) and south (Mexico). Users will also see improved performance on the fringes of the current WAAS service area (southern Texas, southern California, New England states, etc.).

Following is a map of the Iono Grid Points (IGPs) for the current WAAS service area:

Iono Grid Point - GPS

And here’s a map of the IGPs for the expanded WAAS service area:


WAAS Accuracy

Even though the expanded IGPs have yet to be implemented, WAAS accuracy is impressive. The National Satellite Test Bed (NSTB) produces a WAAS Performance Report on a quarterly basis. Each GPS receiver collects about 7,000,000 each quarter, and the NSTB compiles and publishes the test results.

Following are the results of Q1 2007:

For a system that was originally specified to provide 7-meter accuracy, the performance is impressive. At the 95% confidence level, horizontal accuracy for all test sites across the CONUS and some in Alaska are all under a meter. Also, remember that these figures will improve with the addition of the new reference stations later this summer.

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