Differential Global Positioning System (DGPS)
Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard
The position accuracy of the DGPS Service will be 10 meters (95% of the time), or better in all specified coverage areas for suitable user equipment (assuming the full 24 GPS satellite constellation and a HDOP < 2.3). As the DGPS Reference Station surveyed antenna position is referenced to the NAD 83 Coordinate System, the user's differentially determined position solution is inherently transformed into the NAD 83 Coordinate System. The user equipment suite need not perform any datum conversion from WGS 84 when working with NAD 83 Charts within the service area.
A satisfactory DGPS broadcast is defined as one that is
- the PRC time out limit for at least four satellites has not been reached, and
- the DGPS Station ID number checks out against that in the beacon almanac.
A user is primarily concerned with being able to receive a satisfactory DGPS broadcast with minimal disruption. Known as user availability, this parameter is a function of three components:
- the reliability of the DGPS station;
- the effect of atmospheric noise preventing user equipment from receiving an otherwise healthy DGPS broadcast.
- whether a user is in a standard/enhanced coverage or in a multiple coverage area.
The first component of user availability depends on the reliability of the entire set of DGPS broadcast station equipment (see Figure 2). Known as broadcast reliability, it is specified to be at least 99.7 % (see the DEFINITIONS Section for an elaboration of this term.). A user may view this reliability as the probability that the DGPS broadcast is providing healthy DGPS corrections at a specified power when a user selects that particular broadcast.
The second component of user availability is called broadcast availability. As atmospheric noise varies over time and region, modeling to derive signal availability's at the various DGPS broadcast station sites using CCIR noise figures is used. This process also estimates the required effective radiated power for each radiobeacon transmitter and the coverage region in which this availability is assured (see DEFINITIONS Section for details and related paragraph for definition of broadcast coverage). Broadcast availability will be at least 99% at the edge of the advertised coverage area for a DGPS station.
The third component will provide the best service availability since service will still be available from an alternate DGPS station in the event of a total DGPS station failure.
Prolonged empirical DGPS data collection and operational experience will be needed in order to arrive at an accurate user availability figure.
² A DGPS broadcast is considered to be healthy if it meets the following requirements:
- The protection limit has not been exceeded.
- The broadcast is monitored.
- The RS receives a correct RS ID
- The IM position error is within the position threshold
Redundancy of equipment is provided in order to attain the 99.7 % equipment reliability (see Figure 4). Figure 4 is a representative DGPS Station configuration.
Figure 4: Block Diagram of a DGPS Station
The Reference Station (RS) and Integrity Monitor (IM) operate autonomously. In the event of a RS or IM malfunction, the redundant equipment should come on line either through the control station (CS) or by commands from the control monitor (CM). Restoration of DGPS signal transmission in the latter case will take longer as it requires operator instructions.
DGPS coverage performance is adversely affected by atmospheric and man-made noise that falls within the radiobeacon's bandwidth. Experience has shown that the noise level in inland areas is considerably higher than in offshore areas. Therefore, two levels of advertised coverage are provided:
Offshore Coverage: The service area of each DGPS broadcast will be defined as the area within which the DGPS signal strength is at least 75 µv/m or signal availability is at least 99% in average yearly noise conditions, whichever is the more stringent criteria.
Inland Coverage: The service area of each DGPS broadcast will be defined as the area within which the DGPS signal strength is at least 100 µv/m or signal availability is at least 99% in average yearly noise conditions, whichever is the more stringent criteria.
DGPS signal levels may be reduced locally within an advertised coverage area because of obstructive terrain such as fiords and bridges. As long as the receiver continues to track and properly decode the DGPS correction data in the reduced signal, the user receiver position output will not be adversely affected.
All Canadian radio beacons broadcast at 200 bits per second, regardless of whether they provide inland or offshore coverage.
Overlapping coverage from adjacent Canadian and American radiobeacons is available in many areas.
See the Fisheries and Oceans publications (DFO 5470 and 5471) - Radio Aids to Marine Documentation (RAMN) for details of the DGPS Coverage in Canada.
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