Wideband measurement strategies: from RADAR to passive wireless sensors
... and how passive wireless sensors were/are used by intelligence agencies.
RADAR systems are instrinsically wideband devices, with a range resolution inversely proportional to the probe signal bandwidth. Recording wideband signals is a challenging task, with high data rates often yielding low resolution samples and hence poor range. Multiple strategies have been investigated to reduce the recording rate, including stroboscopy (assuming a static environment), downconversion or frequency stepped measurements, all of which are well suited to feed Software Defined Radio applications. In addition to monitoring passive reflectors, cooperative targets can be designed to reflect a signal whose delay is not representative of distance or velocity but a physical quantity. One early application of such an insight has been the bug placed by Russians in the American ambassador house, modulating an incoming continuous wave illumination signal to an amplitude modulated backscattered signal. Although the leaked NSA documents hint at such techniques still being used today, we will be interested in more daily applications in which sensors are designed to return a signal representative of an identifier (ID-tag) or a physical property.
While Software Defined Radio (SDR) is mostly concerned with data transmission, especially for communication purposes, one original aspect of RADAR application of SDR is to consider the complete system, from emitter to receiver, and including the design of dedicated cooperative targets acting as sensors. While the backscattered signal from the target includes a signature representative of a physical quantity (amplitude, frequency, time of flight), the SDR approach provides the flexibility needed to adapt the emitted signal to the target signature. As an example, when the signature from a narrowband resonator is the resonance frequency -- shifting for example with temperature -- the flexibility of SDR allows for focusing on the spectral features under interest and prevents wasting time on regions of the spectrum where the signature is known, from prior measurement, not to lie. While the original Theremin  spying experiment  was using a dielectric cavity resonator whose boundary conditions were varying with a thin membrane position -- vibrating under varying pressure waves from the ambassador voice -- to convert the incoming Continuous Wave (CW) to a backscattered Amplitude Modulated (AM) signal, the signal to noise ratio is plagued by clutter from environmental reflectors. Time gating, as trivially implemented in SDR by a delay between switching between emission and reception for clutter to fade out before the sensor signal is detected, offers the opportunity for improved detection range which is hardly accessible to purely hardware implementation (eg Frequency Modulated Continuous Wave -- FMCW) RADAR strategies. The main challenge of SDR implementation of RADAR techniques is the necessary wideband emitter or receiver: various techniques have been envisioned to overcome the narrowband limitation of high resolution Analog/Digital sampling, including Frequency Stepped Continuous Wave or stroboscopy. These various approaches will be discussed, with hopefully some low cost demonstration of remote sensing using commonly available hardware, including acoustic filters acting as delay lines .
 A. Glinsky, Theremin: Ether Music And Espionage, University of Illinois Press (2005)
 P. Wright, Spycatcher, Heinemann (1987)
 J.-M Friedt, A. Hugeat, A low cost approach to acoustic filters acting as GPR cooperative targets for passive sensing, 8th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), 2015