GPS and WAAS

[oscarmike]

Hi All

There was a thread running a couple of days ago where hand held GPS units were under discussion and I brought up the subject of WAAS.

A couple of posters shot me down and said that the WAAS system was operational in the USA only and did not work in the UK.

I was sure I had read somewhere that it did work here, but doing a Google search revealed evidence to the contrary, and I backed down and said I was wrong.

However, upon browsing the manual for my GPS (a Magellan Spor Trak) I found the following:

quote/

Your Magellan GPS receiver has been upgraded to include the latest WAAS

(Wide Area Augmentation System) technology. With WAAS your GPS receiver

can compute position locations with an accuracy never before possible in a commercial receiver.

What is WAAS?

WAAS is a means of improving the accuracy of GPS beyond
the standard civilian signal. WAAS was developed by the FAA to provide the accuracy needed for aircraft to perform approaches for landings.

WAAS improves the accuracy of GPS by calculating the errors in the GPS signal at several monitoring stations around the country and then transmitting the error corrections to special WAAS satellites. In turn, these satellites broadcast the error corrections to WAAS-capable GPS receivers around the world. I

WAAS is a free service and, with the small exception of two additional satellites displayed on the Satellite Status screen, it will be transparent to you. There is nothing you need do, it’s all ready to go.

How accurate will my GPS receiver be with WAAS?

The FAA reports that expected accuracy can be improved to around 7 meters vertically and horizontally.

Our own testing suggests that Magellan GPS receivers will typically experience accuracy improvements to around 3 meters. You can expect to see this level of accuracy for 95% of the time that you are receiving WAAS signals.

What are some of the problems with WAAS?

The signal coverage with WAAS is not global like GPS satellites. In some locations, receiving WAAS signals can be difficult and intermittent. There are two satellites transmitting WAAS for corrections in the Americas There are two satellites transmitting WAAS for corrections in the Americas and two more over Africa and India providing similar coverage for Europe.

These satellites are geostationary, i.e., they stay in the same spot over the Earth equator. The two satellites for the Americas are over the Pacific and Atlantic Oceans. Chances are then, they will probably be near the horizon from where you are using your GPS receiver. This increases the chance that buildings, foliage, terrain and other obstacles could block the WAAS signal.

Also WAAS is a relatively new system and is still in its developmental stage. So on occasion you may experience service outages. Even during these outages, your GPS receiver will still be working, computing position fixes and helping you get to where you want to go using the GPS signals. The only difference is you will not be getting the precision that WAAS was providing. You can expect to see these outages diminish as the system matures and stabilizes.

/unquote

Regards