We investigated signatures of small unmanned aerial vehicles (UAV) with different sensor technologies ranging from acoustical antennas, passive and active optical imaging devices to small-size FMCW RADAR systems. These sensor technologies have different advantages and drawbacks and can be applied in a complementary sensor network to benefit from their different strengths.
In this paper we present two system approaches for perimeter surveillance with radar techniques focused on the detection of Micro Aerial Vehicles (MAVs). The main task of such radars is to detect movements of targets such as an individual or a vehicle approaching a facility. The systems typically cover a range of several hundred meters up to several kilometers. In particular, the capability of identifying Remotely Piloted Aircraft Systems (RPAS), which pose a growing threat on critical infrastructure areas, is of great importance nowadays. The low costs, the ease of handling and a considerable payload make them an excellent tool for unwanted surveillance or attacks. Most platforms can be equipped with all kind of sensors or, in the worst case, with destructive devices. A typical MAV is able to take off and land vertically, to hover, and in many cases to fly forward at high speed. Thus, it can reach all kinds of places in short time while the concealed operator of the MAV resides at a remote and riskless place.
For the purpose of getting sensitive information relevant to civil or military security, high-resolution airborne Synthetic Aperture Radar (SAR) provides the possibility to organize missions at short notice regardless of the daylight and of the weather conditions. The use of compact millimeter-wave FMCW SAR systems allows reaching these goals more safely and at lower cost using unmanned lightweight platforms. As a counterpart these platforms are relatively unstable, making the data-processing more difficult. In order to reach optimum focusing quality also in unfavorable flight conditions or for highly non-linear tracks we developed a fast Time-Domain Processor that relies on parallelization using the GPU resources. A production areal processing rate as high as 6 km<sup>2</sup>/h using 20 cm ground pixel spacing on a single PC station was achieved. The processing quality and efficiency is demonstrated using real data from the MIRANDA35 Ka-band SAR system.
An FM-CW radar at 94 GHz has been equipped with antennas with high azimuthal gain and a broad beam in elevation and mounted on a rotating pedestal. The radar covers a range interval of 500 m. A change detection algorithm is used to discriminate invading persons from the background. Tracking of non-stationary target objects is maintained by Kalman filtering. The RCS of detected objects is determined using a pre-calibration against a precision corner reflector at a defined range. The measured RCS is compared with a threshold value to discriminate between dangerous and nondangerous persons. Additional information from a video camera is necessary for a first discrimination between target classes.
The paper describes different technological efforts to demonstrate the usefulness of millimeter wave sensors for security applications. The scope of the work covers a miniature radar in a portal geometry using a near field SAR approach for passenger control and the same radar hardware with a slightly modified scanning approach for luggage inspection employing a three dimensional SAR algorithm.
Another approach to detect concealed weapons and explosives is by using radiometric systems. In principle, a scanner using this technique measures the thermal noise of the radiation reflected by the body. This is equivalent to the temperature on the surface of the body. The main difficulty with this technique is the realization of a fast scanning algorithm.
The status of both approaches is surveyed and typical results are discussed.
Sensors used for security purposes have to cover the non-invasive control of men and direct surroundings of buildings
and camps to detect weapons, explosives and chemical or biological threat material. Those sensors have to cope with
different environmental conditions. Ideally, the control of people has to be done at a longer distance as standoff
detection. The work described in this paper concentrates on passive radiometric sensors at 0.1 and 0.2 THz which are
able to detect non-metallic objects like ceramic knifes. Also the identification of objects like mobile phones or PDAs will
be shown. Additionally, standoff surveillance is possible, which is of high importance with regard to suicide bombers.
The presentation will include images at both mentioned frequencies comparing the efficiency in terms of range and
resolution. In addition, the concept of the sensor design showing a Dicke-type 220GHz radiometer using new LNAs and
the results along with image enhancement methods are shown.
2.1 Main principle
Sensors used for Security purposes have to cover the non-invasive inspection of persons, baggage and letters with the aim to detect weapons, explosives and chemical or biological threat material. Currently, emphasis is placed on system concepts and technologies for this type of applications, employing millimeterwave-, submillimeterwave- and terahertz sensors. This is based on the capability of these frequency bands to look through textiles and the possibility to achieve a geometric resolution which is sufficient to resolve critical items within the necessary range. Using multiple frequencies promises to give more detailed information about the structure of the observed objects. Furthermore, to overcome the limitations of passive millimeter- and submillimeterwave sensors which depend on indirect illumination, systems using miniaturized mmw-radar modules are applied as well. This paper describes two approaches for the detection of concealed weapons, the first using a millimeterwave radiometer on a scanner and the second employing a miniaturized radar module based on a synthetic aperture method.
During recent year's research on radiometric signatures, non-imaging, of the exhaust jet of missiles and imaging, on small vehicles in critical background scenarios were conducted by the mmW/submmW-group at FGAN-FHR. The equipment used for these investigations was of low technological status using simple single channel radiometers on a scanning pedestal. Meanwhile components of improved performance are available on a cooperative basis with the Institute for Applied Solid State Physics (Fraunhofer-IAF). Using such components a considerable progress concerning the temperature resolution and image generation time could be achieved. Emphasis has been put on the development of a demonstrator for CWD applications and on an imaging system for medium range applications, up to 200 m. The short range demonstrator is a scanning system operating alternatively at 35 GHz or 94 GHz to detect hidden materials as explosives, guns, knifes beneath the clothing. The demonstrator uses a focal plane array approach using 4 channels in azimuth, while mechanical scanning is used for the elevation. The medium range demonstrator currently employs a single channel radiometer on a pedestal for elevation over azimuth scanning. To improve the image quality, methods have been implemented using a Lorentzian algorithm with Wiener filtering.
We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency.
The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz.
We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.