Altimetry Low Altitude Multipath Target Method

Low-altitude and very low-altitude flying targets are difficult to detect and track, and this kind of the targets is also one of the four great threats propelling radar development. Research on the height measurements of low-altitude targets is of great significance to counter low-altitude air defense penetration. When detecting this kind of targets, the receiver receives the reflected signal from the ground as well as direct path signal from the target, which forms multi-path complex signals.The multi-path targets can be achieved the height measurement and other parameter estimation by applying the relationship among multi-path signals. This thesis recommends the principle of altimetry by the multi-path effect for height measurement adopting linear frequency modulation signal as the transmitting signal. The multi-path signal models are evaluated and it is found that resolving the true target and the mirror target is the fundamental of performing the height measurement. The effects of target range, altitude, velocity and other radar parameters on the measurement accuracy are also discussed.For near-field targets, the radar echoes can be resolved in both the time domain and the Doppler domain. In this case, the distance and/or the Doppler frequency difference between the true target and mirror target can be used to calculate the target height. For far-field targets, it is only possible to resolve the multi-path signals in the Doppler domain. Then the Doppler frequency difference can be used to perform the height measurements.For high-velocity target, the received multi-path signals may result in range migration in coherent accumulating duration. The migration will affect the target accumulation and then some techniques are needed to calibrate the reflected target range. Simultaneity, the angle between radar and line of target sight is changing at all times, which results in the continuous change of Doppler frequency. At this time, Doppler bandwidth will be seriously extended. To solve the problem, the paper performs calibration to the direction of range and azimuth with range-Doppler algorithm, which has good effect to the measurement of target altitude and multi-path target detection according to simulation experiments.