AIS for situational awareness, and what it does not show

What you do on the bridge

1. AIS on the radar
2. Wake the targets up
3. A Class B target
4. The echo with no AIS
5. Check AIS against the radar

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What are the limitations of AIS for collision avoidance?Often asked

   A good answer covers:

   • Not every vessel carries AIS, and it can be switched off: small craft, fishing boats and warships may not show
   • The data are only as good as the other ship's sensors and what her crew entered: position from her GNSS, heading from her gyro, a wrong navigational status
   • Class B transmits less often and at lower priority, so its picture lags more
   • Reports can be lost in a busy area, and positions are not checked against radar
   • So AIS is an aid to the look-out and to identification, never a substitute for radar and a proper visual look-out

   Show me on the bridgeCompare the AIS targets with the radar echoes. Is there an echo with no AIS?

2. What is the difference between AIS Class A and Class B?Often asked

   A good answer covers:

   • Class A is carried by SOLAS ships: higher power, reports every 2 to 10 seconds under way depending on speed and turning, and every 3 minutes at anchor
   • Class B is for small craft: lower power and a slower reporting rate, every 30 seconds above 2 knots and every 3 minutes below (Class B CS)
   • Class B sends less voyage data: no navigational status, destination or draught
   • Both show on the ECDIS and radar, often with a different symbol or label
3. What is the difference between a sleeping and an activated AIS target?

   A good answer covers:

   • A sleeping target shows only its position and heading: no vector and no CPA alarm
   • An activated target shows its vector, its data on selection, and takes part in CPA and TCPA alarms
   • Targets can be activated by hand, or automatically by range or CPA limits that the officer sets
   • Leaving the activation range too short means a closing ship stays asleep until it is close
4. Should you use AIS to identify a ship and call her on VHF to agree how you will pass?

   A good answer covers:

   • AIS helps identify the ship, but VHF agreements are a known cause of collisions: the wrong ship may answer, or the agreement can break the rules
   • Act in line with the COLREGs; do not rely on an agreement in place of them
   • If you do call, identify her positively, and still check that the action taken is working
   • AIS safety-related text messages are not a substitute for a proper call when urgent

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're heading east at eight knots, visibility about three miles. There's traffic around us: some of it on AIS, and some of it not.

   We'll set up AIS on the radar and the ECDIS, see what it tells us, and find what it misses.

2. AIS on the radar

   Switch the AIS overlay on, on the radar. Each AIS target is a triangle pointing along its heading. Where an AIS triangle sits on a radar echo, the two agree.

3. Wake the targets up

   Look at the yacht on the port bow. Her triangle has no vector: she is a sleeping target. The last watch left the ECDIS activating AIS targets only inside two miles, so a ship closing on us stays asleep until she is near.

   In the ECDIS settings, raise the AIS activation range to about six miles. Now she's activated: a vector, and a CPA that counts towards the alarms.

4. A Class B target

   Select her and read the data. She's on Class B, the kind small craft carry. It reports less often and sends less: no destination, no draught, no navigational status. Her position can be half a minute old.

5. The echo with no AIS

   Now look at the radar on the starboard bow, close in. There's an echo with no AIS triangle on it: a fishing boat with no AIS, or with it switched off. The ECDIS target list doesn't know she exists.

   Acquire her on ARPA. That is the only way she gets a vector and a CPA.

6. Check AIS against the radar

   Last, the ship crossing from starboard. AIS gives her name and course at once, but it is her data, from her sensors. Acquire her on ARPA too, let the track settle, and compare the two CPAs. If they disagree, believe your own radar until you know why.

The rules and guidance behind it

• MGN 379 (M+F) Amendment 1: use of electronic navigational aids