A timing offset between sensors, most often between positioning and sonar. Just 50 milliseconds at 8 knots moves soundings about 20 cm along track, and timing errors between attitude and sonar are worse because the vessel is rotating. Synchronizing every sensor to GNSS time with a pulse-per-second signal removes most latency, and the patch test checks what remains by running one line at two speeds over a slope.
Latency check shows the slope moving with speed, the PPS cable must have worked loose.
Also heard as
- time delay
- timing offset
Related terms
Patch test
A set of calibration lines over flat ground and slopes that measures sonar alignment biases and timing error.
GNSS
Global navigation satellite systems: satellite positioning such as GPS, GLONASS, Galileo and BeiDou, used together.
Pitch
Bow-up or bow-down rotation of a vessel, which shifts sonar footprints forward or aft along the track.
MRU
Motion reference unit: the sensor that measures a vessel's roll, pitch and heave to correct sonar data.