The term “submillimeter confocal imaging active module” (SCIAM) denotes a proposed airborne coherent imaging radar system that would be suitable for use in reconnaissance, surveillance, and navigation. The development of the SCIAM would include utilization and extension of recent achievements in monolithic microwave integrated circuits capable of operating at frequencies up to and beyond a nominal radio frequency of 340 GHz. Because the SCIAM would be primarily down-looking (in contradistinction to primarily side-looking), it could be useful for imaging shorter objects located between taller ones (for example, objects on streets between buildings). The SCIAM would utilize a confocal geometry to obtain high cross-track resolution, and would be amenable to synthetic-aperture processing of its output to obtain high along-track resolution.

The SCIAM would initially include two antenna apertures, through each of which radar signals would be both transmitted and received. The antenna structures in each aperture would be frequency-dispersive so that frequency scanning could be used to effect cross-track scanning.
The SCIAM (see figure) would include multiple (two in the initial version) antenna apertures, separated from each other by a cross-track baseline of suitable length (e.g., 1.6 m). These apertures would both transmit the illuminating radar pulses and receive the returns. A common reference oscillator would generate a signal at a controllable frequency of (340 GHz + Δf )/N, where Δf is an instantaneous swept frequency difference and N is an integer. The output of this oscillator would be fed to a frequency-multiplier-and-power-amplifier module to obtain a signal, at 340 GHz + Δf, that would serve as both the carrier signal for generating the transmitted pulses and a local-oscillator (LO) signal for a receiver associated with each antenna aperture.

Because duplexers in the form of circulators or transmit/receive (T/R) switches would be lossy and extremely difficult to implement, the antenna apertures would be designed according to a spatial-diplexing scheme, in which signals would be coupled in and out via separate, adjacent transmitting and receiving feed horns. This scheme would cause the transmitted and received beams to be aimed in slightly different directions, and, hence, to not overlap fully on the targets on the ground. However, a preliminary analysis has shown that the loss of overlap would be small enough that the resulting loss in signal-to-noise ratio (SNR) would be much less than the SNR loss associated with the use of a 340-GHz T/R switch.

NASA Tech Briefs Magazine

This article first appeared in the May, 2009 issue of NASA Tech Briefs Magazine.

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