User's Guide

provide corrections to each rover that are more

accurate than corrections from a single base station.

To use the VRS corrections, the rover sends its position

to the VRS server. The VRS server uses the reference

station data to model systematic errors (such as

ionospheric delay) at the rover position. It then sends

RTCM or CMR correction messages back to the rover.

WAAS

Wide Area Augmentation System. WAAS was

established by the Federal Aviation Administration

(FAA) for flight and approach navigation for civil

aviation. WAAS improves the accuracy and availability

of the basic GPS signals over its coverage area, which

includes the continental United States and outlying

parts of Canada and Mexico.

The WAAS system provides correction data for visible

satellites. Corrections are computed from ground

station observations and then uploaded to two

geostationary satellites. This data is then broadcast on

the L1 frequency and is tracked using a channel on the

GPS receiver, exactly like a GPS satellite.

Use WAAS when other correction sources are

unavailable, to obtain greater accuracy than

autonomous positions. For more information on WAAS,

refer to the FAA website at http://gps.faa.gov.

The EGNOS service is the European equivalent, and

MSAS is the Japanese equivalent of WAAS.