Leading edge

The Leading Edge (LE) is a method for directly measuring the distance to the water-ice boundary using the altimeter principle. The direct distance measurements provide accurate displacement records, which are required for calculating the ice draft.

The instrument measures the distance to the submerged part of the ice sheet directly with the central beam. There are two ways to decide the distance to the boundary interface (water-air or water-ice) using the altimeter profile. The boundary can be detected either using the max peak of the return signal or the leading edge (Figure 1). The max peak method of detecting the surface searches for the largest value in the return signal and is used to detect the water-air boundary when measuring waves. The leading edge method detects the boundary by finding where the return echo has the largest change in value. This method is better suited for echoes that increase at the boundary but do not decrease rapidly. This is typical for water-ice boundaries, and the leading edge method is therefore mostly used to detect ice.

The procedure used by the instrument to detect the boundary is relatively straightforward and can be summarized in the following steps:

1. Transmit a relatively short pulse
2. Specify a receive window covering the range of all possible wave heights
3. Apply a match filter process which uses a leading edge detector over the profile to locate a surface
4. Use quadratic interpolation of the peak point and its neighbors to precisely estimate surface location

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.

Leading edge data from a Generation 1 AWAC

To include LE distance values in your AWAC data, enable the ice mode option during configuration. The AWAC collects altimeter data in pairs, at twice the sampling rate used for velocity and pressure measurements.. The first value (AST distance 1) is detected using the AST algorithm, while the second value (AST distance 2) is detected using the LE algorithm. If ice mode is not enabled, both distance values are calculated using the AST algorithm (see Acoustic surface tracking (AST)).

Leading edge data from a Signature

When altimeter measurements are activated, the instrument will store time series with distance to surface, detected by both the max peak and the leading edge method. In your data you will therefore have one time series called AST (using the max peak) and one called LE (short for leading edge). 

Altimeter quality parameter

In your Signature data, you will find a parameter called Altimeter quality. This parameter gives an estimate of the quality of the altimeter data collected with the LE method and must not be confused with the AST quality parameter (see Acoustic surface tracking (AST)). Altimeter quality is a dimensionless value that is a result of the match filter process used to detect the surface. The limits for what is considered good data depends on the instrument, but if the quality is below the limit for your specific instrument, the LE distance will be set to zero.