Passive VHF radar for ionospheric physics

Recent technological advances enable a new class of passive radar instrumen ts. These radars have no dedicated transmitter, observing serendipitous sca tter of existing sources. Such radars may have very high performance and co st far less than conventional radars. The resulting equipment is essentiall y reduced to simple antennas, desktop computers, and Global Positioning Sys tem equipment. The safety hazards, interference problems, licensing issues, and financial costs associated with high-power transmitters are conspicuou sly absent. We will offer general design considerations and describe our ow n instrument, which observes the scatter of commercial FM broadcasts. Our s ystem provides far better range and Doppler resolution than any conventiona l radar used in ionospheric coherent scatter studies, and is completely fre e of any range or Doppler aliasing problems. There are two principal drawba cks to passive radars: the "front end" signal processing cost is very large , and there is a significant data transport problem. However, spectacular a dvances in low-cost computing and internet bandwidth have rendered these pr oblems quite easy to solve. (C) 2001 Elsevier Science Ltd. All rights reser ved.