Huography and doppler radar

TRANSMITTED Recently the operation of holography has been extended ENVELOPE to the processing of pulse doppler radar signals. For cases where both the range and the range rates of many separate targets must be measured, Arm and King • • • of Riverside Research Institute showed that the holo- graphic procedure would take a sequence of radar returns and store them as holograms, with optical recon- TIME struction yielding the desired information. In this system, good range resolution implies short RECEIVED pulses; range rate data based on doppler frequency- ENVELOPE shift measurements of the return signal demands a long radar pulse. To accomplish both, acoherent pulse burst (top Fig.) is used. A burst consists of a train of N pulses equally spaced T seconds apart with each pulse r seconds long. Pulse duration determines range o resolution; burst duration, NT, determines the doppler frequency-shift resolution of 1/NT hertz. The lower diagram of the Fig. shows that each radar ULTRASONIC PHOTOGRAPHIC FILM LIGHT transmission elicits a return from each of four targets. SIGNAL LENS PLACED IN THIS The delay between each short transmitted pulse of the MODULATOR PLANE train and the time of the corresponding return from one of the targets accurately resolves range. -1 ORDER The range rate of each target usually is obtained 0ORDER by feeding the entire sequence of N returns from the 4-+1 ORDER, ILLUMINATED target into a spectrum analyzer, and determining the BY REFERENCE doppler frequency shift. Since this analysis must be BEAM AT ANGLE performed for each required range element, many spectrum analyzers will be needed. However, with REFERENCE BEAM holography, optical procedures do the same task. In the first step, returns are stored holographically (middle Fig.). A collimated beam of laser light is split into reference and signal components; it's later recom- -BEAM SPUTTER bined to form an interference pattern on avertical strip COLLIMATED LIGHT, of the photographic film. The upper path contains the WAVELENGTH signal beam and records the radar return signal as it passes through an ultrasonic light modulator. The lower path directs the reference beam onto the same film strip. The signal of the ultrasonic light modulator's aper-

ture is the signal reflected from the four targets as FREQUENCY produced by a single transmitted radar pulse. When SHIFT the complete reflected signal (comprising in this case only four echoes) reaches the ultrasonic modulator, the laser light is turned on briefly, and the hologram is recorded. Thus, the hologram occupies a narrow vertical strip on the film. After each pulse, the film is indexed, or moved perpendicular to the plane of the _ figure. When the return signal from the next pulse fills the acoustic cell, the laser is again activated. Thus an RANGE entire sequence of N radar returns is recorded as N holograms side by side on the photographic film. This series of holograms forms, for each target, a grating whose tilt is a function of the target's range FOCAL FOCAL LENGTH LENGTH rate or doppler frequency so that the four gratings are superimposed, and each has a different degree of tilt. After development, the film sheet of many side-by- side holograms is coherently illuminated and the original configuration reconstructed (lower Fig.). Two areas ap- OPTICAL pear in the output or frequency plane containing all AXIS the range-doppler information. In an expanded view of one of these information areas each of the four bright spots of light corresponds to one of the targets interrogated by the N transmitted radar LENS N HOLOGRAMS pulses. Vertical positions corresponds to the target's ON FILM RANGE-DOPPLER range, and the horizontal position its range rate. PATTERNS

86 Electronics October 12, 1970