How It Works - Raymarine Raychart 425 Owner's Handbook Manual

Chapter 1: Overview
The combination of the WAAS, EGNOS and MSAS systems will
provide global satellite based differential GPS augmentation into the
future.

How it Works

The following description is based on WAAS, but the principles apply
equally to the EGNOS and MSAS systems.
WAAS comprises the following components:
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The Ground Reference Stations are located at known positions and
receive data continuously from GPS. The Ground Reference Stations
send their data to the Master Stations which calculate the error of the
GPS-received positions and generate correctional data.
The corrected 'differential' signals are then sent to the two
Geostationary Satellites which broadcast the corrected data on the
standard GPS frequency, making it available to the user's Raystar 120
GPS Receiver.
The Raystar 120 GPS Receiver uses the correctional data transmitted by
the Geostationary Satellites to refine the GPS positional data for greater
accuracy.
WAAS is shown pictorially in Figure 1-2.
Ground Reference Stations across the USA
Master Stations located at East Coast and West Coast
Geostationary Satellites located above the equator
Figure 1-2: The WAAS System
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