Wide Area Augmentation System WAAS 

The US Federal Aviation Authority (FAA) is developing the Wide Area Augmentation System (WAAS)

The WAAS is safety critical navigation system that improves the accuracy integrity and availability of GPS so that GPS can be a primary means of navigation for aircraft en route travel and non precision approaches. 

The WAAS improves real time civil accuracy of GPS to about 7 metres in three dimensions and improves system availability and provides integrity information for the GPS constellation GPS signals are received by about 25 Wide area ground Reference Stations (WRSs) They determine the correction information and assesses the integrity of the system. 

Correction messages are then uplinked to the geostationary communication satellites 

*The aircraft listens to the GPS signals and the message broadcast on the GPS L l frequency by geostationary communication satellites.

*Benefits claimed for the WAAS are that it enables GPS to be the primary means of navigation including take off approach and landing. 

*Aircraft routes are more direct and there will not be the need for aircraft to be routed according to the constraints of existing ground based navigation systems. 

*There will be increased capacity in airspace without increased risk and there will be reduced and simplified navigation equipment on aircraft. 

*As far as the engineering surveyor is concerned WAAS satellites may be useful as additional satellites for pseudo range measurements.