MX575A/B DGPS Compass
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1.1 MX575 D/GPS Beacon Operation
The MX575 D/GPS Satellite Compass is an integrated GPS and beacon receiver
unit that is fully automatic and usually does not require user intervention in most
cases. It will automatically search for available GPS satellites and beacon
differential stations as soon as power is applied.
The internal 2-channel beacon receiver performs a database search for beacon
stations that cover its present position. The first channel tracks the nearest
beacon station frequency while the second channel searches for and collects
beacon almanac data for other available beacons stations. Should it find another
station that is closer, it will automatically switch the primary channel to the new
station.
The MX575 D/GPS automatically selects the closest beacon station or can be
controlled through the MX420 CDU to manually select a specific beacon
frequency.
1.2 Satellite Based Augmentation System (SBAS)
When SBAS is available, the MX575 can be controlled (using the CDU) to track
the Satellite Based Augmentation Systems (SBAS) like the WAAS (US), EGNOS
(European) and MSAS (Japan) satellites. These geostationary satellites transmit
DGPS correction data (just like the Coast Guard beacon stations) using the GPS
frequency.
1.3 Receiver Autonomous Integrity Monitoring (RAIM)
RAIM is a software algorithm in the MX575 program that monitors the GPS system
error constantly. When the GPS system error exceeds a predetermined value (100
meters), the RAIM feature sends an alarm code to the MX CDU to sound a RAIM
unsafe alarm. This feature requires at least five or more GPS satellites to operate
properly. If the position solution error exceeds a preset limit a “RAIM Unsafe (R-)”
or “RAIM Caution (R?)” alarm will be indicated in the MX CDU. This means that the
accuracy of the position cannot be guaranteed at that point in time. The operator is
advised to use the GPS position with caution until the RAIM indicator switches to
(R+) denoting safe RAIM condition. Position errors may be caused by unhealthy
