Microwave holography in detection of hidden objects under the surface and beneath clothes Andrey Zhuravlev 1 , Alexander Bugaev 2 , Sergey Ivashov 1 , Vladimir Razevig 1 , and Igor Vasiliev 1 1 Bauman Moscow State Technical University, Remote Sensing Laboratory, 5, 2 nd Baumanskaya str., 105005 Moscow, Russia, azhuravlev@rslab.ru, sivashov@rslab.ru, vrazevig@rslab.ru, ivasiliev@rslab.ru 2 Moscow Institute of Physics and Technology, Institutskii per., 9, 141700, Dolgoprudny, Moscow Region, Russia, bugaev@cplire.ru Abstract Proposed in the article design of a continuous wave radar with programmable frequency switching and quadrature receiver is targeted as base component to a variety of radars: ground penetrating radar, body scanner, and bio-radar. Algorithms of reconstructing holograms are outlined for planar and circular apertures. High achievable resolution is demonstrated in experiments involving measurements and reconstruction of holograms in air and opaque media. 1. Introduction As an example of successful use of microwave holography it is possible to mention holographic subsurface radar RASCAN [1, 2]. The name holographic was given to this type of radar as it uses the same principle as it used in optical holography. There are two waves that influence the output signal: object wave that was reflected by a buried object and another due to receive-transmit antenna coupling and reflection from the surface. This interference pattern registered over sufficiently extent area of interest reveals highly detailed picture of what is beneath the ground. The radar is very sensitive to buried object depth variation though does not give absolute depth information. Post processing of images is usually not required due to attenuation and antenna directivity pattern. A fragment of a buried object is only visible when it is in close proximity to the antenna. The resulting hologram of a shallowly buried object resemble shadow silhouette of the object as diffraction effects are still weak at small distances and attenuation and near field sensitivity of the antenna result in almost no interference rings away from nadir. As the depth of a buried object increases, diffraction effects deteriorate the picture, making it look like interference pattern with distinctive interference stripes. In such a situation reconstruction algorithms are required. And even if reconstruction is usually not required in subsurface applications at shallow depths, it is necessary when the problem of detecting hidden objects under clothes emerges or the distance to a buried object increases. The rest of the article is organized as follows. The next section covers principal schematic of the radar. In Section 3 a reconstruction algorithm is outlined for the plane aperture with experimental examples. Circular aperture is considered in Section 4. Several application areas suggested in Conclusion. 2. System Design The schematic of the radar is presented in Fig. 1. The design of the radar is based on available IC components: phase-locked loop frequency synthesizers and voltage-controlled oscillators (VCO). Depending on desired frequency range different VCO and synthesizers are available. In this project three radars sharing the same principle schematic were designed and assembled in the following frequency ranges: 3.6 – 4.0, 5.8 – 6.8, and 14 – 15 GHz. The reference frequency for the synthesizer is drawn from a 20 MHz crystal oscillator. Charge pump tuning pulses (CP output) are filtered, amplified, and fed into VCO. One output of VCO feeds transmit antenna and local oscillator input of the frequency mixer while other provides divided by 8 (for 15 GHz radar) feedback frequency. The signal from the receive antenna is mixed with in-phase and shifted by 90º components of the local oscillator, giving I and Q components of the received signal (direct conversion receiver). I and Q components are sampled by an ADC at programmable intervals. The microcontroller unit (MCU) uses SPI bus to initialize and control the synthesizer and ADC. Switching between frequencies is accomplished by programming internal registers of the