DBSAR is an airborne L-band radar system, developed at NASA/GSFC (Goddard Space Flight Center) for the introduction and implementation of digital beamforming radar techniques.
The instrument assembly combines state-of-the-art radar technologies, FPGA-based onboard processing, and advances in signal processing techniques in order to enable new remote sensing capabilities applicable to Earth science and planetary exploration applications.
DBSAR implementation:
DBSAR evolved from early GSFC IRAD (Internal Research and Development) efforts aimed at the development of the RadSTAR, an active/passive spaceborne system concept that combined an L-band imagining scatterometer and a STAR (Synthetic Thinned-Array Radiometer) for the measurement of soil moisture and ocean salinity.
Since then, DBSAR evolved to include state-of-the-art features such as reconfigurable hardware and firmware, a multi-channel data acquisition and real time processor system, reconfigurable waveform generation, and a high resolution navigation system.
The antenna is a corporate fed microstrip patch-array centered at 1.26 GHz with a 20 MHz bandwidth. The patch elements have a separation of one-half wavelength, and are printed on 0.32 mm thick Teflon-fiberglass substrate, as shown in Figure 1.
Although only one feed is used with the present configuration, a provision was made for separate corporate feeds for vertical and horizontal polarization.
DBSAR is a reconfigurable data acquisition and processor system capable of real-time, high-speed data processing. DBSAR uses an FPGA-based architecture to implement digitally down-conversion, in-phase and quadrature (I/Q) demodulation, and subsequent radar specific algorithms.
The processor architecture was a custom design for the implementation SAR algorithms. The core of the processor board consists of an analog-to-digital (A/D) section, three Altera Stratix field programmable gate arrays (FPGAs), an ARM microcontroller, several memory devices, and an Ethernet interface, as shown in Figure . The processor also interfaces with a navigation board consisting of a GPS and a MEMS gyro.
Operational modes:
The processor has been configured to operate in scatterometer, SAR (Synthetic Aperture Radar), and altimeter modes.
1) Scatterometer mode:
In this mode, the radar is capable to generate a wide beam or scan a narrow beam on transmit, and to steer the received beam on processing while controlling its beamwidth and side-lobe level. The radar can operate in tho beamforming modes, each characterized by unique strengths and weaknesses, and each applicable to different measurement scenarios. The scatterometer operational modes are as follows:
2) SAR mode:
In this mode the radar can achieve fine resolutions over large swaths without degrading image quality as in the case of conventional SAR systems. Conventional SAR systems are inherently narrow swath because of imposed ambiguity limitations.
3) SAR altimeter mode:
In this mode altimetry is performed on the nadir beam of the Scatterometer mode or SAR mode, as shown in Figure . The Delay/Doppler technique is employed to reduce the altimeter foot print along track and provide better speckle reduction and precision than conventional altimeters. In, this mode, the onboard processing enables high data reduction, making this technique very suitable for spaceborne applications.
DBSAR is capable in support of non-conventional DBSAR operational modes such as simultaneous left and right imaging of the ground track , and single pass interferometry.
The radar was designed to fly on the NASA P3 aircraft. The radar mounted on the aircraft bomb-bay with the antenna pointing at nadir. The radar processor and power supplies were mounted on a rack which seats directly above in the fuselage.
The two polarimetric upgrades carried out under this effort will provide more accurate estimates of important scientific parameters such as biomass and surface roughness.
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