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This differential GNSS broadcast 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.
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Parameter
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Number of bits
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Description
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Message ID
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6
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Identifier for Message 17; always 17
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Repeat indicator
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2
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Used by the repeater to indicate how many times a message has been repeated. 0-3; 0 = default; 3 = do not repeat any more
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Source ID
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30
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MMSI of the base station
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Spare
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2
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Spare. Should be set to zero. Reserved for future use
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Longitude
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18
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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°
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Latitude
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17
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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°
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Spare
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5
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Not used. Should be set to zero. Reserved for future use
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Data
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0-736
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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
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Number of bits
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80-816
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80 bits: assumes N = 0; 816 bits: assumes N = 29 (maximum value)
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Differential Correction Data Table
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Parameter
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Number of bits
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Description
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Message type
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6
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Recommendation ITU-R M.823
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Station ID
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10
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Recommendation ITU-R M.823 station identifier
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Z count
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13
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Time value in 0.6 s (0-3 599.4)
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Sequence number
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3
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Message sequence number (cyclic 0-7)
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N
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5
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Number of DGNSS data words following the two-word header, up to a maximum of 29
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Health
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3
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Reference station health (specified in Recommendation ITU-R M.823)
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DGNSS data word
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N = 24
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DGNSS message data words excluding parity
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Number of bits
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736
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Assuming N = 29 (the maximum value)
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Notes:
– 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.
– 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.
– 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.
Source: Rec. ITU-R M.1371-5)
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