chapter  18
7 Other GNSS
Pages 1

All these augmentation systems operate on the

principle of numerous ground stations in known

geographical positions receiving GPS signals.

Correction signals are then sent to users in a

variety of ways. The wide area augmentation

system (WAAS) was developed specifically for

aviation users and is intended to enable GPS to be

used in airspace that requires high integrity,

availability and accuracy. WAAS improves a

GPS signal accuracy of 20 metres to

approximately 1.5 metres (typical) in both the

horizontal and vertical dimensions. WAAS is

based on a network of reference stations around

the world that monitors GPS signals and

compares them against the known position of the

reference stations. These reference stations

collect, process and transmit this data to a master

station. Updated data is then sent from the master

station via an uplink transmitter to one of two

geostationary satellites; the aircraft receiver

compares this with GPS data and messages are

sent to the crew if the GPS signal is unreliable.