AIS Target Position Latency

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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AIS Target Position Latency

Postby jimh » Tue Mar 13, 2018 3:11 pm

In the Automatic Identification System (AIS) a vessel transmits its position at intervals that depend on two criteria:

  1. the category of AIS transmitter being used, according to Class A, Class-B Self-organizing, and Class-B Carrier Sensing
  2. the speed of the vessel

The vessels to report most frequently (every two seconds) are Class-A transmitters with a vessel speed greater than 23-nautical-miles-per-hour. The vessels to report least frequently (every three minutes) are Class-A transmitters at anchor or moving not faster than 3-nautical-miles-per-hour.

We can compute the position latency by distance = speed × time:

Least latency:
23-nautical-miles/1-hour × 2-seconds/(3600-seconds/1-hour)= 0.0127-nautical-miles
= 77.7-feet position latency


Most latency:
3-nautical-miles/1-hour × 180-seconds/(3600-seconds/1-hour)= 0.15-nautical-miles
= 911-feet position latency


For a recreational boat with a Class-B Carrier Sensing AIS transmitter and moving at a speed faster than 2-nautical-miles-per-hour, the transmitter interval is 30-seconds. If the boat is moving at 30-MPH, the position latency would be

30-Miles/1-hour × 30-seconds/(3600-seconds/1-hour)= 0.25-miles = 1,320-feet

For a comprehensive and authoritative list of vessel reporting intervals, see TABLE 1 and TABLE 2 in

Recommendation ITU-R M.1371-4
Technical characteristics for an automatic identification system using time-division multiple access in the VHF maritime mobile band
http://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1371-4-201004-S!!PDF-E.pdf