Global navigation-satellite system broadcast binary message

msg_ID: 
17
Description: 
DGNSS corrections provided by a base station
Priority: 
2
Access scheme: 
FATDMA
RATDMA
Communication state: 
N/A
Transmitted by: 
Base station

This message should be transmitted by a base station, which is connected to a DGNSS reference source, and configured to provide DGNSS data to receiving stations. The contents of the data should be in accordance with Recommendation ITU-R M.823, excluding preamble and parity formatting.
TABLE 68

Parameter Number of bits Description
Message ID 6 Identifier for Message 17; always 17
Repeat indicator 2 Used by the repeater to indicate how many times a message has been repeated. See § 4.6.1, Annex 2; 0-3; 0 = default; 3 = do not repeat any more
Source ID 30 MMSI of the base station
Spare 2 Spare. Should be set to zero. Reserved for future use
Longitude 18 Surveyed longitude of DGNSS reference station in 1/10 min (±180°, East = positive, West = negative). If interrogated and differential correction service not available, the longitude should be set to 181°
Latitude 17 Surveyed latitude of DGNSS reference station in 1/10 min (±90°, North = positive, South = negative). If interrogated and differential correction service not available, the latitude should be set to 91°
Spare 5 Not used. Should be set to zero. Reserved for future use
Data 0-736 Differential correction data (see below). If interrogated and differential correction service not available, the data field should remain empty (zero bits). This should be interpreted by the recipient as DGNSS data words set to zero
Number of bits 80-816 80 bits: assumes N = 0; 816 bits: assumes N = 29 (maximum value); see Table 69

The differential correction data section should be organized as listed below:
TABLE 69

Parameter Number of bits Description
Message type 6 Recommendation ITU-R M.823
Station ID 10 Recommendation ITU-R M.823 station identifier
Z count 13 Time value in 0.6 s (0-3 599.4)
Sequence number 3 Message sequence number (cyclic 0-7)
N 5 Number of DGNSS data words following the two word header, up to a maximum of 29
Health 3 Reference station health (specified in Recommendation ITU‑R M.823)
DGNSS data word N = 24 DGNSS message data words excluding parity
Number of bits 736 Assuming N = 29 (the maximum value)
NOTE 1 – It is necessary to restore preamble and parity in accordance with Recommendation ITU‑R M.823 before using this message to differentially correct GNSS positions to DGNSS positions.
NOTE 2 – Where DGNSS corrections are received from multiple sources, the DGNSS corrections from the nearest DGNSS reference station should be used taking into account the Z count, and the health of the DGNSS reference station.
NOTE 3 – Transmissions of Message 17 by base stations should take into account ageing, update rate and the resulting accuracy of the DGNSS service. Because of the resulting effects of VDL channel loading, the transmission of Message 17 should be no more than necessary to provide the necessary DGNSS service accuracy.