Marine Radar Analysis Software
The data collected using a rotating marine radar antenna is stored in a range-azimuth format in cylindrical co-ordinates. Before imagery can be formed for spatial processing or image averaging, the data must first be converted to Cartesian co-ordinates. Our software package, CXfer, performs this operation, interpolating the radially raw data, as shown in the example below. Output is stored for further analysis.
For analysis of data to determine local currents and/or bathymetry, three-dimensional analysis of windows from a series of radar images are processed using FFT-3D.The code performs one dimensional FFT analysis of rows and columns of a square window and retains the output in complex format in memory. The full sequence of (in this case) 32 images is completed, after which the analysis is repeated in the time dimension to produce output in the frequency domain as individual KxKy spectra. For a 32-image sequence, the output frequency FFT will have 16 positive frequencies and 16 negative frequencies that contain redundant information. Below is shown an example of eight of 16 output frequencies for data collected in shallow water. The dispersion rule for shallow water is shown as white circles and for deep water as red circles in the results below. The spectral energy lies close to the shallow water dispersion circles as expected for these data that were collected at the USACE FRF pier.
Processing of radar imagery using FFT analysis requires square sub-images to be extracted for analysis from the CXfer results. The code, SnipIt, uses a parameter file that specifies the size of the square to be extracted, the center range of the window, and the azimuthal angle relative to the start angle of the image acquisition, for each of a series, N, windows of interest. An example for two windows at the same azimuth, but different ranges is shown below.
Use of a series of windows is useful for bathymetry analysis, or choosing windows about the wind direction maximum in the radar screen echo for open ocean data collections. An alternative mode for SnipIt allows it to collect windows serially across a scene, in an along-shore direction, for example, for some fixed offshore distance