The low attenuation of millimeter-wave radiation propagating through sandstorms has created an interest in using
millimeter-wave imagers in desert environments. The ground in desert environments can have significant differences in
polarization properties depending on the angle of observation. Perturbations to the natural desert surface will change
these polarization properties and by using a polarization difference technique these changes are highlighted. This
technique has been applied to millimeter-wave images from a desert environment for several different objects including
holes in the ground, footsteps, and changes to the surface created by digging.
Millimeter-wave (mmW) imaging is presently a subject of considerable interest due to the ability of mmW radiation to
penetrate obscurants while concurrently exhibiting low atmospheric absorption loss in particular segments of the
spectrum, including near 35 and 94 GHz. As a result, mmW imaging affords an opportunity to see through certain
levels of fog, rain, cloud cover, dust, and blowing sand, providing for situational awareness where visible and infrared
detectors are unable to perform. On the other hand, due to the relatively long wavelength of the radiation, achieving
sufficient resolution entails large aperture sizes, which furthermore leads to volumetric scaling of the imaging platform
when using conventional refractive optics. Alternatively, distributed aperture imaging can achieve comparable
resolution in an essentially two-dimensional form factor by use of a number of smaller subapertures through which the
image is interferometrically synthesized. The novelty of our approach lies in the optical upconversion of the mmW
radiation as sidebands on carrier laser beams using electro-optic modulators. These sidebands are subsequently stripped
from the carrier using narrow passband optical filters and a spatial Fourier transform is performed by means of a simple
lens to synthesize the image, which is then viewed using a standard near-infrared focal plane array (FPA).
Consequently, the optical configuration of the back-end processor represents a major design concern for the imaging
system. As such, in this paper we discuss the optical configuration along with some of the design challenges and
present preliminary imaging data validating the system performance.
The unique ability of the millimeter-wave portion of the spectrum to penetrate typical visual obscurants has resulted in a
wide range of possible applications for imagers in this spectrum. Of particular interest to the military community are
imagers that can operate effectively in Degraded Visual Environments (DVE's) experienced by helicopter pilots when
landing in dry, dusty environments, otherwise known as "brownout." One of the first steps to developing operational
requirements for imagers in this spectrum is to develop a quantitative understanding of the phenomenology that governs
imaging in these environments. While preliminary studies have been done in this area, quantitative, calibrated
measurements of typical targets and degradation of target contrasts due to brownout conditions are not available. To
this end, we will present results from calibrated, empirical measurements of typical targets of interest to helicopter pilots
made in a representative desert environment. In addition, real-time measurements of target contrast reduction due to
brownout conditions generated by helicopter downwash will be shown. These data were acquired using a W-band,
dual-polarization radiometric scanner using optical-upconversion detectors.
Passive millimeter wave (mmW) imagers have improved in terms of resolution sensitivity and
frame rate. Currently, the Office of Naval Research (ONR), along with the US Army Research,
Development and Engineering Command, Communications Electronics Research Development
and Engineering Center (RDECOM CERDEC) Night Vision and Electronic Sensor Directorate
(NVESD), are investigating the current state-of-the-art of mmW imaging systems. The focus of
this study was the performance of mmW imaging systems for the task of small watercraft / boat
identification field performance. First mmW signatures were collected. This consisted of a set of
eight small watercrafts; at 5 different aspects, during the daylight hours over a 48 hour period in
the spring of 2008. Target characteristics were measured and characteristic dimension, signatures,
and Root Sum Squared of Target's Temperature (RRSΔT) tabulated. Then an eight-alternative,
forced choice (8AFC) human perception experiment was developed and conducted at NVESD.
The ability of observers to discriminate between small watercraft was quantified. Next, the task
difficulty criterion, V50, was quantified by applying this data to NVESD's target acquisition
models using the Targeting Task Performance (TTP) metric. These parameters can be used to
evaluate sensor field performance for Anti-Terrorism / Force Protection (AT/FP) and navigation
tasks for the U.S. Navy, as well as for design and evaluation of imaging passive mmW sensors for
both the U.S. Navy and U.S. Coast Guard.
Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security
markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or
sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer
wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a
result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum
achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is
used in which the effective aperture size can be increased without the associated volumetric increase in imager size.
However, such systems typically require high frequency (~ 30 - 300 GHz) signal routing and down conversion as well
as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical
upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse
aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the
optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design
tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be
discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system
operating at 35 GHz.
Passive millimeter-wave (mmW) imaging has many specific defense, security and safety applications, due to the fact
that all terrestrial bodies above absolute zero are emissive, and these wavelengths are not scattered by normal obscurants
such as haze, fog, smoke, dust, sandstorms, clouds, or fabrics. We have previously demonstrated results from the
construction of a 94 GHz passive mmW far-field imaging system utilizing optical upconversion, which imaged in only
horizontal polarization. The effective radiometric temperature of an object is a combination of the object's surface and
scattered radiometric temperatures. The surface radiometric temperature is a function of the object's emissivity, which
is polarization dependent. Imaging with radiometric temperature data from both polarizations will allow a greater
identification of the scene being imaged, and allow the recognition of subtle features which were not previously
observable. This additional functionality is accomplished through the installation of added equipment and programming
on our system, thus allowing the simultaneous data collection of imagery in both polarizations. Herein, we present our
experimental procedures, results and passive mmW images obtained by using our far-field imaging system, a brief
discussion of the phenomenology observed through the application of these techniques, as well as the preliminary details
regarding our work on a 3-D passive mmW simulator capable of true physical polarization dependent effective
emissivity and reflectivity rendering, based on the open-source Blender engine.
Millimeter wave (mmW) imaging is continually being researched for its applicability in all weather imaging. While
previous accounts of our imaging system utilized Q-band frequencies (33-50 GHz), we have implemented a system that
now achieves far-field imaging at W-band frequencies (75-110 GHz). Our mmW imaging approach is unique due to the
fact that optical upconversion is used as the method of detection. Optical modulators are not commercially available at
W-band frequencies; therefore, we have designed our own optical modulator that functions at this frequency range.
Imaging at higher frequencies increases our overall resolution two to three times over what was achieved at Q-band
frequencies with our system. Herein, we present imaging results obtained using this novel detector setup, as well as key
imager metrics that have been experimentally validated.
Millimeter-wave imaging is very interesting due to its unique transmission properties through a broad range of atmospheric obscurants such as cloud, dust, fog, sandstorms, and smoke, which thereby enables all-weather passive imaging. Unfortunately, the usefulness of millimeter-wave imagers is often limited by the large aperture sizes required to obtain images of sufficient resolution, as governed by the diffraction limit. To this end, we previously proposed a distributed aperture system for direct non-scan millimeter-wave imaging using an optical upconversion technique. In this proposed approach, an antenna array is employed to sample image signals in the millimeter-wave domain. The sampled millimeter-wave signals are then upconverted to the optical domain using electro-optic modulation techniques. These optical signals are mapped into a similar array on the entrance pupil of the following optical system for direct imaging. Although distributed aperture imaging is not new in both radio astronomy and conventional optical inteferometric imaging, the proposed approach is different in that it physically samples image in the millimeter-wave domain and directly forms the image in the optical domain. Therefore, specific analysis and evaluation techniques are required for the design and optimization of the proposed system. In this paper, we will address these issues, develop techniques to evaluate and enhance the system imaging performance and present methods to optimize the geometric configuration.
Millimeter-wave imaging has the unique potential to penetrate through poor weather and atmospheric conditions and
create a high-resolution image. In pursuit of this goal, we have implemented a far-field imaging system that is based on
optical upconversion techniques. Our imaging system is passive, in which all native blackbody radiation that is emitted
from the object being scanned is detected by a Cassegrain antenna on a rotating gimbal mount. The signal received by
the Cassegrain is passed to an optical modulator which transfers the radiation onto sidebands of a near-infrared optical
carrier frequency. The signal is then passed to a low-frequency photodetector that converts remaining sideband energy to
a photocurrent. Even though optical upconversion can produce loss, our system demonstrates low noise equivalent
powers (NEP) due to the low-noise of the photodetection process. Herein, we present our experimental results and
images obtained by using the far-field scanning system, which was assembled with commercially available components.
In addition, we detail efforts to increase the resolution of the image and to compact the imaging system as a whole.
In previous publications, we have described a novel technique for millimeter-wave detection based on optical
upconversion, carrier suppression, and photodetection. Using these techniques, we have been able to achieve NETD's as
low as 1 K /√Hz in both 35 GHz and 95 GHz atmospheric transmission windows. These results were obtained without
the use of millimeter-wave LNA's or cryogenic cooling, which have previously been requirements for reaching these
In this proceeding, we detail efforts to create a scanning single-pixel imager based on this detector technology. The
configuration developed uses a larger 60 cm aperture in a Cassegrain configuration, which is mounted on a gimbal for
far-field imaging. The described system has been used to collect data for perception experiments on the identification of
small watercraft and some of the imagery collected in that experiment is presented herein. In addition, we discuss
phenomenological observations noted during this data collection.