Acoustic surface tracking (AST)

Acoustic Surface Tracking (AST) is a method for directly measuring the distance to the sea surface using the altimeter principle. The direct surface measurements provide accurate surface displacement records, which are required for time-series analysis (see Time-series analysis (Zero-crossing method)).

The AST option was first introduced in 2002 with the release of the Nortek AWAC. It was implemented on the first-generation AWACs and later carried over to the second generation. It is also available on the Nortek Signature instruments with operating frequencies of 1000, 500, and 250 kHz.

AST uses the vertically oriented center transducer to measure the two-way travel time of the acoustic signal, thereby estimating the distance to the surface for each ping. Direct measurements with AST have several advantages when measuring waves. Most importantly, AST removes practical depth limitations in coastal waters and enables coverage of a larger portion of the wind-wave frequency band than indirect measurements based on pressure and velocity alone. In addition, AST measurements allow both time-series and spectral analyses to be carried out, making it possible to determine design parameters such as H10 and Hmax directly.

Detection procedure

The raw AST data produced by the instrument consist of a time series of distances from the instrument to the sea surface. The procedure used by the instrument to detect the surface is relatively straightforward and can be summarized in the following steps:

1. Transmit a short acoustic pulse.
2. Define a receive window that covers the full range of possible wave heights.
3. Divide the receive window into discrete cells (≈ 5 cm) to achieve high resolution.
4. Apply a matched filter across the series of cells to identify the maximum peak, corresponding to the sea surface.
5. Use quadratic interpolation of the peak and its neighboring cells to refine the surface location estimate, achieving a final distance resolution of approximately 1 mm.

The detection procedure is performed automatically by the instrument and requires no user interaction. The description above is provided for reference only. The resulting time series of surface distances is included in the raw wave data.

To obtain wave parameters such as wave height and period from the raw wave data, an additional processing step must be applied. This can be done in Nortek post-processing software. For live operation, an internal wave processing license can be purchased so that the instrument can process wave data internally and output wave parameters directly.

Altimeter window

As described in Step 2 above, the surface detection procedure includes defining a receive window that covers the full range of expected wave heights. This window is referred to as the AST or Altimeter window and is the range within which the instrument is looking for the surface. If the sea surface lies outside the defined window, the instrument cannot detect the surface position, and the resulting AST time series will consist only of noise. The definition of the AST/Altimeter window varies slightly between instrument types.

Generation 1 AWACs

The AST Window is adaptively defined for each wave burst. The window range is determined from the pressure readings collected during the preceding current profile, ensuring that it covers the full range of expected wave heights. The mean and minimum pressure readings from the current profile measurements indicate the location of the wave trough and the mean water level (Figure 2). The equations used to determine the AST window are given in Equations 1, 2, and 3. The equations for \(Cell\ size\) and \(V_{end}\) define the extent and location of the wave velocity cell used to obtain orbital velocity measurements for directional wave parameters (see Wave velocity cell). As shown in the equations below, the window location varies from burst to burst depending on the pressure values measured during the preceding current data collection.

Generation 2 AWACs and Signatures

Generation 2 AWACs and Signature instruments define the AST window in a similar way. During configuration, a constant altimeter window is defined by the Altimeter Start and Altimeter End parameters. These values are not manually set but follow the user-selected instrument depth. The Altimeter Start and Altimeter End values are shown in the Effect tab during configuration.

This configured window defines the overall range within which the instrument can search for the surface. If storage of raw altimeter data is enabled, the altimeter return signal from the full extent of the configured altimeter window is saved.

During measurements, the instrument uses the pressure sensor when estimating the AST value. For each altimeter ping, it reads the pressure sensor and places the AST window around the measured pressure value. This adaptive positioning ensures that the window is sufficient for all expected wave conditions.

AST quality parameter

The raw wave data contain a parameter called AST quality. This parameter gives an indication of the quality of the altimeter data collected with the AST method. The definition of the parameter varies between instrument types:

Generation 1 AWACs

The quality parameter is a result of the match filter process used when detecting the surface, and is a dimensionless value. As a rule of thumb, the data quality is considered acceptable if the quality parameter is above 100. If it is over 200, the data is considered very good, and if the quality parameter is below 100, it indicates potential issues with the data set that require further investigation. 

Generation 2 AWACs and Signatures

For these instruments, the quality parameter has a more physical meaning and represents the amplitude from the point where the altimeter has detected the surface using the AST method. The amplitude is given in steps of 0.01 dB; for example, a quality value of 8000 means that the amplitude of the surface return signal is 80 dB, which is considered good.