Abstract : The work performed under this contract was related to the testing and procurement of the phase and, in particular, frequency processor boards that are needed in Trex Enterprises' second generation real-time passive millimeter-wave camera (PMC). In addition, an imaging demonstration performed under this contract demonstrated the feasibility of building such a PMC and that it is possible to obtain near diffraction limited imagery. The Trex Enterprises Corporation's PMC imager uses a patented flat panel antenna and millimeter-wave signal processors to enable a 30 by 20 degree instantaneous field of view true real time (30 Hz) sensor with a 2 Kelvin instantaneous thermal sensitivity, to be built in a highly compact package suitable for concealed weapons detection. The millimeter-wave (MMW) portion of the electromagnetic spectrum is chosen as it offers a good balance between clothing penetration as well as resolution and, therefore, allows compact, practical sized systems suitable for law enforcement to be built. By measuring only natural thermal emissions (from living beings and inanimate objects), and reflections of natural ambient sources (such as the cold sky), passive millimeter-wave imaging is intrinsically safe and suitable for imaging people.

W-band direct detection circuits have been designed and fabricated for use in a passive millimeter wave camera. The circuits are based on the recently developed Sb-heterostructure diode. We measure record voltage responsivities in test circuits, up to 8,000 mV/mW from 75 to 93 GHz, with input power from -50 to -30 dBm. Performance was similar in an actual camera frequency processor board with 128 tuned channels. 72% of detectors showed responsivity at or above 6,000 mV/mW and 3% of channels were above 10,000 mV/mW. Since tens of thousands of Sb-heterostructure diodes can be reproducibly and inexpensively fabricated, this demonstrates for the first time the feasibility of large-scale detector arrays utilizing zero bias direct detection circuitry.

With the current upsurge in domestic terrorism, suicide bombings and the like, there is an increased interest in high technology sensors that can provide true stand-off detection of concealed articles such as guns and, in particular, explosives in both controlled and uncontrolled areas. The camera discussed in this paper is based upon passive millimeter-wave imaging (75.5-93.5 GHz) and is intrinsically safe as it uses only the natural thermal (blackbody) emissions from living beings and inanimate objects to form images with. The camera consists of four subsystems which are interfaced to complete the final camera. The subsystems are Trex's patented flat panel frequency scanned phased array antenna, a front end receiver, and phase and frequency processors to convert the antenna output (in phase and frequency space) into image space and in doing so form a readily recognizable image. The phase and frequency processors are based upon variants of a Rotman lens.

Trex Enterprises is developing a 2nd generation passive millimeter wave imaging system which operates in real time with a 20×30 degree field of view and a 2 K temperature sensitivity. This system is based on a pupil-plane aperture architecture. A pupil-plane architecture allows for a lower system volume and reduced number of receivers compared to focal plane systems. The system has low noise W-band amplifiers, a flat panel dielectric antenna, high sensitivity diodes and will serve as the basis for a commercially available system. Millimeter wave imaging provides enhanced vision capabilities to pilots, allowing them to see through fog and smoke and aiding in navigation, landing, and taxiing in low-visibility situations. Unlike radar, passive millimeter wave imaging provides the pilot with a display based on angle-angle data rather than range-angle data which must be interpreted into an angle-angle display. The passive system avoids problems with multi-path reflections that can create problems for active systems in close proximity to the ground. The fact that the sensor does not emit also makes it useful for military applications where the pilot wishes to avoid detection.

Real time passive millimeter-wave imaging systems have a wide variety of uses from aircraft navigation and landing in fog to detection of concealed weapons. A useful imaging system for flight platforms requires a large number of pixels and a high frame rate combined with a small antenna volume and a low cost. We present a millimeter-wave imaging system which uses 32 MMIC low noise amplifiers to display a 60/spl times/75 pixel image at a 30 Hz frame rate. The system's pupil-plane phased array architecture allows for a relatively thin large aperture antenna. A remotely located processor utilizes microwave guiding circuit boards to perform phase and frequency discrimination on the radiation received by the antenna array. The imaging system was mounted on a helicopter platform and used to gather real-time imagery of a variety of targets. Targets include improved and unimproved runways, roadways, vehicles on roadways, and power lines. We present video-rate millimeter-wave imagery of ground features which are obscured from visible and infrared sensors by fog. The helicopter is shown to be able to follow and navigate ground features while travelling above a layer of fog using only the millimeter-wave imagery.

Millimeter-wave thermal imaging provides a unique autonomous capability for aircraft landing in adverse weather, giving a pilot a comprehensive view of runway location and availability in real time with high fidelity. ThermoTrex Corporation has reported previous results from a Passive Millimeter-wave Camera (PMC) demonstration device. The addition of W-band low-noise amplifiers into the front end of this sparse phased-array thermal imaging camera has improved system thermal sensitivity by 5 dB over that previously reported. Processing upgrades have increased system frame update rate to about 1 Hz, and remote site field testing has established phenomenology relevant to aircraft landing guidance applications. Next-generation hardware design has addressed the issue of aircraft integration using an innovative lightweight, X-band antenna for 89 GHz thermal imaging. A flightworthy demonstration imager using this antenna is currently under construction for 10 Hz operation.

Passive Millimeter-wave Imaging (PMI) technology provides a powerful sensor capability for military and commercial imaging applications, during day or night, and in adverse weather. Recent advances in high-frequency antennas, MMW electronics, and high-speed signal processing, have brought real-time, high-contrast, high-resolution, wide-field PMI into the realm of technological feasibility. However, the substantial size, weight, and cost of previous PMI architectures have proved impractical for all but a few scientific implementations, creating a barrier to large-volume production. This reality has precluded PMI usage in several applications with demonstrable benefits, such as aircraft navigation and landing, radio-silent airborne surveillance/battle damage assessment, concealed weapons detection (CWD), or through-wall imaging. A new PMI architecture has been demonstrated which allows this wide-area, near-real-time staring capability with significant reductions in size, weight, and cost relative to previous designs. Specifics of this new PMI architecture will be presented along with a host of imaging data representing its current capability for airborne imaging, CWD, and through-wall imaging.

Although radiometry has been used in imaging applications for many years, it is only recently that radiometric imaging at millimeter-wave frequencies has become practical and relatively common. Unfortunately while millimeter-wave radiometers offer much better cloud, fog and solid object penetration than either visible or infrared systems, the obtainable spatial resolution is often too poor to produce a useful imagery. In this paper the authors present an array-based concept that has already demonstrated good image performance and is scalable to very large sparse apertures (for high spatial resolution) while still preserving a high frame rate. Approaches and techniques to help mitigate the effects of array sparseness are discussed in some detail.

ThermoTrex Corporation (TTC) has developed an imaging radiometer, the passive microwave camera (PMC), that uses an array of frequency-scanned antennas coupled to a multi-channel acousto-optic (Bragg cell) spectrum analyzer to form visible images of a scene through acquisition of thermal blackbody radiation in the millimeter-wave spectrum. The output of the Bragg cell is imaged by a standard video camera and passed to a computer for normalization and display at real-time frame rates. One application of this system could be its incorporation into an enhanced vision system to provide pilots with a clear view of the runway during fog and other adverse weather conditions. The unique PMC system architecture will allow compact large-aperture implementations because of its flat antenna sensor. Other potential applications include air traffic control, all-weather area surveillance, fire detection, and security. This paper describes the architecture of the TTC PMC and shows examples of images acquired with the system.

A novel microwave imaging technique that is completely passive and has the ability to image objects and scenes that are obscured by smoke, fog, clouds, building materials, and opaque barriers has been developed. The system employs a electrooptic approach to reduce system complexity and cost by orders of magnitude over competing real-time high-resolution microwave imaging systems. A one-dimensional, non-real time passive microwave camera that operates at 17-21 GHz using a 1-meter slotted waveguide antenna was constructed and tested as a proof-of-principle for this approach. Potential applications include aircraft landing, air traffic control, all-weather area surveillance, fire detection, and security.< >