Differential Global Positioning System (DGPS)
Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard
User Equipment Capabilities
In order to ensure end-to-end system integrity, the user equipment suite should have the following capabilities:
- receive and process all the various types of messages broadcast by the DGPS station as described in Section 2.
- receive and process all the various messages at 200 bps, 100 bps and 50 bps.
- receive and process the various messages arising from out of tolerance and alarm conditions detected at the broadcast station site as described above;
- alert the user for pseudo range level alarms if an inadequate satellite constellation exists at that time in that user location.
- alert the user when the protection limit is exceeded (see paragraph 4.1 for details);
- select automatically the appropriate radio beacon with priority given to proximity first and signal strength second.
- have a minimum of nine parallel channels to be used for GPS reception (recommended for navigation in restricted waterways); otherwise a minimum five channel receiver should be used for normal navigation;
- combine the UDRE values with localized error factors such as user receiver noise, interference, multipath, HDOP, and PRC latency in order to provide a confidence level about the user's displayed position;
- detect the absence of RTCM messages containing pseudo range corrections in the data stream and if available tune to a different marine radio beacon in advance of the “PRC Time Out Limit”. The broadcast of alternating ones and zeros should not cause any false acquisitions since the subject broadcast will be listed as unhealthy by the Type 7 Message;
- display a textual message based on information in the header of any broadcast DGPS type message concerning unhealthy or unmonitored conditions existing at the reference station. Additionally, unhealthy or unmonitored conditions should cause a visual alarm to activate;
- Type 16 messages should also be treated in the same manner as in j);
- if a marine radiobeacon is utilized beyond 260 nautical miles the user equipment suite should display this condition in order to indicate that additional unaccounted for error components are present. The use of the broadcast signal beyond its specified range is further discussed in Section 7;
- retain the ability to process Type 9-1 messages at 50 bps rate in the event that SA is withdrawn permanently;
- retain the ability to process Type 1 messages in the event that SA is withdrawn permanently;
- discard all pseudo range corrections from the previous broadcast when switching broadcasts before utilizing any pseudorange corrections from the new broadcast
- immediately stop applying any PRC derived information for a satellite until the alarm condition ends, when any pseudorange alarm messages are received from the DGPS station. This is accomplished by the setting of the PRC (to) field to a value of binary 1000 0000 0000 0000 and the RRC field to a value of 1000 0000;
- alert the user of positional alarms generated by the DGPS station due to a lack of healthy pseudo ranges because of insufficient satellites or a failure of the pseudo range weighting or monitoring functions. These conditions are indicated by the message header which allows the broadcast of an alarm without breaking frame synchronization;
- alert the user of an unmonitored condition alarm (e.g. IM failure). This is indicated by the message header and will generally occur for duration's of only several minutes. During this time the redundant IM is used to perform an initial assessment of the broadcast before the status of the system returns to the monitored condition. As the ‘hot’ and standby reference stations usually maintain a time base to within 15 ns of each other, the IM may be able verify the broadcast health status for the new reference station in a few seconds. However, if both IMs malfunction, the unmonitored condition can last for a prolonged period;
- stop using the pseudorange correction if its age exceeds 30 seconds since this PRC should not be applied to the user's navigation solution. When Type 9 3 Messages are broadcast at 200 bps the user would have to miss nine consecutive updates before the time out limit is reached for a given pseudo range; and
- exit the differential navigation mode and revert to GPS if there are insufficient satellites with valid pseudorange corrections
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