Terrain profile radar system

A radar altimeter system having a transmitter section, a receiver section and a digital processor section wherein:

• the transmitter section comprises:
• a transmitting oscillator producing a high frequency carrier;
• a phase modulator capable of sequentially changing the phase of the carrier signal between 0.degree. and 180.degree. according to a code at a first constant frequency during a period defined by an amplitude modulator and randomly changing to other sequences of phase changes during the following periods;
• an amplitude modulator producing a periodic pulsed signal having a period and, within it, a pulse having a pulse width (a chip);
• a power amplifier;
• and
• a transmitting antenna;
• the receiver section comprises:
• a receiving antenna;
• a radio frequency amplifier;
• means to heterodyne the received signal to an intermediate frequency;
• an intermediate frequency amplifier;
• quadrature multipliers which multiply the signal by the cosine and sine of the intermediate frequency carrier, thereby creating two quadrature components;
• and
• analog to digital conversion means acting upon the two quadrature components which sample the signal at a rate which is at least twice the frequency of the code to produce at least two samples per chip;
• and the digital processor section comprises:
• input gate means for the two quadrature components, having a temporal width which is controllably adapted to allow only the passage of a received pulse as well as sufficient additional time to allow a plurality of correlation operations to be conducted at closely spaced time intervals;
• correlation means for each of the two quadrature components to add the received quadrature component to the transmitted code, then to time shift the transmitted code the closely spaced time interval and re-add the code to the received quadrature component, then repeatedly to time shift and re-add the transmitted code until the plurality of correlation operations is completed and repeating the correlation operations for a plurality of pulses;
• squaring means to square the outputs of the correlation means;
• summation means which add together the squared outputs of the correlation means for the two quadrature components for transmission to a pulse summing means;
• pulse summing means which sum the squared outputs of the correlation means such that the individual pluralities of pulses are summed for each closely spaced time interval for which the correlation operation is done;
• detection logic means which receives the output of the pulse summing means, finds the closely spaced time interval having the highest sum (peak value), said sum exceeding a preset false alarm threshold level and provides to the processor section the signal level of the highest sum;
• gate control means which shift the temporal position of the input gate means in response to command signals from received signal acquisition or re-acquisition means;
• and
• altitude calculation means which provide the altitude of the system above a surface based upon the time interval between the transmission of the pulse and the time of reception of the peak value of the received pulse.