Signal Processing Techniques for a Lunar Radar System

The NRL lunar radar system is being used to make long term, precision Earth-Moon distance measurements. A pulse compression ratio of 104 permits a high signal to noise ratio for a range resolution of 1.2 ??s (180 meters). The high pulse compression ratio is achieved through the use of a pseudorandom (P-N) code, phase reversal modulation of the carrier, and a multichannel cross-correlation receiver. The transmitted signal is phase modulated by the P-N code. This code has a correlation function which permits separation of the echo in range. In each correlator, the echo is multiplied by a replica of the code with an appropriate delay, and the product is passed through a narrow band filter. The correlator output measures the energy of the echo from a range interval corresponding to the delay chosen. A separate correlator is used for each of 40 adjacent range channels. The correlation circuitry consists of ring modulators, crystal filters, and envelope detectors. The ring modulators permit compact and economical packaging of the 40 correlators. The multiple outputs are recorded on magnetic tape for analysis and are also displayed in realtime.