Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard
DGPS Station Selection
Table of Content
The latency of a pseudorange correction increases and the position accuracy consequently degrades, when there is a weak and less reliable DGPS signal. This will usually occur when a vessel navigates beyond the edge of an advertised DGPS coverage. Another error contribution due to spatial decorrelation, becomes pronounced when a broadcast emanating from a DGPS Station more than 260 nautical miles from the vessel is used. The user should be aware of these factors when using a broadcast from a DGPS Station.
An unhealthy broadcast should not be used under any circumstances.
The user equipment suite will encounter three DGPS station selection scenarios in Canadian waters:
- user is within the advertised coverage of one CG designated DGPS Station
- user is within the advertised coverage of two or more CG designated DGPS Stations
- user is outside the advertised coverage of any DGPS Station
In the first scenario, the user equipment suite should use the broadcast from the designated DGPS station and continually monitor its health and various parameters. If the broadcast becomes unhealthy, the user equipment should follow the guidelines described below.
The second scenario occurs in a multiple coverage area. In a limited number of locations where several broadcasts are available, the closest one may not necessarily be the one with the highest received power. The closest station, provided it is healthy, should be used even if its signal strength is low relative to other received signals. The alternate designated DGPS Station(s) will be selected in the event that the broadcast from the closest station becomes unhealthy or unmonitored. If the alternate designated DGPS Station also becomes unhealthy, it should attempt to check out the status of the “faulty” designated DGPS Station(s) before attempting to scan for a satisfactory broadcast from a non-designated DGPS Station, as in the guidelines described below.
In the third scenario where there are no designated DGPS Stations, the user equipment suite should use a satisfactory broadcast from the nearest DGPS Station, subject to the applicable constraints in the guidelines described below.
The following guidelines should be used in seeking a satisfactory broadcast:
- scan for and use a satisfactory broadcast from the nearest non-designated DGPS Station;
- a satisfactory broadcast emanating from a DGPS Station more than 260 nautical miles away should be used with caution; and
- an unmonitored broadcast of any DGPS Station should be used with caution, only after the procedures described in 1) and 2) have been attempted unsuccessfully.
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