Canadian Marine Differential Global Positioning System (DGPS) Broadcast Standard

Table of Content


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)
Date modified: