In this work a novel approach to collision avoidance radar is presented. Leveraging the chipsets currently in development for the automotive industry, with an operational band of 77 to 81 GHz a new frontend has been developed using a distributed frequency swept antenna. The frequency swept antenna is able to steer the beam based on the transmit frequency. Multiple sub-array elements are distributed across the bumper of a vehicle increasing the aperture size of the system for improved beam resolution, thereby leading to better system sensitivity. To this end, system modeling was implemented to study the tradeoff between system sensitivity or range and power, gain, antenna aperture size and number of sub-array elements. The sub-arrays were designed, optimized, manufactured, and characterized using a conformal, flexible, low loss high frequency Rogers 3003 material. The measured far field patterns of the developed antenna array demonstrated consistent angular steering characteristics of -4° to 8° over the frequency from 75 to 85GHz with minimum reflection. The developed sub-array elements are cascaded and then synchronized using electronically controlled, high resolution, wideband, low loss W-band phase shifters. To drive such a large distributed array, we also focused on the development of high-resolution or analog phase shifters with 360 degrees coverage from 77-81GHz. The phase shifter chip was designed based on three-vector method and manufactured by leveraging SiGe foundry run. RF integration of the fabricated chips along with the control circuit was also conducted to demonstrate fully phase control over the band of interest. The packaged antenna subarrays and phase shifters are integrated together to form a distributed array. Through synchronization, coherent operation of the system can be established, enhancing the angular resolution of the system. The developed antenna array will be integrated with a Frequency Modulated Continuous Wave (FMCW) transceiver for applications of automobile radars.
Sensors operating in the millimeter wave region of the electromagnetic spectrum provide valuable situational awareness in degraded visual environments, helpful in navigation of rotorcraft and fixed wing aircraft. Due to their relatively long wavelength, millimeter waves can pass through many types of visual obscurants, including smoke, fog, dust, blowing sand, etc. with low attenuation. Developed to take advantage of these capabilities, ourmillimeter wave imager employs a unique, enabling receiver architecture based on distributed aperture arrays and optical upconversion. We have reported previously on operation and performance of our passive millimeter wave imager, including field test results in DVE and other representative environments, as well as extensive flight testing on an H-1 rotorcraft. Herein we discuss efforts to improve RF and optical component hardware integration, with the goal to increase manufacturability and reduce c-SWaP of the system. These outcomes will allow us to increase aperture sizes and channel counts, thereby providing increased receiver sensitivity and overall improved image quality. These developments in turn will open up new application areas for the passive millimeter wave technology, as well as better serving existing ones.
This paper will discuss the development of a millimeter-wave (mm-wave) receiver module used in a sparse array passive imaging system. Using liquid crystal polymer (LCP) technology and low power InP low noise amplifiers (LNA), enables the integration of the digital circuitry along with the RF components onto a single substrate significantly improves the size, weight, power, and cost (SWaP-C) of the mm-wave receiver module compared to previous iterations of the module. Also comparing with previous generation modules, the operating frequency has been pushed from 77 GHz to 95 GHz in order to improve the resolution of the captured image from the sparse array imaging system.
Degraded visual environments create dangerous conditions for aircraft pilots due to loss of situational awareness and/or ground reference, which can result in accidents during navigation or landing. Imaging in millimeter wave spectral bands offers the ability to maintain pilot's situational awareness despite DVE with a "see-through" imaging modality. Millimeter waves exhibit low atmospheric attenuation as well as low scattering loss from airborne particulates, e.g. blowing sand, dust, fog, and other visual obscurants. As such, Phase Sensitive Innovations (PSI) has developed a passive, real-time mmW imager to mitigate brownout dangers for rotorcraft. The imager consists of a distributed aperture array with conversion of detected mmW signals to optical frequencies for processing and image formation. Recently we performed operationally representative flight testing of our sensor while imaging various natural and manmade objects. Here we present imagery collected during these tests as it confirms the performance of the sensor technology and illustrates phenomenology encountered in the mmW spectrum.
The transmission characteristics of millimeter waves (mmWs) make them suitable for many applications in defense and security, from airport preflight scanning to penetrating degraded visual environments such as brownout or heavy fog. While the cold sky provides sufficient illumination for these images to be taken passively in outdoor scenarios, this utility comes at a cost; the diffraction limit of the longer wavelengths involved leads to lower resolution imagery compared to the visible or IR regimes, and the low power levels inherent to passive imagery allow the data to be more easily degraded by noise. Recent techniques leveraging optical upconversion have shown significant promise, but are still subject to fundamental limits in resolution and signal-to-noise ratio. To address these issues we have applied techniques developed for visible and IR imagery to decrease noise and increase resolution in mmW imagery. We have developed these techniques into fieldable software, making use of GPU platforms for real-time operation of computationally complex image processing algorithms. We present data from a passive, 77 GHz, distributed aperture, video-rate imaging platform captured during field tests at full video rate. These videos demonstrate the increase in situational awareness that can be gained through applying computational techniques in real-time without needing changes in detection hardware.
In this presentation we will discuss the performance and limitations of our 220 channel video rate passive millimeter wave imaging system based on a distributed aperture with optical upconversion architecture. We will cover our efforts to reduce the cost, size, weight, and power (CSWaP) requirements of our next generation imager. To this end, we have developed custom integrated circuit silicon-germanium (SiGe) low noise amplifiers that have been designed to efficiently couple with our high performance lithium niobate upconversion modules. We have also developed millimeter wave packaging and components in multilayer liquid crystal polymer (LCP) substrates which greatly improve the manufacturability of the upconversion modules. These structures include antennas, substrate integrated waveguides, filters, and substrates for InP and SiGe mmW amplifiers.
KEYWORDS: Imaging systems, Upconversion, Extremely high frequency, Antennas, Sensors, Image processing, Near field optics, Cameras, Optical scanning systems, Control systems
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, 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 desired for many applications. Accordingly, lens-based focal plane systems and scanning systems tend to require large aperture optics, which increase the achievable size and weight of such systems to beyond what can be supported by many applications. 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. This distributed aperture system is realized through conversion of the received mmW energy into sidebands on an optical carrier. This conversion serves, in essence, to scale the mmW sparse aperture array signals onto a complementary optical array. The side bands are subsequently stripped from the optical carrier and recombined to provide a real time snapshot of the mmW signal. Using this technique, we have constructed a real-time, video-rate imager operating at 75 GHz. A distributed aperture consisting of 220 upconversion channels is used to realize 2.5k pixels with passive sensitivity. Details of the construction and operation of this imager as well as field testing results will be presented herein.
The demand for all-weather, day-night imaging systems has been spurred by calls for persistent surveillance in security
and defense applications, and increased safety in military aviation, such as carrier landings in fog and helicopter landings
in sand and dust. To meet these demands requires systems that offer robust imaging capabilities. Whereas visible and
infrared systems can provide high resolution imagery in a small-sized package, they are hindered by atmospheric
obscurants, such as cloud cover, fog, smoke, rain, sand, and dust storms. Millimeter wavelengths, on the other hand, are
not and passive millimeter wave imaging may be one method to reduce, or perhaps even eliminate, the impact of low
visibility atmospheric conditions. In this paper we examine the scattering from rotorcraft induced dust clouds using
Sandblaster dust particle density data. We examine the effect of Mie scattering as a function of particle size and
operating wavelength and conclude that W-band operation yields the highest resolution imaging while still maintaining
"see-through" imaging capability.
Beam steering is an enabling technology for establishment of ad hoc communication links, directed energy for infrared
countermeasures, and other in-theater defense applications. The development of nonmechanical beam steering
techniques is driven by requirements for low size, weight, and power, and high slew rate, among others. The
predominant beam steering technology currently in use relies on gimbal mounts, which are relatively large, heavy, and
slow, and furthermore create drag on the airframes to which they are mounted. Nonmechanical techniques for beam
steering are currently being introduced or refined, such as those based on liquid crystal spatial light modulators;
however, drawbacks inherent to some of these approaches include narrow field of regard, low speed operation, and low
optical efficiency. An attractive method that we explore is based on optical phased arrays, which has the potential to
overcome the aforementioned issues associated with other mechanical and nonmechanical beam steering techniques.
The optical array phase locks a number of coherent optical emitters in addition to applying arbitrary phase profiles
across the array, thereby synthesizing beam shapes that can be steered and utilized for a diverse range of applications.
Currently, brownout is the single largest contributor to military rotary-wing losses. Millimeter-wave radiation
penetrates these dust clouds effectively, thus millimeter-wave imaging could provide pilots with valuable situational
awareness during hover, takeoff, and landing operations. Herein, we detail efforts towards a passive, video-rate
imager for use as a brownout mitigation tool. The imager presented herein uses a distributed-aperture, optically-upconverted
architecture that provides real-time, video-rate imagery with minimal size and weight. Specifically, we
detail phenomenology measurements in brownout environments, show developments in enabling component
technologies, and present results from a 30-element aperiodic array imager that has recently been fabricated.
We report on our initial results of passive, real-time imaging in the Q-band using a distributed aperture and optical
upconversion. The basis of operation is collection of incident mmW radiation by the distributed aperture, as embodied
by an array of horn antennas, which is then amplified and upconverted to optical frequencies using commercially
available electro-optic modulators. The non-linear mixing of the modulators creates sidebands containing the mmW
signal with both amplitude and phase preserved. These signals are relaunched in the optical domain with a homothetic
mapping of the antenna array. The optical carrier is stripped via dielectric stack filters and imagery is synthesized from
the sidebands using the Fourier transform properties of a simple lens. This imagery is collected using a standard nearinfrared
camera with post-processing to enhance the signal of interest and reduce noise. Details of operation and
presentation of sample imagery is presented herein.
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.
KEYWORDS: Imaging systems, Calibration, Sensors, Temperature metrology, Data modeling, Visualization, Antennas, Polarization, Absorption, Data acquisition
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) 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 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 (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 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.
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