Lawrence Hilliard

NASA's L-Band Digital Beamforming Synthetic Aperture Radar

The Digital Beamforming Synthetic Aperture Radar (DBSAR) is a state-of-the-art L-band radar that employs advanced radar technology and a customized data acquisition and real-time processor in order to enable multimode measurement techniques in a single radar platform. DBSAR serves as a test bed for the development, implementation, and testing of digital beamforming radar techniques applicable to Earth science and planetary measurements. DBSAR flew its first field campaign on board the National Aeronautics and Space Administration P3 aircraft in October 2008, demonstrating enabling techniques for scatterometry, synthetic aperture, and altimetry.

Lightweight linear broadband antennas enabling small UAV wing systems and space flight nanosat concept

The RadSTAR initiative merges an interferometric radiometer with a digital beam forming scatterometer, providing Earth surface backscatter and emission measurements. Heretofore these instrument developments have been designed for low flying brown platforms such as the NASA P-3. Commercial-off-the-shelf design materials can be used to inexpensively build antennas that approximate free-space permittivity, enabling remote sensing of soil moisture levels locally using small Unmanned Aerial Vehicles (UAVs). A foam/free-space sandwich can be used to minimize the weight of the dielectric backing structure. This technique enables rapid prototyping with space-grade materials. A low-mass 3-element antenna array of this design is already baselined for a University nano-satellite mission. A light-weighted version of the L-band Imaging Scatterometer (US) radar electronics is being developed for a Small Business Innovative Research (SBIR) program. This lightweight wing antenna has a large potential payoff to NASA. For example, it may enable an active/passive hydrology mission using a fleet of low cost small UAVs. A mesh ground plane can further reduce the overall mass and stowability of the very large antennas required for spaceborne observations at L-Band. The cross track scan success criterion was met at L-Band frequencies for radar and radiometry. That is, we designed and prototyped a wideband antenna patch tunable in this range and additional work is being earned out to improve the cross polarization isolation. Making the present broadband design into arrays will be limited to one dimension due to array spacing and the aspect ratio of the patch elements. A fore and aft Doppler beam synthesis and the US cross-track beam forming concept will be considered for potential application to surface hydrology measurements using these arrays

Real-time beamforming synthetic aperture radar

This paper discusses the concept and design of a real-time Digital Beamforming Synthetic Aperture Radar (DBSAR) for airborne applications which can achieve fine spatial resolutions and wide swaths. The development of the DBSAR enhances important scientific measurements in Earth science, and serves as a prove-of-concept for planetary exploration missions. A unique aspect of DBSAR is that it achieves fine resolutions over large swaths by synthesizing multiple cross-track beams simultaneously using digital beamforming techniques. Each beam is processed using SAR algorithms to obtain the fine ground resolution without compromising fine range and azimuth resolutions. The processor uses an FPGA-based architecture to implement digital in-phase and quadrature (I/Q) demodulation, beamforming, and range and azimuth compression. The DBSAR concept will be implemented using the airborne L-Band Imaging Scatterometer (LIS) on board the NASA P3 aircraft. The system will achieve ground resolutions of less than 30 m and swaths of 10 km from an altitude of 8 km.

NASA's L-Band Imaging Scatterometer

The L-Band Imaging Scatterometer (LIS) is an airborne radar developed at NASA Goddard Space Flight Center which combines electronic beam steering and digital beamforming allowing the implementation of different scanning techniques. The LIS efforts are part of the RadSTAR initiative intended to develop the technology that will enable a combined radar/radiometer system that jointly uses a single, dual frequency antenna with cross-track scanning capabilities, but no moving parts. The new technology will enable single aperture measurements of important Earth Science Enterprise climate applications such as ocean salinity, soil moisture, sea ice, and surface water among others. The LIS instrument will be flown along with existing NASA Synthetic Thinned Array Radiometers (STAR) in order to prove the concept of coregistered data and to provide a prototype for a spaceborne, single aperture radar/radiometer system.

RadSTAR L-band imaging scatterometer- performance assessment

L-band Imaging Scatterometer (LIS), developed at NASA/Goddard Space Flight Center as part of the RadSTAR initiative, is an airborne imaging radar that combines phased array technology and digital beam forming techniques for the measurement of important scientific parameters. The instrument operates at 1.26 GHz, horizontal polarization, and employs a real-time processor capable of synthesizing multiple beams over a scan range of +/-50 degrees. LIS was flight tested in May 2006 and in January 2007 on board of the NASA P3 aircraft over the Delmarva Peninsula, VA. In this paper we describe the RadSTAR system and present some preliminary analysis of the radar data collected during the test flights.

Prototype cryospheric experimental synthetic aperture radiometer (CESAR)

Present satellite microwave radiometers typically have a coarse spatial resolution of several kilometers or more. This is only adequate only over homogenous areas. Significantly enhanced spatial resolution is critically important to reduce the uncertainty of estimated cryospheric parameters in heterogeneous and climatically-sensitive areas. Examples include: (1) dynamic sea ice areas with frequent lead and polynya developments and variable ice thicknesses, (2) mountainous areas that require improved retrieval of snow water equivalent, and (3) melting outlet glacier or ice shelf areas along the coast of Greenland and Antarctica. For these situations and many others, an Earth surface spot size of no more than 100 m is necessary to retrieve the information needed for significant new scientific progress, including the synthesis of field observations with satellite observations with high confidence. At Goddard Space Flight Center, active/passive microwave remote sensing calibration and validation programs have resulted in instrumentation that uses the underside of the fuselage and wing space of Uninhabited Aerial Vehicles (UAVs) as small remote sensing platforms. This research takes advantage of prototype antennas developed for an L-Band frequency survey (nanosat), instrumentation developed for soil moisture and salinity measurements (RadSTAR), and finally a mission concept to study sea ice topography and its interaction with snow layers. The Cryospheric Experimental Synthetic Aperture Radiometer (CESAR). CESAR is a NASA proposal to fly K-Band and Ka-Band thinned arrays on an Uninhabited Aerial Vehicle (UAV) in order to measure at 100 meter ground resolution. Prior to the flight of CESAR, a prototype CESAR will be tested with elements that will fly at lower altitudes to begin the system level testing of the synthetic array with commercial-off-the-shelf (COTS) components. The prototype 4-element CESAR will precede the 10 element CESAR, and an L-Band version (Little CESAR) will precede that by using Ultrastable Radiometer (USR) components and techniques from the previous SBIR, and RadSTAR research. The K-Band and Ka-Band COTS prototype receivers will be augmented with monolithic microwave integrated circuits (MMICs) and multi-chip module receivers to be developed at Colorado State University. These miniaturized MMIC-based receivers have been designed to be installed inside the wing of the CESAR UAV, a specialized vehicle developed specifically for this high spatial resolution research. The antenna element positions along the wing will be monitored to allow for correction in software of deviations from a planar collecting array. In addition to the MMIC-based receivers, CESAR will employ a correlator chip developed for the Lightweight Rainfall Radiometer (LRR). This rad-hardened low power and low mass system was developed for Synthetic Thinned Array Radiometer (STAR) systems where mass and power are minimized to result in the largest collecting aperture possible on a given platform.

Quadruple-ridged flared horn feed with internal RFI band rejection filter

This paper presents a new technique to reject the radio frequency interference (RFI) from the nearby radar at 9.4 GHz, which causes about 20% blockage of the sky coverage in the Very Long Baseline Inter ferometry (VLBI) telescope at the Goddard Geophysical Astronomical Observatory (GGAO), in Greenbelt, MD. An internal notch filter is proposed by inserting and optimizing two-quarter wavelength slots within the wide band Quad-Ridge Flared Horn (QRFH) feed to achieve RFI band rejection at 9.4 GHz. The simulated result shows about 95 % rejection at 9.4 GHz. The estimated attenuation due to the band rejection slots is of the order of .01 dB at frequencies distant from the rejection frequency. This technique will open the door for designing wideband feeds with RFI band rejection characteristics for different RFI sources.

Hyperspectral microwave atmospheric sounder (HyMAS) - New capability in the CoSMIR/CoSSIR scanhead

MIT Lincoln Laboratory and NASAs Goddard Space Flight Center have teamed to adapt an existing instrument platform, the CoSMIR/CoSSIR system for atmospheric sensing, to develop and demonstrate a new capability in a hyperspectral microwave atmospheric sounder (HyMAS). This new sensor comprises a highly innovative intermediate frequency processor (IFP), that provides the filtering and digitization of 52 radiometric channels and the interoperable remote component (IRC) adapted to CoSMIR, CoSSIR, and HyMAS that stores and archives the data with time tagged calibration and navigation data. The first element of the work is the demonstration of a hyperspectral microwave receiver subsystem that was recently shown using a comprehensive simulation study to yield performance that substantially exceeds current state-of-the-art. Hyperspectral microwave sounders with ~100 channels offer temperature and humidity sounding improvements similar to those obtained when infrared sensors became hyperspectral. Hyperspectral microwave operation is achieved using independent RF antenna/receiver arrays that sample the same area/volume of the Earths surface/atmosphere at slightly different frequencies and therefore synthesize a set of dense, finely spaced vertical weighting functions. The second, enabling element is the development of a compact 52-channel Intermediate Frequency processor module. A principal challenge of a hyperspectral microwave system is the size of the IF filter bank required for channelization. Large bandwidths are simultaneously processed, thus complicating the use of digital back-ends with associated high complexities, costs, and power requirements. Our approach involves passive filters implemented using low-temperature co-fired ceramic (LTCC) technology to achieve an ultra-compact module that can be easily integrated with existing RF front-end technology. This IF processor is applicable to other microwave sensing missions requiring compact IF spectrometry. The unit produces 52 channels of spectral data in a highly compact volume (<;100cm3) with low mass (<;300g) and linearity better than 0.3% over a 330K dynamic range.

Microwave receiver prototype development for the Hyperspectral Microwave Atmospheric Sounder (HyMAS)1

Recent technology advances have significantly changed the landscape of modern radiometry by enabling miniaturized, low-power, and low-noise radio-frequency receivers operating at frequencies up to 200 GHz. These advances enable the practical use of receiver arrays to multiplex multiple broad frequency bands into many spectral channels. We use the term “hyperspectral microwave” to refer generically to microwave sounding systems with approximately 50 spectral channels or more. We present the design and analysis of the receiver subsystem for the Hyperspectral Microwave Atmospheric Sounder (HyMAS), with focus on the ultra compact Intermediate Frequency (IF) processor module. HyMAS comprises multiple receivers operating near the oxygen absorption line at 118.75GHz and the water vapor absorption line at 183.31GHz. The hyperspectral microwave receiver system will be integrated into a scanhead compatible with the NASA GSFC Conical Scanning Microwave Imaging Radiometer (CoSMIR) airborne system to facilitate demonstration and performance characterization. HyMAS is designed to have a 52-channel hyperspectral microwave receiver subsystem with four temperature sounding bands (two antennas) near 118.75GHz and two moisture sounding bands (one antenna) near 183.31GHz. Both polarizations are measured (although at slightly different IF passbands) to increase the total channel count. Subharmonic mixers will be pumped by phase-locked oscillators, and single-sideband operation will be achieved by waveguide filtering of the lower sideband. Size/volume constraints on the receiver subsystem led to a relatively high IF frequency (18 - 29GHz) to facilitate miniaturization of the IF processor module. Broadband operation over such a relatively high intermediate frequency range is a technical challenge for the front-end receiver sys

Design and analysis of a hyperspectral microwave receiver subsystem

Recent technology advances have profoundly changed the landscape of modern radiometry by enabling miniaturized, low-power, and low-noise radio-frequency receivers operating at frequencies near 200 GHz and beyond. These advances enable the practical use of receiver arrays to multiplex multiple broad frequency bands into many spectral channels. We use the term “hyperspectral microwave” to refer generically to microwave sounding systems with approximately 50 spectral channels or more. In this paper, we report on the design and analysis of the receiver subsystem (lensed antenna, RF front-end electronics, and IF processor module) for the Hyperspectral Microwave Atmospheric Sounder (HyMAS) comprising multiple receivers near the oxygen absorption line at 118.75 GHz and the water vapor absorption line at 183.31 GHz. The hyperspectral microwave receiver system will be integrated into a new scanhead compatible with the NASA GSFC Conical Scanning Microwave Imaging Radiometer/Compact Submillimeter-wave Imaging Radiometer (CoSMIR/CoSSIR) airborne instrument system to facilitate demonstration and performance characterization under funding from the NASA ESTO Advanced Component Technology program. Four identical radiometers will be used to cover 108-119 GHz, and two identical receivers will be used to cover 173-183 GHz. Subharmonic mixers will be driven by frequency-multiplied dielectric resonant oscillators, and single-sideband operation will be achieved by waveguide filtering of the lower sideband. A relatively high IF frequency is chosen to facilitate miniaturization of the IF processor module, which will be fabricated using Low Temperature Co-fired Ceramic (LTCC) technology. Corrugated feed antennas with lenses are used to achieve a FWHM beamwidth of approximately 3.5 degrees. Two polarizations are measured by each feed to increase overall channel count, and multiple options will be considered during the design phase for the polarization diplexing approach. Broadband operation over a relatively high intermediate frequency range (18-29 GHz) is a technical challenge of the front-end receiver systems, and a receiver temperature of approximately 2000-3000K is expected over the receiver bandwidth. This performance, together with approximately 100-msec integration times typical of airborne operation, yields channel NEDTs of approximately 0.35K, which is adequate to demonstrate the hyperspectral microwave concept by comparing profile retrievals with high-fidelity ground truth available either by coincident overpasses of hyperspectral infrared sounders and/or in situ radiosonde/dropsonde measurements.