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Many radar systems need to be able to accurately determine a target's height above terrain. In airborne early warning (AEW) systems, the radar antenna is limited in its vertical aperture, thus producing a broad beamwidth in elevation. This reduces its curacy for height fmding. An alternate method of height fmding is to measure the delay of a ground-bounce (multipath) signal from the target, referenced to the direct line of sight path. This has been demonstrated successfully with AEW radars which operate over water in certain situations, but has had limited success over land. The problem has been the lack of an accurate, robust signal processing algorithm for determining the time delay between closely spaced direct and ground bounce returns. Our basic objective was to make credible the premise that an airborne AEW radar can accurately determine height of targets by processing the delayed echo due to the multipath ground bounce. Current AEW systems use this technique, but in a limited way -usually only over water, where the surface reflection is strong and predictable and when it is well separated from the direct reflection. It has been recognized for some time that land also can cause multipath reflections, but due to its irregular nature, this technique has not been exploited thoroughly. Therefore, our objectives were to show height finding can be done accurately when the direct pulse overlaps the specular return (over water or land). The signal processing problem is essentially one of performing time-of-arrival estimation of two or more pulse returns; the direct plus one or more ground bounce echoes. For typical AEW scenarios, the echoes may overlap the direct return and are usually of lower amplitude. Therefore, the algorithm must make use of the maximum information content of the signal and should have as low a threshold signal to noise ratio as possible in order to apply in the greatest number of conditions. This problem will also occur in laser radar systems, and has an exact analog for the phased-array direcfion-of-arrival estimation. The signal processing results of this paper can be applied to pulsed or frequency modulated continuous wave radar/ladar systems. We assume that the target has been detected and that it is of interest to estimate the target's height above terrain. This processing would occur at baseband using a complex demodulated channel. In general, some form of averaging is often used to enhance the SNR of small signals, such as Doppler processing (the technique is described later in this paper) and the algorithms are shown to be compatible with these operations. The algorithm could be used in some situations for enhanced detection as well. Using knowledge of the terrain, the surveillance aircraft altitude and the time delay estimates of the echoes, we can infer target height. Concurrent with the basic objective, we gathered an extensive, fairly high-resolution data set of the surface reflection of various terrain types at two radar hand frequencies: 430 MHz (UHF) and 1255 MHz (L-band). Data were to be collected for each polarization: horizonial, vertical, and cross-polarization. The main terrain of interest was rugged terrain; the predominant view being that the echo data taken in very rugged terrain would be the most difficult to extract. Calibration data were collected over water, and much data of fairly flat, predictable terrain were also taken.
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