J. L. Garry

Investigations toward multistatic passive radar imaging

Potential for imaging in passive multistatic radar systems is investigated primarily in terms of illuminator type and coherency. A typical set of transmitters in the UHF and VHF bands based upon the local illuminators in the Columbus, Ohio region is presented to constitute a realistic passive imaging environment. A passive radar signal model is then developed and demonstrates one possible processing implementation for imaging across multiple distributed illuminators. From this, the spatial frequency representation of an airborne target traversing a common flight path is presented. This k-space formulation is assessed as a tool for predicting imaging performance, along with potential limitations of the approach for accurately modeling realistic imaging scenarios of multistatic passive radar systems. Finally, simulation results show the -3 dB point target response to be <;1 m for FM transmitters and <;0.2 m for DTV, for the realistic Columbus-based imaging environment.

Direct signal suppression schemes for passive radar

Passive radar systems must detect the presence of a target response many orders of magnitude weaker than the direct signal interference. Even when digitally modulated waveforms with favorable ambiguity surfaces are employed, the floor of the ambiguity surface sets significant limitations on the minimum detectable signal. Suppression of this direct path and close-in clutter from the surveillance waveform is crucial for maximizing the dynamic range which increases the useful detection range of the system. Presented here is an evaluation of various direct signal suppression schemes - both block and adaptive filtering - tested against various metrics on experimental collected passive radar data using North American digital television (DTV) waveforms. Results show the fast block least-mean squares adaptive filter to be significantly faster than existing algorithms with superior suppression performance. Strategies for selecting filtering schemes depending on the task at hand are also discussed.

A narrow band imaging technique for passive radar

In this paper we report on initial results into Doppler imaging, a technique by which both down and cross-range resolution are obtained with low bandwidth waveforms. Conventional radar imaging utilizes aperture synthesis to obtain cross-range resolution and relies on wideband waveforms for the downrange resolution. More recently, tomography has been proposed an alternative able to generate high-resolution imagery but without necessarily requiring wideband waveforms. Here we introduce the theory behind a third alternative, Doppler imaging and consider how it might be applied to passive bistatic radar. We present results that include the 2D image of two point targets separated by just 0.6 m illuminated using a pure tone signal in an anechoic chamber. The resolution limits using the technique are also shown, using a Fourier domain representation.

Wideband DTV passive ISAR system design

Advanced operational modes for passive radar, including inverse synthetic aperture radar (ISAR), require particular consideration with respect to the receiver hardware. The spatial and spectral diversity of passive illuminators necessitates a wideband system with a large dynamic range beyond the already demanding requirements for single channel system due to direct signal interference. Here, a novel, hybrid high/low side downconversion scheme is shown to maximize the number of illuminators without sacrificing receiver dynamic range. Features of American digital television (DTV) signals are demonstrated along with their impact on passive systems. An extension to the efficient calculation of the range-Doppler surface is presented. This technique preserves integration gain and exceeds the unambiguous range/Doppler tradeoff of conventional pulsed radar systems. Finally, initial experimental ISAR results for a commercial airliner are shown to demonstrate discernible structure using a single 6 MHz DTV illuminator.

Doppler imaging for passive bistatic radar

In this paper we report on initial results into Doppler imaging, a technique by which both down and cross-range resolution are obtained with low bandwidth waveforms. Conventional radar imaging utilizes aperture synthesis to obtain cross-range resolution and relies on wideband waveforms for the downrange resolution. More recently, tomography has been proposed an alternative able to generate high-resolution imagery but without necessarily requiring wideband waveforms. Here we introduce the theory behind a third alternative, Doppler imaging and consider how it might be applied to passive bistatic radar. We present results that include the 2D image of two point targets separated by just 0.6 m illuminated using a pure tone signal in an anechoic chamber. The technique lends itself well to passive radar imaging.

Utilization of terrestrial navigation signals for passive radar

Passive radars use transmissions from signals of opportunity in order to locate targets. The absence of a dedicated transmitter reduce the cost, complexity and detectability of the radar as compared to active radar systems. This paper investigates — for the first time — the use of signals of a metropolitan-wide terrestrial positioning system for passive radar. We present simple theoretical analysis predicts the efficacy of these terrestrial navigation signals for this application. Additionally, we demonstrate that aircraft can be observed in a passive radar system exploiting a terrestrial positioning transmission at 10 km bistatic range.

Evaluation of Direct Signal Suppression for Passive Radar

Passive radar (PR) systems must be able to detect the presence of a target signal many orders of magnitude weaker than the direct signal interference (DSI). Due to the continuous nature of most PR signals, this interference, rather than thermal noise, determines the sensitivity of the system. Suppression of DSI and clutter prior to range-Doppler processing is crucial for maximizing the effective dynamic range, to increase detection range and improve overall system performance. A number of time-domain adaptive filtering techniques have been proposed to mitigate the effects of DSI, with varying levels of success. As such, an investigation of the primary factors affecting suppression performance is presented, using Advanced Television Systems Committee digital television (DTV) waveforms as an example, through simulation and extensive experimental trials. A number of spectral and spatially diverse DTV signals are considered to analyze suppression performance under a wide range of realistic scenarios. In particular, the fast block least mean squares filter is shown to provide good suppression performance with low computational requirements. Results of this analysis can be used to predict PR performance and stability. Practical metrics, such as suppression runtime and ease of implementation, also serve to counsel selection of DSI mitigation algorithms for experimental systems.

Array based passive radar target localization

In this paper, a procedure for target localization with digital television based passive radar is developed. Considerations for beamforming, including array calibration and range-Doppler processing are presented, which establish a comprehensive framework for determining the 3D position of a target in space. Experimental validation of these concepts using a 6 element linear array demonstrates angular localization accuracy far exceeding the resolution capabilities of the array itself. These results were verified with air truth provided by an automatic dependent surveillance-broadcast (ADS-B) system on board a commercial flight. Assuming knowledge of the targets elevation angle, location accuracies within 100 m were obtained for an Airbus A320 air target at a range of 6 km.

Subarray processing for passive radar localization

The addition of an array to a passive radar increases the possible ways to process data for different purposes. The digital beamforming capability, used in a passive radar environment with multiple transmitters and geometries, can be used to increase gain in a particular direction, reduce signal interference in another, or to estimate the direction of a transmitting source. This paper presents a method to achieve multiple objectives simultaneously, in this case improving both detection and localization, by processing data from subsets of receiver channels at different stages. It is demonstrated using experimental data that through this scheme, a relatively small array of 6 antenna elements can further reduce direct signal interference by 35 dB in a scenario where the air target is in a near forward-scatter bistatic configuration; at the same time retaining its ability to estimate the target direction of arrival.

Distributed multipath effects with passive radar

Previously unreported phenomena of distributed target multipath observed with passive radar systems are presented and discussed. Trail-like signatures in the range-Doppler surface are shown from UHF Digital Television passive experimental trials with commercial airliner targets. Simulated results for distributed ground multi-bounce paths demonstrate a high degree of similarity to the empirical observations, validating the cause of this atypical range-Doppler response. Both experimental and simulated results demonstrate the phenomenology which can arise as a result of the multi-bounce distribution, such as Doppler expansion and inversion. Potential methods for exploiting the multipath components to improve radar system performance are also discussed.