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Differential Global Positioning System (DGPS)
Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard
Glossary
Acronyms
bps: bits per second
CM: control monitor
CS: control station
dB: decibel
DGPS: Differential Global Positioning System
GPS: Global Positioning System
HDOP: Horizontal Dilution of Precision
Hz: Hertz
IM: Integrity Monitor
IOD: Issue of Data
KHz: Kilo Hertz
m: metre
MF: Medium Frequency
MSK: Minimum Shift Keying
NAD 83: North American Datum of 1983
nm: nautical mile
ns: nano second
PR: pseudorange
PRC: Pseudorange Correction
RRC: Range Rate Correction
RS: Reference Station
RTCM: Radio Technical Commission for Maritime Services
SNR: Signal to Noise Ratio
SPS: Standard Positioning Service
µV/m: Micro Volt/metre
UDRE: User Differential Range Error
Definitions
Accuracy:
Absolute accuracy is defined as the expected maximum error in the geographical position as computed by the DGPS user equipment within some specified statistical limit. For DGPS systems the limit is usually the horizontal two dimensional error measure called 2 drms (twice the root mean square error). For the Canadian DGPS system, the error limit is 95%, which is the minimum 2 drms value for bivariate normal error distribution. The position accuracy of the DGPS Service will be 10 meters, 95% of the time; or better in all specified coverage areas (assuming the full 24 GPS satellite constellation and a HDOP < 2.3).
Availability broadcast:
The percentage of time during which a proper DGPS broadcast (i.e. healthy and at specified signal power) can provide at a specified location, a sufficient signal-to-noise level to enable good quality user equipment to detect and demodulate the signal.
Availability user:
The percentage of time in which a DGPS signal is available to a user at any given part of an coverage area which allows the position accuracy specification to be met. It is a product of broadcast availability and broadcast reliability.
Broadcast Coverage:
The area where a user can expect DGPS service provided by a particular DGPS station. It has a limit defined either by a specified signal level (e.g. 75 or 100 μv/m) or a specified broadcast availability (normally 99%); whichever is more stringent. For a DGPS station, this is called the advertised coverage area.
Broadcast Reliability:
It is a function of the expected failure rate i.e. mean time between failure (MTBF) of the DGPS and transmitter equipment at a site and the time to repair the failure i.e. mean time to repair (MTTR). In statistical terms:
Reliability = MTBF / ( MTBF + MTTR )
Broadcast Reliability can also be expressed as the probability of a healthy broadcast being on the air at specified power when a user randomly selects it.
Data Rate: The number of information bits per second that are broadcast.
Datum: A geodetic coordinate system that is specific to a given geographical region.
Integrity: The ability of a system to provide timely warnings to users when it should not be used for navigation and also to verify the validity of the DGPS broadcast.
Latency: The difference between the time at which the first bit of a given message is broadcast and the time tag in the header of the pseudo range correction messages. The time tag in the message header is the Z Count that is closest to the time of last measurement upon which a correction is based. Latency is specified as an average in order to take into account the difference between the Z Count and the time of measurement that can be up to 0.6 seconds.
Protection Limit: The user position error as measured by an IM, which shall not be exceeded for a specified interval without the broadcast of an alarm.
Time to Alarm: The maximum allowable time between the appearance of an error outside the protection limit at the integrity monitor and the broadcast of the alarm.
Transmission Rate: The total number of bits per second that are broadcast.
UDRE: A one sigma estimate of the pseudo range correction error due to ambient noise and residual multipath.
Unhealthy: Unable to operate within tolerance.
Unmonitored: Not monitored by an integrity monitor (IM)
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