GW 100 Baud FSK

This mode is one of several used by a commercial communications company to contact ships at sea and to send data to them. The shore stations transmit "channel free markers" which are short bursts of 100 baud FSK every few seconds. In addition it appears that contact between ships and shore stations is initiated using 100 baud FSK then if the link is good enough the actual data is sent using the higher speed PSK or OFDM modulation types. It appears that if the link is to poor for those modes to be reliable then only then is the data sent using 100 baud FSK.

GW data transmissions use two frequencies. One is used by the shore station to transmit data (the ships listen to this frequency) and other by the ships to transmit. The shore side frequency is easy to recognise as the shore station transmits what are called free channel marker transmissions every few seconds. These look like this when decoded by Rivet ..

GW Free Channel Marker from Station Code 0x9c (VCS, Halifax, Canada) 20 38 a3 1f 6c 35 3b 4b f2 f2 f2 9c 9c 9c 9c 9c 9c ff

From time to time you will see shore stations calling ships to tell them that there is traffic for them. The transmissions look like this ..

18:46:48 GW Type=5 Count=0 Subtype=41 (10010100101001) (0x9d,0xfa,0xfa,0xcc,0x9a,0xf9)
Traffic For : MMSI 235050665 (LISA C-MVOL5,Great Britain)
Command=330

Notice how in this case Rivet recognises the ships name. This is because this ships details are in the ships.xml file. This is an add on file for Rivet which can be downloaded from here. To use it download the file and put it into the same folder as Rivet.

Rivet can also decode interesting activity on ships GW channels although these are harder to find. The trick is to find an active GW shore side channel transmitting the free channel markers then to tune to its associated ship frequency. You will hear less traffic on these channels since the ships transmit a signal which is much less powerful than the shore side transmitters. However every so often you will see a ship transmit its position such as this ..

13:25:01 GW Type=5 Count=0 Subtype=41 (10010100101001) (0xfb,0xeb,0x66,0x66,0x1e,0x39)
Traffic for : 545800000
Command=775
13:25:02 GW Type=2 Count=1 Subtype=101 (10001011100101) (0x84,0x63,0x22,0x32,0x66,0x66)
MMSI : 235011150 (ANJA C-MMTT9,Great Britain)
13:25:02 GW Type=5 Count=0 Subtype=102 (10010101100110) (0x78,0x78,0x78,0x78,0x78,0x78)
<<<<<<
13:25:02 GW Type=5 Count=1 Subtype=118 (10010111110110) (0x20,0x20,0x3e,0x4e,0x78,0x78)
11<<<<
$GLOBE,203610.00,A,4927.7702,N,00019.0063,E,000.0,000.0,250113,04.8,W,N*11<<<<
13:25:02 GW Type=2 Count=0 Subtype=106 (10001001101010) (0xf8,0x78,0x78,0x78,0x78,0x78)
<<<<<<

To learn more about the GW network then this blog post is well worth a read as is this on how GW encodes MMSIs in packets.