Tuesday, January 24, 2006

Differential GPS: WAAS

I can't look at 3/4 letters acronyms and not start to wonder what is hidden behind them. Sometimes I can get satisfied with the full name, other times the curiosity continues taking you down their paths. This time I had to stop at some point to write this post.

Let's start with the need first: before handheld GPS devices became available so we all could play geocaching, hike Yosemite or drive around LA, airplanes were trying to land in airstripes with very poor visibility and the FAA wanted to get them to do it safely. The problem was that GPS signals lacked the necessary precision for pilots to use them.

"Remember that GPS receivers use timing signals from at least four satellites to establish a position." But thanks to changes in the atmosphere and ionosphere plus other sources of errors these times have delays added to them and will add inaccuracy to the positions calculated based on them.

The FAA engineers came up with the idea of having a fixed receiver station with a known position that would calculate time instead of distance: "It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an 'error correction' factor." Then it broadcasts to the GPS satellites these correction values so they can adjust its measurements.

The plan was called "Wide Area Augmentation System" (or WAAS), a way to improve accuracy in the North American continent with Differential GPS. It translates today in 25 ground reference stations and two geostationary satellites with a fixed position over the Equator providing correction information and better accuracy to any compatible GPS receiver.

With the collaboration between continents with the European Space Agency (ESA) (and under a different acronym, SBAS for Satellite Based Augmentation System) EGNOS (European Geostationary Navigation Overlay Service) is another set of satellites and ground stations that can also be used (if you happen to be under their satellites coverage) which adds a total of 44 stations.

From PocketGPSWorld: "Both EGNOS and WAAS systems are designed against the same [...] standards [...] both WAAS and EGNOS receivers [can] be interoperable, and, therefore, a receiver able to process WAAS will also process EGNOS and vice versa."

If you want more check this very well done tutorial from Trimble (Flash player recommended). Check also Dale DePriest who published an even better explanation of WAAS usage in Garmin receivers and Sam Worley's website who really knows his stuff.

No comments: