How Zora judges an AIS target
Zora checks every vessel it hears on AIS against the limits you set, and warns you about the ones that will pass too close. This is how it decides, and why it sometimes stays quiet. To set the limits, see Set your AIS limits.
What AIS can show you
AIS only shows vessels that broadcast it. A boat with no transponder, or one that is switched off, is not on the AIS screen at all, however close it is. An empty screen does not mean empty water. Keep a lookout.
AIS also arrives at the pace the other vessel sends it. A Class A set reports its position every 2 to 10 seconds underway, and only every 3 minutes at anchor. A vessel's name, size and destination come every 6 minutes, so a vessel that has just appeared can show its MMSI in place of a name for a while.
Zora shows each vessel where it last reported. It does not move it forward between reports. A vessel not heard for 3 minutes turns grey as lost, and after 5 minutes Zora drops it and clears its notifications.
Closest point of approach
For each vessel Zora works out two numbers, assuming both boats hold their course and speed:
- CPA, the closest point of approach: how near the vessel will come.
- TCPA: how long until then.
The moment either boat changes course or speed, those numbers change, so Zora checks every vessel again every second, against where you are now. A vessel that could not reach your widest limits even if it came straight at you is skipped; every other vessel gets a full CPA.
The screens do not wait for that check: each AIS message is passed on as it arrives.
When a level goes off
Your limits have three levels, alarm, warn and alert, each with a distance and a time. A level goes off only when the CPA is inside its distance and the TCPA is inside its time. Alarm is checked first, then warn, then alert.
Zora stays quiet when:
- the vessel has already passed its closest point and is drawing away
- the vessel is your own AIS transponder, which Zora recognises by your boat's MMSI. Without an MMSI set, it shows up as another vessel.
- no limits have been saved
The alarm sounds once per encounter. After that the notification updates quietly, its time counting down, as the vessel closes or the level changes. Once the vessel drops out of every level, it starts afresh: if it reaches alarm again, the alarm sounds again.
When you save new limits, Zora clears its AIS notifications and judges every vessel again.
Underway or stopped
Zora picks one of your two sets of limits from the boat's vessel state:
| Vessel state | Limits used |
|---|---|
| Sailing, motoring, motor-sailing, drifting | underway |
| Docked | none, AIS notifications are off |
| Every other state, at anchor, on a mooring or hauled out | stopped |
With the stopped limits, a vessel has to be moving faster than half a knot and still closing before it sets anything off. A boat lying still next to you stays quiet, however close.
Crossing markers
The dot on a vessel's course line on the AIS screen is worked out separately from your limits. It appears when:
- you and the vessel are both moving, faster than about 1 knot
- the vessel is ahead of you, relative to your course, and within 10 nautical miles
- your courses cross ahead of you, and you reach the crossing before the vessel does
There is no distance or time limit on the crossing beyond that. The dot shows where you pass ahead of the vessel's bow when you reach its track. Zora works it out again each time AIS data arrives.