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What are the prerequisites for an AIS transceiver to transmit?
05-04-20, 09:14 PM
Post: #1
What are the prerequisites for an AIS transceiver to transmit?
There are a number of requirements that have to be met in order for an AIS transceiver to transmit position data to other vessels:

  1. The AIS must have an MMSI number programmed in
  2. The Silent Mode override wires on the power cable not connected together (orange and light green on Raymarine AIS650 and 700)
  3. Silent Mode not enabled by PC software over USB or your MFD
  4. The AIS must have a GPS position fix from its own dedicated antenna (external NMEA2000 or NMEA0183 GPS is not sufficient; the internal receiver is RAIM-compliant and this is required by the governing standards)
  5. The AIS must have been able to measure a reasonable VSWR from the connected radio antenna
  6. The specified transmission interval between messages must have elapsed. This is different for different AIS message types (static vessel data, position reports etc.) and for different vessel speeds. For example, static vessel data (message 24) is sent as two messages, 1 minute apart, every 6 minutes, and both must be received by the other station in order for it to be able to display that vessel static data.

On antenna VSWR, it's worth pointing out that:
  • just because an antenna had a good VSWR when installed, that doesn't mean it still is
  • just because the manufacturer's documentation specifies a certain VSWR, that doesn't mean that the particular antenna measures up to that specification
  • the installed connectors and cabling change the VSWR, so whilst the antenna in isolation might have a good VSWR, once you connect up to other cabling run through the boat that may no longer be the case
  • VSWR is a function of frequency and many VHF antennas are quite narrow-band. A manufacturer's specification that says it has a VSWR of 2:1 might well mean that it has 2:1 VSWR at the VHF channel 16 frequency of 156.8MHz, but it can have a much higher VSWR even at the ends of the VHF band, let alone at the AIS bands at around 162MHz. If you are using an antenna dedicated to the AIS, it's better to choose an AIS-tuned antenna that has its best VSWR at that 162MHz frequency. To verify that an antenna is good for AIS, its VSWR needs to be measured at 162MHz.


Raymarine since 1999.
Interests: Diagnosis of problems in sonar/fishfinders, NMEA2000, ethernet comms, autopilots, thermal cameras
Location: Sydney, Australia.

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