Airborne ash generated by explosive volcanic eruptions presents a significant danger to aviation. Accurate modelling and predictions of the dispersal of hazardous ash into the atmosphere are currently hampered by uncertainties in the ‘source term’ parameters associated with the initial eruption plume, specifically the amount and size of ash particles released into the atmosphere. Ground based radar offers the means to remotely measure ash reflectivity, however estimation of source term parameters from reflectivity measured by single frequency radar is limited by ambiguity between the contribution of particle size distribution (PSD) and ash concentration in the plume. This means that one of these parameters must be assumed rather than measured directly, leading to uncertainties in forecasting eruption hazards. We report on R4AsH, a close range FMCW radar designed to resolve this ambiguity by simultaneous characterization of falling volcanic ash in a laboratory-controlled environment at three different frequencies: 10, 35 and 94 GHz. The R4AsH design uses a single DDS based chirp generator as a common source, multiplied and upconverted to feed three sets of transmit-receive horn antennas directed at a common target volume such that measurements will give spatially and temporally coincident measurements of falling ash. In addition, there will be independent measurement of the PSD using optical imaging and logging of the landing particle mass to calibrate results and inform analysis. The aim of R4AsH is to develop a triplefrequency inversion algorithm to enable simultaneous retrieval of PSD and ash concentration from radar data suitable for future volcano monitoring systems.
We have completed a 16-channel 340 GHz 3D imaging radar for next-generation airport security screening under the European Union funded CONSORTIS (Concealed Object Stand-Off Real-Time Imaging for Security) project. The radar maps a 1 x 1 x 1 m3 sense volume with ~1 cm3 voxel resolution at multi-hertz frame rates. The radar has been installed in the CONSORTIS system enclosure and integrated with a passenger control system and command module. The full system will ultimately also incorporate a dual-band passive submillimeter wave imager and automatic anomaly detection software for reliable, ethical detection of concealed objects. A large data collection trial on targets of interest has been conducted to support the development of automatic anomaly detection software. Initial threat detection analysis indicates promising results against aviation-relevant objects including simulant dielectric threat materials.
Proc. SPIE. 10189, Passive and Active Millimeter-Wave Imaging XX
KEYWORDS: Radar, Transceivers, Beam steering, Extremely high frequency, Real time imaging, Optical sensors, Imaging systems, Image resolution, High dynamic range imaging, Radar imaging, 3D image processing
The EU FP7 project CONSORTIS (Concealed Object Stand-Off Real-Time Imaging for Security) is developing a demonstrator system for next generation airport security screening which will combine passive and active submillimeter wave imaging sensors. We report on the development of the 340 GHz 3D imaging radar which achieves high volumetric resolution over a wide field of view with high dynamic range and a high frame rate. A sparse array of 16 radar transceivers is coupled with high speed mechanical beam scanning to achieve a field of view of ~ 1 x 1 x 1 m3 and a 10 Hz frame rate.
We present a 220 GHz 3D imaging ‘Pathfinder’ radar developed within the EU FP7 project CONSORTIS (Concealed Object Stand-Off Real-Time Imaging for Security) which has been built to address two objectives: (i) to de-risk the radar hardware development and (ii) to enable the collection of phenomenology data with ~1 cm3 volumetric resolution. The radar combines a DDS-based chirp generator and self-mixing multiplier technology to achieve a 30 GHz bandwidth chirp with such high linearity that the raw point response is close to ideal and only requires minor nonlinearity compensation. The single transceiver is focused with a 30 cm lens mounted on a gimbal to acquire 3D volumetric images of static test targets and materials.
Optoelectronic methods are promising for rapid and highly reconfigurable beam steering across the microwave to the terahertz range. In particular, the photo-injected Fresnel zone plate antenna (piFZPA) offers high speed, wide angle, precise beam steering with good beam quality, to enable video rate millimeter wave imagery with no moving parts. We present a piFZPA demonstrator based on a commercial digital light projector (DLP) and high power laser which achieves steering rates up to 17,500 beams per second at 94 and 188 GHz. We also demonstrate radar imaging at 94 GHz at frame rates of 40 Hz (2D PPI) and 7 Hz (3D volumetric).
The second generation AVTIS ground-based millimeter wave instruments designed for monitoring topography of volcanic lava domes are solid state 94 GHz FMCW rastered, real beam radars operating at ranges of up to ~7 km with a range resolution of ~2.5 m. Operating ten times faster than the prototype with reduced power consumption suitable for battery powered portable use as well as installation at a telemetered site under solar power, we examine their performance as tools for monitoring topography over time and report on the operational statistics both as a radar sensor and as a means of generating digital elevation maps.
Screening crowds for threats requires a stand-off sensor with wide area coverage, high spatial resolution and a high
temporal update rate. We have assessed the capability of the NIRAD high speed 94 GHz FMCW surveillance radar
against this requirement. NIRAD’s sub-degree beamwidth, 25 cm range bins and 10 Hz azimuthal frame rate yield high
resolution radar videos of scenes over ranges from tens to hundreds of meters, capable of tracking people walking or
running around the scene. We present how people are detected and tracked in the scene to enable analysis of their radar
cross section images to reveal signatures which may indicate the presence of a carried threat item.
We report on the use of the All-weather Volcano Topography Imaging Sensor (AVTIS) 94 GHz dual mode radar/radiometric imager for environmental monitoring. The FMCW radar yields 3D maps of the terrain whilst the passive radiometer records brightness temperature maps of the scene. AVTIS is a low power portable instrument and has been used operationally to survey terrain at ranges up to 6 km. AVTIS was originally developed for the ground-based measurement of active volcanoes and has been used successfully to measure the Arenal Volcano in Costa Rica and the Soufrière Hills Volcano on Montserrat. However, additional environmental remote sensing applications are emerging for this technology and we will present details of how the instrument is used to perform terrain mapping and thermal surveys of outdoor scenes. The extraction of digital elevation maps is the primary function of the AVTIS radar mode. We review this process covering range drift compensation, radar cross section (RCS) histogram analysis and thresholding, and georeferencing to GPS. Additionally, we will present how careful calibration enables RCS imaging of terrain and the extraction of the intrinsic reflectivity of the terrain material (normalized RCS, or sigma-nought) which can potentially be used to classify terrain types. We have validated the passive mode imagery against infrared thermal imagery and they show good agreement once the differences in spatial resolution are accounted for. This comparison also reveals differences in propagation due to obscurants (steam, gas, ash) in the two wavebands.
We present a 340 GHz 3D radar imaging test bed with 10 Hz frame rate which enables the investigation of strategies for
the detection of concealed threats in high risk public areas. The radar uses a wideband heterodyne scheme and fast-scanning
optics to achieve moderate resolution volumetric data sets, over a limited field of view, of targets at moderate
stand-off ranges. The high frame rate is achieved through the use of DDS chirp generation, fast galvanometer scanners
and efficient processing which combines CPU multi-threading and GPU-based techniques, and is sufficiently fast to
follow smoothly the natural motion of people.
AVTIS is a compact portable remote sensing instrument originally designed for ground based surveying of active
volcanic lava domes. Its primary mode of operation is active, using a monostatic 200mW 94GHz FMCW radar. The
94GHz signal is provided via a multiplied 7GHz source. Careful choice of a low-noise, highly linear 7GHz source has
extended the range of the radar to at least 7km whilst retaining a range resolution of 1m. We will present results
showing the range resolution and discrimination of separated targets for both natural topography at far ranges (>1km)
and man made targets at close ranges (<1km). For close range imaging, the signal bandwidth can be increased to
improve the range resolution allowing finer quality imagery to be retrieved.
We report on the continued development of our 94GHz dual-mode radar/radiometric imager, AVTIS. To date we have concentrated on refining the radar mode and can now acquire state-of-the-art long range, high resolution radar images. More recently we have worked to improve and integrate the radiometric mode, to complete the dual-mode functionality. One notable problem of the monostatic architecture is that of radar transmitter leakage via the circulator/antenna assembly, even with the transmitter signal heavily attenuated. A related issue is the high level of AM noise in the local oscillator signal due to the IMPATT multipliers and power amplifiers used. This far-off-the-carrier noise, in
combination with the leakage, desensitises the receiver and raises the noise floor to give a receiver noise temperature of
approximately 6000K. This yields a thermal resolution of 2K in 4ms that is considered adequate for the intended
volcanological application. In comparison, tests with a high power Schottky diode multiplier chain as the LO source has
yielded a noise temperature of 750K in a separate radiometer. Thermal calibration is also of concern and we have
implemented an IF noise adding circuit in which the radiometer alternates between the scene and hot and cold IF noise
references. This accounts for dominant receiver fluctuations in the IF amplifiers but sky tipping curves are used to
calibrate the overall response. Whilst the radar is already capable of producing high resolution topographic maps with
surface reflectivity overlaid, it is hoped that co-aligned radiometric brightness temperature data will lead to a better
understanding of the emissivity and surface roughness of the terrain being surveyed.
We present the development and field testing results of a dual-mode radar/radiometric imager operating at 94GHz. This instrument combines an FMCW radar with a total power radiometer in a compact, portable unit which is designed for ground based remote sensing. The radar produces range maps with a range bin resolution of 1m out to a maximum range of approximately 5km and target reflectivity can also be retrieved. The radiometer produces co-aligned thermal images of the scene with a thermal resolution of the order of one kelvin. The instrument, which uses a single 0.45m Cassegrain antenna, is rastered over the scene using a commercial pan and tilt gimbal. Image acquisition times are of the order of tens of minutes. The principal application for which the instrument was designed is to survey volcanic lava domes, irrespective of weather conditions - it has been named AVTIS for All-weather Volcano Topography Imaging Sensor. We will present results obtained with AVTIS from the Soufrière Hills Volcano, Montserrat, as well as more general terrain mapping imagery gathered locally in Scotland. Besides volcano surveying, AVTIS could be deployed for other remote sensing applications.
The design and testing of a close range Passive Millimeter Wave (PMMW) scanning thermal imager is described. While close range PMMW imaging has previously been applied to concealed weapon detection at ranges of a few meters, the imager under development here is designed to focus on targets at a range of a few tens of centimeters. In particular, the main design aim is to produce high resolution thermal maps suitable for medical imaging applications. Imaging at MMW frequencies offers greater- penetration depths in lossy dielectric media than conventional infrared (IR) imagers, although there is an obvious trade-off in spatial resolution.