Wondering about WAAS

[hish747]

As I understand it, GPS location can sometimes have errors for a variety of factors and WAAS uses fixed ground stations of known location in order to calculate a correction the GPS signal.

My question is where is the error correction processing done?
Do the ground stations correct the GPS error and broadcast a corrected GPS signal or do they just broadcast their location or correction signal which your WAAS receiver gets and then calculates a correction in the box?

 

[Htaylor]

As I understand it, GPS location can sometimes have errors for a variety of factors and WAAS uses fixed ground stations of known location in order to calculate a correction the GPS signal.

My question is where is the error correction processing done?
Do the ground stations correct the GPS error and broadcast a corrected GPS signal or do they just broadcast their location or correction signal which your WAAS receiver gets and then calculates a correction in the box?

http://www.avionicswest.com/Articles/How WAAS Works.html

 

[wilkersk]

https://www.faa.gov/about/office_or...its/techops/navservices/gnss/waas/howitworks/ "The GPS information collected by the WRS sites is transmitted to WAAS Master Stations (WMS). The WMS generates a WAAS User Message every second. These messages contain information enabling GPS/WAAS receivers to remove errors in the GPS signal, allowing for a significant increase in location accuracy and integrity."

 

[arnoha]

It must be the end unit that does the correction with the way GPS works. GPS is a one-way system, including WAAS, as you're probably aware. The satellites send signals but receive none from the GPS device. (Obviously, they do receive signals from ground stations, so the satellite isn't one-way, but the actual position determination is.) That signal illuminates the Earth in a cone extending from the satellite antenna to the ground. Every point in the resulting cone has a slightly different correction, but that entire cone is only producing one timing signal. So, that means only the device itself has all the information to make the correction. You need to combine three signals to get WAAS: timing from 4 or more satellites, the WAAS correction table, and the satellite almanac. The satellite has the last two, but only the device has the first one.