ULTRA: Wideband Ground Penetrating Radar Filippo Parrini 1 , Matteo Fratini 1 , Massimiliano Pieraccini 1 , Carlo Atzeni 1 , Gaetano De Pasquale 2 , Piernicola Ruggiero 2 , Francesco Soldovieri 3 , Adriana Brancaccio 4 1 Department of Electronics and Telecommunications, University of Florence, Florence, Italy 2 IDS - Ingegneria dei Sistemi S.p.A., Pisa, Italy 3 Istituto per il Rilevamento Elettromagnetico dell’Ambiente – CNR, Naples, Italy 4 Dipartimento Ingegneria dell’Informazione - Seconda Università di Napoli, Aversa (CE), Italy Abstract — The frequency range employed in Ground Penetrating Radar (GPR) systems is generally limited to 2GHz because media loss increases dramatically at higher frequencies. Nevertheless, a series of applications exists in the Cultural Heritage field where the development of high frequency systems would significantly benefit, in terms of resolution, from an increased operating band. This article presents ULTRA, a wideband GPR (800MHz - 4000MHz) system, using a CW-SF technique developed for this type of surveying. Index Terms — Ground Penetrating Radar, GPR, CW-SF, Ultra Wideband, I/Q. I. INTRODUCTION The frequencies commonly used in Georadar applications cover the range 100MHz-2GHz [1], the lower operating frequencies allowing deep surveying, but with the great disadvantage of a corresponding reduction in resolution. This problem is particularly evident in high loss mediums (particularly in wet solids), where attenuation can exceed 50dB/m. However a series of applications exists, mainly in the field of masonry and Cultural Heritage, in which the presence of very low water content makes surveying possible with high frequency systems [2]. These factors have motivated the development of a wideband system for these types of applications. II. BACKGROUND The developed system employs the CW-SF (Continuous Wave Stepped Frequency) technique, which is preferable to a pulsed one because it allows to obtain a controlled bandwidth system with predictable performance. Moreover, this technique can solve most of the problems related to the temporal behaviour of the antenna, which is a key problem of pulsed systems [3]. In addition the CW-SF technique can increase penetration depth due to its extended system dynamic. The dynamic range obtained in CW-SF systems (using a narrow bandwidth tuned receiver) is much higher if compared to pulsed systems. Increasing the scan repetition frequency, and therefore the acquisition speed of the receiver, in a pulsed system, up to several hundred scan/sec, the theoretical advantage in system performance of a CW-SF is approximately 30-40dB. III. TRANSMITTER ARCHITECTURE On this basis, we analyzed the stepped frequencies generation technique. Standard solutions are based on Phase Locked Loop Synthesizers (PLL), Direct Digital Synthesizers (DDS), or mixed techniques employing both types of synthesizers [4], however the necessity of generating a bandwidth greater than 3GHz strongly limits the use of the DDS based architecture. The developed system bases the frequency synthesis on a couple of PLLs that cover a bandwidth of approximately 900MHz, while the entire range of operating frequencies is subdivided for architectural reasons into four bands which are alternatively covered either by the single PLLs or by a signal obtained by mixing both PLLs output signals. IV. RECEIVER ARCHITECTURE The receiver is based on a single frequency conversion scheme, with a successive base band I/Q (in phase/quadrature) demodulation. Three methods of demodulation are generally employed in CW-SF systems: wideband I/Q demodulation (typical of homodyne receivers), sampled IF with software I/Q demodulation [5], and finally the I/Q demodulation at fixed IF (typical of heterodyne receivers). The last solution is preferable to wideband I/Q demodulation because even if it is technically possible to use wideband I/Q demodulators, their overall performance is variable across the entire bandwidth (phase and gain errors) [6] and they strongly bind the GPR performance. However the development of a wideband system contrasts with the necessity of using only one fixed IF: based on these considerations, a system has been developed with a specific IF for each sub-band and a single I/Q demodulator. In this way, we always use the I/Q demodulator at the same frequency (in each sub-band). The demodulation section is preceded by a frequency conversion stage, necessary for intermediate frequency conversion of the received signals (different for each sub- band). Using this architecture, the entire system can be developed using just two PLLs, a I/Q demodulator, and three Proceedings of the 3rd European Radar Conference 2-9600551-7-9  2006 EuMA September 2006, Manchester UK 182