In order to measure common wave parameters the instrument needs to measure (1) the displacement of the sea surface and (2) the orbital velocities. For more details about wave measurements, please refer to Principles of Operation - Waves. Both the AWAC and the Signature 1000, 500, and 250 uses its central beam and Acoustic Surface Tracking (AST) to get a direct measurement of the distance to the surface.

The window or range wherein the instrument is looking for the surface when using the AST is called the AST window. The cell from which the orbital velocities are collected is called the wave velocity cell. Both the AST window and the wave velocity cell are decided based on the pressure readings to ensure that the whole wave is captured. The equations for the location and size of the AST window and the wave velocity cell are as follows:

The AST window is centered around the mean pressure level. The values will vary with time, and be different from burst to burst. You can find both the AST window location and the wave velocity cell size and location in you data.

AWAC

The equations above are calculated before each individual wave burst, based on the pressure readings from the preceding current profile. Both the AST window position and the wave velocity cell size and position will therefore adjust and change from burst to burst. The wave velocity cell is only one cell where velocities are measured. These values can be found in your data

Signature

The Signatures does not use the equation above for AST window. The window is rather set to a relatively large range during configuration with the Altimeter Start and Altimeter End parameters. The selection of the wave velocity cell size and location is done in post processing, as the instrument measures a longer profile of burst data. The pressure data is then used to decide where in the profile the orbital velocities should be collected.