To learn more about these sentences I captured the NMEA-0183 output of a modern digital selective calling
radio which had just received a call and was transmitting data from the call to a chart plotter via NMEA-0183.
The call was from a vessel with the following characteristics
MMSI = 338040079
Latitude = 42-degrees 31.456-minutes North
Longitude = 081-degrees 12.451-minutes West
The call was sent about 4:21 PM EDST.
The
radio sent the following two NMEA-0183 sentences to the chart plotter:
$CDDSC,20,3380400790,00,21,26,1423108312,2021,,,B, E*73
$CDDSE,1,1,A,3380400790,00,46504437*15
The first sentence is NMEA-0183
DSC; the second is DSE.
Let's look at the structure of the DSC data. It is believed to contain the MMSI of the sender, and the position of the sender to whole minute resolution:
$CDDSC,20,3380400790,00,21,26,1423108312,2021,,,B, E*73
The code
$ is the start delimiter, marking the beginning of the data packet. Other elements of the packet are separated by commas.
The code
CD is the Talker ID. This indicates the talker is a
Communication device of the type
Digital selective calling.
The code DSC identifies the type of packet. I don't know the formal name for this in
NMEA documentation.
The code
3380400790 is the
marine mobile
service identity of the station that sent the data to the radio, with an extra "0" appended.
The code
00 is unknown to me.
The code
21 is unknown to me.
The code
26 is unknown to me.
The code
1423108312 appears to contain the latitude and longitude to a resolution of whole minutes, with a "1" prefixed. The latitude is 42-degrees 31-minutes and the longitude is 083-degrees 12-minutes. The "1" appears to be an indicator perhaps of the N-S nature of the latitude or perhaps the quadrant.
The code
2021 appears to be the time of the message in GMT. (This was sent about 1621 local time, keeping GMT -4, so GMT time would be 2021.)
The code
,,, is unknown to me; it appears to represent missing parameters that were not sent.
The code
S is unknown to me. Perhaps it means the position was obtained from a
satellite signal source, such as
GPS.
The code
E*67 appears to be a check sum to allow the packet to be checked for accuracy by parsing of all the elements.
Now we look at the DSE packet. This is believed to contain expansion data for the position. The code is
$CDDSE,1,1,A,3380400790,00,46504437*15
Again the code
$CDDSE is the start delimeter, talker type, and packet name.
The code sequence
1,1,A, may be as follows: the first "1" is the number of packets that are contained in this expansion message in total, the second "1" is the number of this packet in the sequence, and the "A" may be some further indcator of type of message.
The code
3380400790 is the
marine mobile
service identity of the station sending the data, with a zero appended.
The code
00 is unknown to me.
The code
46504437 appears to contain the decimal minutes of the latitude and longitude in the other packet. The 4560 is the decimal minutes of longitude to 0.0001, and the 4437 is the decimal minutes of longitude to 0.0001, both to a ten-thousandths of a minute.
The code
*15 appears to be a check sum to allow the packet to be checked for accuracy by parsing of all the elements.
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