Unmanned Aerial Vehicle (UAV) Radar
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D.W. Baden, E.M. Kutrzyba, A.P. Desrosiers, J.O. Alatishe, S. Talapatra, and M.G Parent
Radar Division
Introduction: The Unmanned Aerial Vehicle (UAV) Radar project is designing and demonstrating a synthetic aperture radar (SAR) and moving target indicator (MTI) radar to (1) detect and track moving ground vehicles and small boats to a range of 80 nmi, (2) provide a simultaneous SAR ground map, (3) provide targeting quality data for weapons, (4) provide for low false track rates, (5) use low microwave frequencies for foliage penetration, and (6) fit in a Navy vertical take-off UAV. This proof-of-concept program supports the Navy's Tactical Ultra-Light Unmanned Aerial Vehicle Program, sponsored by PMA-263.
Algorithms: The different algorithms that have been analyzed for use with this radar include: Displaced Phase Center Antenna (DPCA), Space Time Adaptive Processing (STAP), and Velocity SAR (VSAR). Data are being collected to verify that Velocity SAR will be the optimal algorithm. VSAR assumes that input data from each antenna element is pulse-compressed, motion-compensated, and is in baseband in-phase and quadrature (I and Q) form. The primary difference between VSAR and standard SAR is that VSAR initially requires the formation of multiple (N) SAR images, one from each antenna in an antenna array, while standard SAR forms just one image from one antenna. To detect moving targets, a given pixel is processed over all N images, and this process is repeated for every pixel location. For moving targets, the radar echo phase shift from antenna element to antenna element for each range/azimuth pixel location may be detected.1,2
System Hardware: The radar is a low-power (300 W peak, expandable to 1 kW), L-band (1290-1315 MHz) that is capable of detecting small targets out to an unambiguous range of 25 nmi (expandable to 80 nmi). A field-programmable gate array (FPGA)-based digital signal processor (DSP) board is used to generate the low-power transmit waveform at the first intermediate frequency (IF) of 45 MHz. Typical waveforms outputted by the FPGA board include a bandwidth (BW) = 1 to 10 MHz, pulse repetition interval (PRI) = 1 ms (adjustable), and pulse width (PW) = 8 to 67 µs. The primary waveform will be a linear FM with 67 µs PW, 1.024 ms PRI, 7.5 MHz BW, and 45 MHz IF chirp.
The receiver subsection consists of 16 IF channels (at 45 MHz center frequency, 10 MHz BW). Radar echo data are acquired/sampled via eight dual-channel Pentek digital receivers, which are housed in a 21-slot VME chassis. Data are transferred from the digital receivers via fiber channel to a RAID (Redundant Array of Inexpensive Disks) for post processing. The acquisition subsystem is capable of collecting 20 s of data per run. The current radar system configuration uses a temporary antenna consisting of six receivers and four transmit ports. The final antenna design will have 24 receive ports but only 16 will be active. Each port contains one vertically half-wavelength dipoled antenna element. The transmit dipoles are half-wavelength spaced in both directions, while the receive dipoles have a horizontal spacing of a quarter-wavelength.
Field Testing: The platform chosen for this iteration of testing is a truck. The radar system is housed in an 8 × 7 × 7-ft aluminum shelter with the antenna housing secured to the roof. This shelter is secured to the stake-body truck and powered by a 20 kW generator also located aboard the truck. For the initial testing of the radar system last summer, the radar was stationary at NRL's Chesapeake Bay Detachment (CBD), looking toward Tilghman Island. Returns from the newly working radar system at CBD are consistent with radar data collected from other systems. Radar returns have been identified as Tilghman and Sharps Islands and also a large ship that was noted during data collection. The radar system is still maturing as initial testing begins. The system used only three receive channels during the early stages of development. The first test with the moving radar occurred in Hancock, Maryland, on elevated areas overlooking low-lying country roads where the truck could safely and legally travel at 60 mph. Along these country roads, a target was placed at several designated locations traveling at various speeds. As data processing continues, images will be produced with the target in and out of the field of view. Figure 4 shows preliminary data from the Hancock, Maryland, processing. The radar was also used to collect more data in a similar site at Afton Mountain near Charlottesville, Virginia. The radar system had six receive elements at that time. Figure 5(a) shows a digital orthographic view merged with digital terrain elevation data (DTED) reflectance and shed fan from the transmit area on Interstate Highway I-64 near Afton, Virginia. Figure 5(b) is an image of data collected with the UAV radar. Notice the similarities between the DTED data and the radar data, such as the hook on the left-hand side of the image at 5 statute miles and the mountain ridges from 6 to 10 statute miles (with the area in between hidden because of near-range landscape).
Geographic scatters in radar returns.
(a) Digital orthographic view merged with DTED reflectance and shed fan from transmit area on Interstate Highway I-64 near Afton, VA. (b) Image of data collected with UAV radar.
Summary: The data prove that the Radar Division has a new working radar system. As the radar matures and the data are analyzed, the VSAR algorithm will be used to show that the radar will be able to detect a small moving target. The system will be populated with 16 elements in January 2004, and the radar will continue to be tested at the previous test sites. Future plans will be to locate funding to fly a similar system in either a helicopter or small aircraft.
[Sponsored by ONR]
References
1M. Picciolo, B. Cantrell, E. Kutrzyba, S. Schutz, M. Parent, J. Alatishe, and S. Talapatra, "UAV Radar," presented at Military Sensing Symposia (MSS), National Institute of Standards and Technologies, Gaithersburg, MD 27-29 November 2001.
2B. Friedlander and B. Porat, "VSAR: A High Resolution Radar System for Detection of Moving Targets," IEEE Proc. Radar, Sonar, Navig. 34(3) 205-218 (1997).