Charles Le

The UAVSAR instrument: Description and first results

The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in the ESTO Instrument Incubator Program and after 3 years of development has begun collecting engineering and science data. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In fact a Ka-band single-pass radar interferometer for making high precision topographic maps of ice sheets is being developed based to a large extent on components of the UAVSAR L-band radar. By designing the radar to be housed in an external unpressurized pod, it has the potential to be readily ported to many platforms. Initial testing is being carried out with the NASA Gulfstream III aircraft, which has been modified to accommodate the radar pod and has been equipped with precision autopilot capability developed by NASA Dryden Flight Research Center. With this the aircraft can fly within a 10 m diameter tube on any specified trajectory necessary for repeat-pass radar interferometric applications. To maintain the required pointing for repeat-pass interferometric applications we have employed an actively scanned antenna steered using INU measurement data. This paper presents a brief overview of the radar instrument and some of the first imagery obtained from the system.

First P-band results using the GeoSAR mapping system

GeoSAR is a program to develop a dual frequency airborne radar interferometric mapping instrument designed to meet the mapping needs of a variety of users in government and private industry. Program participants are the Jet Propulsion Laboratory (JPL), Calgis, Inc., and the California Department of Conservation with funding provided initially by DARPA and currently by the National Imagery and Mapping Agency. Begun to address the critical mapping needs of the California Department of Conservation to map seismic and landslide hazards throughout the state, GeoSAR is currently undergoing tests of the X-band and P-band radars designed to measure the terrain elevation at the top and bottom of the vegetation canopy. Maps created with the GeoSAR data will be used to assess potential geologic/seismic hazard (such as landslides), classify land cover, map farmlands and urbanization, and manage forest harvests. This system is expected to be fully operational in 2002. In this paper we describe an experiment conducted at Californias Latour State Demonstration Forest located near the city of Redding. This experiment marks the first operation of the-P-band radar in a vegetated area.

Onboard FPGA-based SAR processing for future spaceborne systems

We present a real-time high-performance and fault-tolerant FPGA-based hardware architecture for the processing of synthetic aperture radar (SAR) images in future spaceborne systems. In particular, we discuss the integrated design approach, from top-level algorithm specifications and system requirements, design methodology, functional verification and performance validation, down to hardware design and implementation.

A digital beamforming processor for the joint DoD/NASA space based radar mission

The space based radar (SBR) program includes a joint technology demonstration between NASA and the Air Force to design a low-earth orbiting, 2/spl times/50 m L-band (1.26 GHz) radar system for both Earth science and intelligence-related observations. A key subsystem aboard SBR is the electronically-steerable digital beamformer (DBF) network that interfaces between 32 smaller sub-antenna panels in the array and the on-board processing electronics for synthetic aperture radar (SAR) and moving target indication (MTI). In this paper, we describe the development of a field-programmable gate array (FPGA) based DBF processor for handling the algorithmically simple yet computationally intensive inner-product operations for wideband, coherent beamforming across the 50 m length of the array. Tests with an antenna array simulator demonstrate that the beamformer performance metrics (0.07/spl deg/ rms phase precision per channel, -39.0 dB peak sidelobe level) will meet the system-level requirements for SAR and MTI operating modes.

Observations and mitigation of RFI in ALOS PALSAR SAR data: Implications for the DESDynI mission

Initial examination of ALOS PALSAR synthetic aperture radar (SAR) data has indicated significant radio frequency interference (RFI) in several geographic locations around the world. RFI causes significant reduction in image contrast, introduces periodic and quasi-periodic image artifacts, and introduces significant phase noise in repeat-pass interferometric data reduction. The US National Research Council Decadal Survey of Earth Science has recommended DESDynI, a Deformation, Ecosystem Structure, and Dynamics of Ice satellite mission comprising an L-band polarimetric radar configured for repeat-pass interferometry. There is considerable interest internationally in other future L-band and lower frequency systems, as well. Therefore, the issues of prevalence and possibilities of mitigation of RFI in these crowded frequency bands are of considerable interest. RFI is observed in ALOS PALSAR in California and Hawaii, USA, and in southern Egypt in data examined to date. Application of several techniques for removing it from the data prior to SAR image formation, ranging from straight-forward spectral normalization to time-domain, multi-phase filtering techniques, are considered. Considerable experience has been gained from the removal of RFI from P-band acquired by the GeoSAR system. These techniques applied to the PALSAR data are most successful when the bandwidth of any particular spectral component of the RFI is narrow. Performance impacts for SAR imagery and interferograms are considered in the context of DESDynI measurement requirements.

Removal of RFI in wideband radars

The least-mean-square (LMS) adaptive filter is applied to suppress narrow-band radiofrequency interference (RFI) in wideband synthetic aperture radar (SAR) signals. Simulation is used to show the working principles of the adaptive filter and to obtain the optimum filters parameters. The algorithm is tested with P-band synthetic aperture radar (SAR) data collected by the NASA/JPL airborne SAR (AIRSAR) in different noisy environments.

ISAAC - a case of highly-reusable, highly-capable computing and control platform for radar applications

ISAAC is a highly capable, highly reusable, modular, and integrated FPGA-based common instrument control and computing platform for a wide range of instrument needs as defined in the Earth Science National Research Council (NRC) Decadal Survey Report. This paper presents its motivation, technical approach, and the infrastructure elements. It also describes the first prototype, ISAAC I, and its application in the design of SMAP L-band radar digital filter.

On-board processor for direct distribution of change detection data products [radar imaging]

We are developing an on-board imaging radar data processor for repeat-pass change detection and hazard management. This is the enabling technology for NASA ESE to utilize imaging radars. This processor enables the observation and use of surface deformation data over rapidly evolving natural hazards, both as an aid to scientific understanding and to provide timely data to agencies responsible for the management and mitigation of natural disasters. Many hazards occur over periods of hours to days, and need to be sampled quickly. The new technology has the potential to save many lives and millions of dollars by putting critical information in the hands of disaster management agencies in time to be of use. The processor architecture integrates two key technologies by combining a field programmable gate array (FPGA) front-end with a reconfigurable computing back-end. A searchable on-board data archive stores the reference data sets needed for the change detection processing. In this paper, we present an overview of the change detection processing algorithm and the preliminary hardware architecture.

Residual motion estimation for UAVSAR: Implications of an electronically scanned array

The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in the ESTO Instrument Incubator Program and after 3 years of development has begun collecting engineering and science data. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In order to make geophysically useful repeat pass interferometric measurements it is necessary to reconstruct the repeat pass baseline with millimeter accuracy, however onboard motion metrology systems only achieve 5–15 cm accuracy. Thus it is necessary to recover the residual motion from the data itself. Algorithms for recovering the motion based on along-track offsets between the repeat pass interferometric pair of images were described in [3], [1] and [4]. Later these techniques were extended to use azimuth subbanded differential interferograms in [5]. This paper provides a derivation for the formula for the along-track offsets (or corresponding the subbanded differential phase), i.e. the relative displacement between two SAR images in a interferometric pair in the along track direction, as a function of baseline for systems employing an electronically scanned antenna. The standard formula for systems not employing electronically scanned antenna for the along-track offsets, Δs, has the form in given equation where bc is the cross-track baseline, bh is the vertical baseline, θℓ is the look angle, θaz is the azimuth or squint angle, ρ is the range and d = 1 for left looking systems and d = −1 for right looking systems. A key feature of this formula is the along-track offsets only range dependency is from the derivatives of the baseline with respect to along-track position. In the electronically scanned case this in no longer true and an additional range dependency arises that is a function of the electronic steering angle.

On-board fault-tolerant SAR processor for spaceborne imaging radar systems

A real-time high-performance and fault-tolerant FPGA-based hardware architecture for the processing of synthetic aperture radar (SAR) images has been developed for advanced spaceborne radar imaging systems. In this paper, we present the integrated design approach, from top-level algorithm specifications, system architectures, design methodology, functional verification, performance validation, down to hardware design and implementation.