The ScanFish uses an altimeter to track the seabed in Follow Seabed mode and as a safety measure in all flight modes, so it does not hit the seabed or obstacles. The altimeter can be a Tritech PA500 or PA200 fitted inside the ScanFish, or the HNAV message from a Sonardyne SPRINT-Nav U (SNU). Some older ScanFish systems still have a Benthos altimeter. Benthos is still supported, but it is not recommended for new installations.

Setting the altimeter type

  • Under Altimeter in the ScanFish controller web interface, select the type that matches the altimeter fitted. Available types are PA500, PA200, Benthos and SPRINT-Nav (HNAV).
  • If the wrong altimeter type is selected, the ScanFish will not pass the preflight check.
  • See the ScanFish manual, section 3 (web interface, Altimeter tab, around page 91).
  • A Tritech altimeter from a G-882 magnetometer cannot replace the ScanFish altimeter without reconfiguration. It runs on 24 V DC, while the ScanFish supplies 12 V DC.

Using a SPRINT-Nav U as the altimeter

  • Set the HNAV output from the SPRINT-Nav U to the ScanFish controller to 10 Hz. This is a recommended setting. The ScanFish runs internally at 10 Hz and the flight control is tuned to it. A lower rate makes the flight unstable, and a higher rate gives no benefit.
  • Loss of HNAV is handled exactly like loss of the PA500 (see below).

What happens if altimeter data is lost?

  1. When the altimeter data stops or becomes invalid, the ScanFish assumes the seabed is rising and climbs at the Seabed Approx rate set in the flight software (for example 0.5 m/s).
  2. If no valid altimeter data returns within the timeout, the ScanFish goes into System Failure.
  3. If valid altimeter data returns before the ScanFish is recovered, set it back to any operating mode and the System Failure clears. You do not need to bring it on deck or set it to On Deck.

Whether you get a warning before System Failure is triggered depends on the flight software. ScanFish Flight does not give a separate warning such as "Altimeter data lost". In both cases, keep an eye on the altimeter value and the alerts during operation.

Unexpected surfacing or poor seabed tracking

If the ScanFish passed the preflight check but surfaces unexpectedly or will not hold the seabed during flight, the altimeter is often the cause:

  • The altimeter is struggling to track the seabed and outputs erroneous values (seen with some altimeters in difficult conditions, for example soft seabed, high turbidity or steep slopes).
  • With a SPRINT-Nav U: HNAV is not set to 10 Hz, or the HNAV stream is interrupted.

Check the altimeter value and return quality in the flight software before relying on Follow Seabed mode. For the SPRINT-Nav U, also confirm the HNAV rate.

If the problem persists, contact EIVA Support at support@eiva.com with the flight log files and a description of when the issue happens.