Low cost radar altimeter with accuracy enhancement

A low cost frequency modulated (FM) radar altimeter system with enhanced accuracy for continuously providing an altitude of a target, comprising:

• (a) transmitter means for transmitting a carrier modulated output signal with a slope, s.sub.m, and a frequency, f.sub.t, toward the target, the transmitter means further comprising:
• (1) a transmitting antenna having a bandwidth, b.sub.t, for receiving and directing the carrier modulated output signal toward the target;
• (2) sweep modulating means connected to the first output of the searching means for providing a linear modulating signal;
• (3) carrier modulating means responsive to the sweep modulating means for receiving the linear modulating signal and for generating a carrier modulated output signal;
• and
• (4) an isolator connected between the carrier modulating means and the coupling means for improving the linearity of the carrier modulating means, as the frequency, f.sub.t, of the carrier modulated output signal is swept across the bandwidth, b.sub.t, of the transmitting antenna;
• (b) receiver means having an altitude output signal for receiving the carrier modulated output signal reflected from the target after a time delay and having a frequency, f.sub.r, the receiver means further comprising:
• (1) a receiving antenna;
• (2) mixer means having a first input for the receiving antenna and a second input for receiving a portion of the carrier modulated output signal for providing a frequency shift, f.sub.d, corresponding to a difference between the frequency, f.sub.t, of the carrier modulated output signal directed to the target and the frequency, f.sub.r, of the reflected carrier modulated output signal;
• (3) amplifier means for receiving and amplifying the mixer output signal;
• (4) fixed filter means having a bandwidth, b.sub.w, and a center frequency, f.sub.c, for passing the amplified mixer output signal when the frequency shift, f.sub.d, corresponds to the slope, s.sub.m, of the carrier modulated output signal and the time delay;
• (5) envelope detector means for receiving and passing the amplified mixer output signal from the fixed filter means and for providing a pulsed video output signal with each upsweep and downsweep of the searching means so that a first pulse corresponding in time to the frequency shift, f.sub.d, during the upsweep and a second pulse corresponding in time to the frequency shift, f.sub.d, during the downsweep may be alternately and repetitively generated;
• (6) post detection filter means having a filtered pulsed video output signal for receiving and filtering the pulsed video output signal so that a signal-to-noise ratio of the pulsed video output signal is improved;
• (7) open loop error correction means responsive to the filtered pulsed video output signal for correcting errors resulting from nonlinearities in the transmitter means and signal processing delay variations in the receiver means so that the altitude determined is accurate, the open loop error correction means further comprising:
• (a) pulse generating means having a series of pulses as an output signal for receiving the filtered pulsed video output signal and for providing a preset threshold level whereby only the pulses equal to or greater than the preset level are generated;
• (b) sampling means having a first altitude output with an amplitude, A.sub.1, and a second altitude output with an amplitude, A.sub.2, for sampling the search output signal at times corresponding to the pulses generated by the pulse generating means;
• (c) averaging means for receiving the first altitude output and the second altitude output of the sampling means, and for averaging the amplitudes A.sub.1 and A.sub.2 so that the errors are reduced and the altitude of the target determined is accurate;
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• (d) integration means for integrating the altitude over a period of time corresponding to a maximum expected altitude;
• (c) coupling means for directing a portion of the carrier modulated output signal to the receiver means;
• and
• (d) searching means having a first output directed to the receiver means and a second output directed to the transmitter means and having a symmetrical search output signal with a slope, s.sub.s, and a fixed period for providing continuous and repeated sweeping from a low altitude to a high altitude, an upsweep, and then back down to the low altitude, a downsweep, so that the radar altimeter is in a constant searching mode, and for automatically altering a search rate of the radar altimeter with changes in the slope s.sub.s, of the search output signal so that the sensitivity of the radar altimeter increases as the slope, s.sub.s, decreases.