DGPS (Differential Global Positioning System) data is mainly transmitted in the low frequency band e.g., 285 - 315 kHz. It is a correction signal for GPS receivers, and is used to increase the accuracy of the satellite-based GPS signal. DGPS is based on the transmission of correction data by a reference station, the position of which has been determined with high accuracy by traditional position-finding measurements. With the correction data, an absolute accuracy to within 4 meters can be achieved.

Transmissions use MSK (Minimum Shift Keying) with speeds of 100 or 200 baud.

DGPS has been implemented according to RTCM SC-104 "Recommended Standards for Differential Navstar DGPS Service" v.2.0, Jan. 1990.

•      These messages are decoded: 1, 3, 7, 9, 13, 16.

•      These messages are NOT decoded, but their type is indicated: 2, 4, 5, 6, 8, 10, 11, 12, 14, 15, 17, 60-64.

•      Compared to M.823-2 messages 4 and 5 are not decoded by my implementation neither are the equivalent GLONASS.

DGPS data, which is formatted according to RTCM v.2.0 or 2.1, is continuously transmitted in frames consisting of a varying number of data words. The two first words of each frame contain the reference station id, the message type, a sequence number, the frame length and the health of data. A data word has a length of 30 bits: 24 data bits and 6 parity bits. The last two bits of a word are used as an EXOR function for selected bits of the succeeding data word. The value of the last bit indicates whether the next data word is sent with inverse or normal polarity.

If Normal 3, 5, 7, 16 is chosen in the Message Type options field, messages containing ASCII text are decoded, in case of numbers their meaning is displayed as well. The message types 1, 6 and 9 containing the real DGPS information are not displayed in this mode of operation. Selecting Raw excl. 1, 6, 9 offers a raw bit display except of the message types 1, 6 and 9 and Diff. corrections displays the corrections messages. By selecting the All frame headers option, all frame headers are displayed regardless of the message type. In case of a parity-error the data transferred in the related data word are displayed in red characters to indicate a potential error.

RTCM v.2.0 and 2.1 are not completely compatible, but both systems are used. This may lead to erroneous interpretation of certain frame types.

More detailed information may be found in "RTCM Recommended Standards for Differential NAVSTAR GPS Service 2.0" (RTCM paper 134-89/SC104-68).