In a typical frequency modulated continuous waveform (FMCW) radar altimeter system, a continuous radio frequency (RF) signal is repetitively swept by a fixed frequency modulating signal to produce a carrier modulated output signal for transmission toward a target. The FM signal is then reflected off the ground back to a receiver. This reflected signal is mixed with a portion of the transmitted signal to produce a frequency shift indicative of the distance from the transmitter to the target. By controlling the modulation of the transmitted signal and by detecting the frequency shift, readings of the distance to a target can be determined.
Modulation non-linearities and signal processing delay variations in the receiver, however, introduce errors into an FMCW radar altimeter system, and thus reduce the system's accuracy. Current systems incorporate costly and complex closed loop circuitry to eliminate such errors and improve the system's accuracy. There are generally two methods currently used for improving the accuracy of these systems. One method involves using a fixed frequency bandpass filter. The frequency shift is detected by adjusting the slope of the modulating signal so that the frequency shift stays within the bandwidth of the filter. The other method, on the other hand, involves using a variable center frequency filter while keeping the slope of the modulating signal constant. The frequency shift is detected by controlling the movement of the center frequency of the filter in a closed loop fashion so that it changes with corresponding changes in the frequency shift. State of the art implementations of this technique incorporate a digital Fast Fourier Transform system so that a plurality of selectable filters may be realized. Systems using such techniques are described in U.S. Pat. Nos. 4,509,049, 4,568,938, and 4,568,938.
Such methods, however, add both to the complexity of the altimeter and to its cost. They also impact on the construction of the altimeter. Present radar altimeter systems must be quite large in order to accommodate the above-mentioned additional circuitry.
Accuracy also depends upon the system's sensitivity, especially at higher altitude ranges (i.e., long range targets). Present systems lack the ability to alter the sensitivity of the altimeter by automatically decreasing the search rate at higher altitudes.
The present invention has therefore been developed to provide a low-cost FM radar altimeter system with enhanced accuracy which overcomes the limitations of the above-known and similar techniques.