Stepped-Frequency Distributed Radar for Through-the-Wall Sensing

A technical analysis of the effectiveness of distributed radar for through-the-wall sensing applications.

April 1, 2023

Army Research Laboratory, Adelphi, MD

The image shows a stepped-frequency radar transceiver: (a) RFSoC by Xilinx with radar firmware by Alion/ HII, and (b) custom transmitter/receiver (Tx/Rx) filter-and-amplifier PCBs powered by 28 VDC. (Image: Army Research Laboratory)

The authors are studying the application of distributed radar to through-the-wall sensing. The intended operational scenario for this technology is detection and identification of personnel and weaponry located inside of a building from a (safe) remote standoff distance outside of that building.

The radar architecture and signal-processing algorithms used for this study are similar to the designs implemented by the U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL) for buried-and concealed-surface target detection; the current radar transmits and receives higher frequencies. For this study, experiments were conducted at ARL’s Adelphi Laboratory Center (ALC) in Building 507 (the “sandbox” area) using the indoor low-metal two-story plywood structure. The controlled environment used to test the distributed radar is the same as the low-metal environment used to test ARL’s harmonic radar against electronic targets.

The radar transceiver was developed by Alion Science & Technology, recently acquired by Huntington Ingalls Industries (HII). The waveform generation-and-capture core of the radar is the Zynq UltraScale+ radio-frequency system-on-a-chip (RFSoC) manufactured by Xilinx. The RFSoC evaluation board, packaged by Alion/HII and controllable by a graphical user interface (GUI) over Ethernet, is shown in Fig. 1a. The GUI is labeled “non-linear radar” because the hardware and firmware were originally developed with the ability to transmit a (lower) band of frequencies while receiving a harmonic (higher) band of frequencies. Two such PCBs are visible in Fig. 1b; both are powered from a single 28-VDC power supply.

The RFSoC-based radar transmits and captures stepped-frequency pulses, appending a single file with two channels of received pulses, for as long as the user desires. Short data collections, when no targets were present or moving in the scene, lasted under 1 minute. Longer data collections, when both authors walked back and forth inside of the building, lasted about 8 minutes.

For the experiments reported here, the radar was always run in “linear” mode (i.e., the transmitted and received frequencies were identical). The radar captures data as in-phase and quadrature modulation on the instantaneous stepped carrier (I/Q data), and it computes an inverse fast Fourier transform (iFFT) to generate individual range profiles.

The data collected in this study indicates that antenna pairs looking into a low-metal building and at right angles to each other are able to detect multiple moving targets when those targets are otherwise not visible from outside the building. Mapping distance over time reveals the path that a target follows, ambiguity regarding a target’s path tracked in one channel may be mitigated by tracking the same target on another channel.

Work remains to coherently combine the IQ amplitudes from the simultaneous data collections to resolve multiple targets. One goal is to map target locations on a 2-D (down-range and cross-range) image, presented perhaps as a video animation overlaid on an overhead view of the scene (i.e., a typical floorplan for the building being imaged). It remains to be determined whether a bistatic pairing of transmitter and receiver antennas provides an advantage (over the standard mono-static transmitter-antenna pairing) when imaging moving targets.

This work was performed by Gregory Mazzaro for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Sensors category. ARL-9627