Full-Polarization Characterizations of Time-Varying Targets via Software-Defined Radar

Dual-polarization radar is typically associated with high-cost national assets such as NASA’s SMAP satellites and NOAA’s weather radar. However, with the advent of software defined radio (SDR) technologies, platforms are becoming available that provide capability to implement low-cost radars having tunable operations, multiple channels and programmable signal processing. While SDR platforms are often not well-suited for radar implementation, radar operation can be achieved with significant customization efforts.

Many researchers have demonstrated SDR-based radar, but very few have considered dual-polarized (dual-pol) SDR-based radar, and fewer yet, exploit the diverse polarization responses obtained from a coherent, full-polarization radar operation. This latter implementation typically requires consistent phase (and gain) relationships among all transmit and receive ports as a function of frequency and enables repeatable, phase-consistent complex-valued characterizations, constituting the target response matrix. Very few SDR radar researchers, if any, attempt to implement a phase-aligned radar system, which preserves the exact phase relationships among the components of the target response matrix. Such phase-aligned operation offers capability to synthesize and analyze signals with an accurate sense of the polarization, assuming proper antenna orientations.

The full-polarization target matrix is commonly referred to as the Sinclair backscatter matrix in monostatic radar configurations or the forward (back-) scattering matrix in bistatic configurations for orthogonal transmit polarizations and orthogonal receive polarizations. This target matrix can reveal more information than is possible with a single-polarization (single-pol) radar, but requires more sophisticated multiport radar operation and signal processing. In particular, phase coherent (or phasealigned) transmission using two orthogonal polarization states is required, and these must be received on coherent (or phase-aligned) orthogonally-polarized receive ports. Furthermore, the implementation must limit cross-coupling between the transmittoreceive channel pairs.

An important constraint associated with radar operation is the signal isolation between the transmit and receive functions. Poor isolation limits the range at which targets can be sensed. Conventional systems operate in a monostatic mode and employ circulators along with carefully designed waveforms to limit interference induced by transmit signal reflections from the antenna interface and the local environment. These architectures are known to provide reciprocal target responses between the cross-polarized responses (such as V/H and H/V transmit/receive polarization components) for reciprocal (e.g., non-ferrous) targets. For off-the-shelf circulators

and continuous-wave waveform designs, these traditional architectures lead to poor isolation. To overcome the limitation of circulators, we instead employ a pseudomonostatic architecture that incorporates a septum between the transmitter and the receiver. The architecture achieves suitable isolation, limiting blanking to the first few range gates in our experiments, and when coupled with complementary waveform designs, the architecture provides an additional diversity through non-reciprocal VH and HV responses.

Typical characterizations for a conventional single-pol radar system include target position (range and angle), and velocity. Common examples of these types of radars include air traffic control radars and early-generation Doppler weather radars. In the literature, one relatively new type of target characterization is based on microDoppler target features, or target motions exhibiting small displacements. In singlepol radars, the micro-Doppler phenomenon has successfully been exploited to characterize target vibrations. However, this approach is known to have a prominent limitation: target vibrations can induce polarization-dependent responses that are largely mismatched to the polarization of a single-pol radar.

In contrast to the micro-Doppler approach, a dual-polarization radar can utilize polarization-based microvibrometry to measure changes in the target’s polarization while also providing polarization-diverse micro-Doppler features. Complex target reflections often exhibit unpredictable (i.e., arbitrary) polarization states. Obtaining a full-polarization characterization of a target allows for transmit and receive polarization dependencies to be leveraged in post-processing. This permits a more complete analysis of target attributes, like vibration, and offers a capability not possible with a single-pol radar system. This approach is shown experimentally to generate more diverse responses, relative to a single-pol radar characterization.

Five different radar prototypes of varying complexities have been designed, developed, built, and/or integrated using SDR platforms to demonstrate the capabilities of coherent, dual-polarization radars. The prototypes are based on architectures incorporating single and multiple SDRs. Different commercial-off-the-shelf (COTS) SDR platforms and architectures offer various trade-offs in radar design related to cost, signal isolation, range resolution, and magnitude/phase coherency. To achieve phase coherency, a calibration procedure that does not require an external signal source, was designed and implemented to provide magnitude/phase coherence across all transmit and receive channels. Laboratory experiments were performed to validate the technique’s effectiveness.

The developed radar systems were also used in numerous field experiments conducted on the campus of the University of Notre Dame and a steel manufacturing plant in New Carlisle, IN. Full-polarization operation of these systems were demonstrated and compared. The most capable of the radar systems was utilized in various experiments to analyze dynamic responses from a host of different targets. In each instance, target features of interest were hypothesized and then verified with simulated and experimental results. Where possible, field tests were also compared against non-radar-based sensor systems (e.g., vibration measurement systems) to provide a baseline comparison of target behaviors.

Each testing location provided unique challenges that necessitated the use different signal processing schemes to combat undesirable attributes in target behavior and/or environmental clutter. Techniques which highlight time-varying attributes are considered alongside longer-term statistical analyses, including a novel method based on polarimetric-vibrational entropy, to help characterize steady-state target responses. The results from these experiments demonstrate capabilities of dual-pol radars and further support the advantage of polarimetric characterizations over those possible with single-pol radars.