What are the Technical Characteristics of DGPS?
DGPS is provided from radiobeacons by modulating a second
carrier with the correction data and other information on the
system:
- the modulation is Minimum Shift Keying (MSK), a special form of Frequency Shift Keying (FSK)
- the modulation rate is usually 100 or 200 bits per second
- in Europe the second carrier is 500Hz away from the radiobeacon direction-finding signal and the identification code is included in the data
- DGPS receivers should comply with the International Electrotechnical Commission (IEC) Standard 1108-4 (BS EN 61108-4) and may be combined with the GPS receiver; the messages provided include information about the health of the reference station and the transmitter.
The International Telecommunications Union (ITU)
Recommendation M.823 incorporates the RTCM SC-104 protocol and the
International Association of Lighthouse Authorities has adopted
these standards for use with maritime radiobeacons to transmit DGPS
corrections. The radiobeacon system uses the LF/MF band 283.5 - 315
kHz in Europe (285 - 325 kHz in other parts of the world). The
Radio Regulations governing the use of this band allow for
supplementary information to be transmitted - such as DGPS
signals.
A DGPS display onboard ship
The GLAs' DGPS is Year 2000 compliant, but no guarantee can be given concerning GPS which may also be susceptible to the effects of solar wind activity which is a cyclic phenomenon.
A DGPS display onboard ship
The GLAs' DGPS is Year 2000 compliant, but no guarantee can be given concerning GPS which may also be susceptible to the effects of solar wind activity which is a cyclic phenomenon.