A random frequency hop code technique for improved range-Doppler imagery

With the discoveries of polar caps on Mercury and the stealth region on Mars, there is a renewed interest in generating global range-Doppler images of the inner planets and the Galilean satellites from Earth-based radar systems. This task represents a significant engineering challenge due to the large spreading of the associated scattering functions. A technique for radar imaging based on coherent random frequency modulation is presented. This approach, which is being implemented at the Jet Propulsion Laboratory, makes use of digitally programmed oscillators to generate a continuous FM waveform. The waveform is shaped with a random set of frequency hops spanning a bandwidth /spl beta/. A total of N discrete frequencies are allowed. The probability of occurrence for any given frequency is N/sup -1/. The statistical behavior of the waveform's spectrum and autocorrelation is analyzed. It is highly desirable to provide a nearly perfect autocorrelation with a sharp peak at zero delay and a uniform self-noise pedestal. Also desirable is a spectrum with white characteristics over the bandwidth /spl beta/ and fast sidelobe rolloff. Values of /spl beta/T (T is the duration of each subpulse) and N conducive to these characteristics are determined.