What are the Technical Characteristics of DGPS?

DGPS is provided from radiobeacons by modulating the 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
. 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 full description of the technical characteristics of the system can be found in ITU-R Recommendation M.823 (1996).