In Echoview, instrument scanning data is partitioned into frames to facilitate school detection. A frame represents a single scan of a volume of space (water).

For the purpose of school detection the platform must be stationary during the scan. Due to GPS imprecision and movement of a vessel at anchor during a scan the actual measured platform position may vary over the time of an instrument scan. You will generally need to prepare your data for school detection.

When using the instrument scanning school detection algorithm, only pings that are within frames are used. Variable data is divided into frames as far as possible (not all pings will be in a frame) to prepare for detection.

The following steps are used to identify frames:

1. Find the first ping in the variable which is not in a target-locked ping pair. This is the first ping in the first frame, and the frame has no scan direction.
(note: this is not necessarily the first ping in the detection)
2. Repeat for all subsequent pings in time order until no pings are left:
1. If the frame currently has two pings, note the direction of scanning
(from the change in bearing or tilt between the first two pings in the frame)
2. Add the ping to the current frame if it meets the following requirements:
1. the position of the ping (latitude and longitude) is the same as the last ping added
2. it is not in a target-locked ping pair
3. the geometry of the ping is the same as the last ping added
4. the tilt and/or bearing angle is not the same as the last ping added
5. the tilt and/or bearing angle is not more than 10° different to that of the last ping added
6. if this is a V-Mode bearing scan then:
if the sector angle is less than or equal to 90° then:
If the bearing is not more than 360° different to that of the first ping added
Otherwise if the sector angle is greater than 90°:
If the bearing is not more than 180° different to that of the first ping added
7. the direction of scanning is the same as that of the frame (ignored if the frame has no scan direction yet)
Otherwise:
1. if the current frame has less than 5 pings, discard it
2. find the next ping in the variable which is not in a target-locked ping pair. This is the first ping in a new frame, and the frame has no scan direction.

Notes:

1. Pings which are in a target-locked ping pair, cannot also be in a frame.
2. A frame cannot overlap in time with any target-locked ping pairs
3. The instrument scanning school detection algorithm will not detect schools in pings which are not in frames, it will only detect schools in frames.
4. If the Scan position operator is used to prepare for school detection, the frames and their composition are reported on the Notes page of the Variable Properties dialog box.