The ASCII telemetry data that is transmitted by the Signature series of instruments is quality controlled before it is sent, and any data that is deemed unreliable is flagged. The criteria for one data point being flagged as unreliable is the correlation being less than 50% for the measurement, and if more than 50 % of the data points within an average interval is considered bad the whole cell is flagged in the outputted data. The flag values are as follows:

• Velocity: -32.767 m/s
• Speed: 46.34 m/s
• Direction: 225°

All ASCII telemetry data formats go through this QC and it is not possible to disable this. Telemetered binary data does not undergo any QC. If you need the unflagged data, this must be retrieved from the instrument recorder.

Example reasons that the correlation might be <50% (this is not an exhaustive list):

• Sidelobe interference near the surface
• Acoustic or electrical interference
• Obstructions in the path of the beams