MPC User Manual Rev 0D 95
Appendix C WAAS Overview
The Wide Area Augmentation System (WAAS) is a safety-critical system that provides a quality of
positioning information previously unavailable. The WAAS improves the accuracy, integrity, and
availability of the basic GPS sig nals. In the future, the wide area of coverage for this system will
include the entire United States and some outlyin g areas. At th e tim e of publication, there are two
WAAS satellites over the western Atlantic Ocean and the Pacific (PRN 122 and PRN 134
respectively) and one EGNOS satellite over the eastern Atlantic Ocean (PRN 120). WAAS data is
available from any of these satellites and more satellites will be available in the future.
The primary functions of WAAS include:
•data collection
•determining ionospheric corrections
•determining satellite orbits
•determining satellite clock corrections
•determining satellite integrity
•independent data verification
•WAAS message broadcast and ranging
•system operations & maintenance
As shown in Figure on Page 96, the WAAS is made up of a series of Wide Area Reference Stations,
Wide Area Master Stations, Ground Uplink Stations and Geostationary Satellites (GEOs). The Wide
Area Reference Stations, which are geographically distributed, pick up GPS satellite data and route it
to the Wide Area Master Stations where wide area corrections are generated. These corrections are
sent to t he Ground Uplink Stations which up-link them to the GEOs for re-transmission on the GPS
L1 frequency. These GEOs transmit signals which carry accuracy and integrity messages, and which
also provide additional ranging signals for added availability, continuity and accuracy. These GEO
signals are available over a wide area and can be received and processed by OEM4 family GPS
receivers with appr opriate firmware. GPS user receivers are thus able to receive WAAS data in-band
and use not only dif ferential corrections , but also integrity, residual errors and ionospheric information
for each monitored s atellite.
The signal broadcast via the WAAS GEOs to the WAAS users is designed to minimize modifications
to standard GPS receivers. As such, the GPS L1 frequency (1575.42 MHz) is used, together with
GPS-type modulation - e.g. a Coarse/Acquisition (C/A) pseudorandom (PRN) code. In addition, the
code phase timing is maintained close to GPS time to provide a ranging capability.