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Differential Global Positioning System (DGPS)
Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard
System Operations
System Integrity
DGPS System Failure / Default Mode
System Integrity
System Integrity depends on the ability of:
- the DGPS station to provide a satisfactory broadcast;
- the system to alert the user of any out of tolerance or unhealthy conditions in the DGPS corrections; and
- the user equipment suite to process the DGPS alarms
The reference station is augmented by a collocated integrity monitor (IM). The IM verifies the accuracy of the broadcast PRCs at the pseudorange and positional levels. The process utilized by the IM is shown in Figure 5. In essence, by knowing its own surveyed (GPS antenna) location, the IM is able to assess whether the broadcast PRCs and derived DGPS positions are within certain specified limits, and if not, will alert the 24 hour staffed control monitor (CM) and the user. As part of augmenting system integrity, the DGPS station sends routine station status messages every half hour and any alarm messages to the CM.
Figure 5 Integrity Monitor Process
The integrity monitor (IM) performs integrity verification of pseudorange and positional accuracies, and takes appropriate action as described below:
- if the pseudorange residual is high, a pseudorange alarm is broadcast by the setting of the PRC (to) Field to a value of binary 1000 0000 0000 0000 along with the setting of the RRC Field to a value of 1000 0000. Should the user equipment suite detect either one of these settings it should immediately stop applying any PRC derived information for that satellite until the alarm condition ends. This broadcast will occur for a maximum thirty second period.
- for positional verification, a protection limit for the over determined (position) solution shall be set at the IM to warn the user in the event that the accuracy is out of specified tolerance. The IM utilizes the broadcast UDRE values to weight the pseudoranges in computing the over determined solution. The known position of the IM is not included in the over determined solution that it computes. If the set value is exceeded after a defined interval, the broadcast will become unhealthy. The time from when a protection limit is exceeded to when the user equipment suite/user is alarmed by the broadcast will not exceed ten seconds. This time includes the length of the longest possible message and the header of the following message. The use of the Type 9 message as the exclusive PRC message results in these low time to alarm values.
DGPS System Failure / Default Mode
A system failure is said to occur when any one of the following conditions apply:
- unhealthy broadcast;
- absence of pseudo range corrections in message transmission;
- unmonitored broadcast (e.g. integrity monitor failure)
- no broadcast
Should any of the above mentioned prolonged system failure conditions occur, appropriate action will be taken by the Operator to alert the mariner. If a radio beacon is identified as being “unhealthy” or “unmonitored” over a prolonged period, this condition will be broadcast to the user as an automatic Type 16.
An RTCM Type 6 message will be broadcast when the reference station has no other message to send e.g. no Pseudo range corrections.
The Almanac of a defective station will be automatically updated upon instant recognition of an “unmonitored” or “unhealthy” indication. The updating of the adjacent station almanacs will be provided when full networking capability is implemented.
Reference station modulator failures may result in the broadcast of alternating ones and zeros, a single carrier tone, or no output at all.
If an out of tolerance condition (e.g. station is “off the air”) is expected to continue for more than two hours or as advised by Marine Aids Superintendent, a “Notice To Mariners” and/or “Notice to Shipping” will be issued as appropriate.
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