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Freeman, A.

Publications and source records attributed to Freeman, A..

82 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Calibration of NASA/JPL DC-8 SAR data

The paper describes a number of improvements in calibration of NASA/JPL DC-8 SAR (synthetic aperture radar) data, intended to make calibrated data from the system available to the majority of the investigators. The improvements include a move to a better site for calibration of the system at the start of each campaign season, the release of software to allow users to calibrate their data, the supply of trihedral corner reflectors and instructions on how to deploy them, and the setting up of an archive of calibrated data. Analysis of the calibration performance of the DC-8 SAR over a number of sites during 1990 suggests that the system is sufficiently stable to allow calibration of the standard data products without the presence of calibration devices within the scene. The limiting factor on DC-8 SAR data calibration would now appear to be the knowledge of the altitude of the aircraft, which determines the relationship between the measured slant range coordinates and the antenna pattern in elevation.

Freeman, A.↗
NASA NTRS1991

Development of a calibration workstation for SIR-C

The paper describes the status of a comprehensive synthetic aperture radar (SAR) software package developed specifically to aid the calibration of a multifrequency and multipolarization radar like the Shuttle Imaging Radar, SIR-C. The primary data types processed are engineering telemetry, point targets (corner reflectors and polarimetric active radar calibrators), distributed targets (clutter), tone-generators, and ground receivers. The parameters derived to aid the calibration of the production processor include measurements of relative radiometric channel to channel imbalance, absolute calibration constants, antenna pattern corrections, noise levels, polarimetric cross-talk, and geometric offsets. The software reports standard performance measurements such as resolution, peak and integrated sidelobes, radiometric response accuracy, and geometrical distortions such as scale errors, position bias and standard deviation, and image skew. Furthermore, the inclusion of software to allow updates of the shuttle-state vector (position and velocity) is considered.

Madsen, S. N.↗
NASA NTRS1989

Geometric accuracy in airborne SAR images

Uncorrected across-track motions of a synthetic aperture radar (SAR) platform can cause both a severe loss of azimuthal positioning accuracy in, and defocusing of, the resultant SAR image. It is shown how the results of an autofocus procedure can be incorporated in the azimuth processing to produce a fully focused image that is geometrically accurate in azimuth. Range positioning accuracy is also discussed, leading to a comprehensive treatment of all aspects of geometric accuracy. The system considered is an X-band SAR.

Blacknell, D.↗
NASA NTRS1989

Results of the 1988 NASA/JPL airborne SAR calibration campaign

During the spring of 1988, the NASA/JPL multifrequency, multipolarization SAR (synthetic aperture radar) flew in series of calibration experiments over the Goldstone dry lake bed. An array of calibration devices was deployed, including dihedral and trihedral corner reflectors, polarimetric active radar calibrators, passive receivers, and CW (continuous wave) tone generators. The aim of the campaign was to calibrate both amplitude and phase of the resulting SAR images, over long and short time scales. The results of the analysis of the calibration data collected in the spring of 1988 are presented. Trihedral corner reflector signatures and certain image background measures are used to externally calibrate relative amplitude and phase between polarizations at a given frequency, and to calibrate across frequencies. Assessments are made of the calibration accuracy as a function of image frame position for each frequency, and the stability of the radar calibration over long and short time-scales.

Freeman, A.↗
NASA NTRS1988

The effects of undersampling on multipolarization SAR images

The scattering matrix and Stokes matrix formats for multipolarization synthetic aperture radar (SAR) images are introduced. The effects of converting to the Stokes matrix format, without first doubling the sampling rate to allow for the conversion from a complex to an intensity format is quantified and discussed. It is shown that for most applications the Stokes matrix format is acceptable, since errors introduced by undersampling tend to average out when large numbers of pixels are averaged. For those applications requiring analysis of single image pixels, the scattering matrix format is recommended.

Freeman, A.↗
NASA NTRS1988

Design considerations for advanced multi-polarisation SAR

System design and verification of an airborne imaging radar (AIR) system are discussed. Issues of importance to multipolarization systems (e.g., mutual coherence and amplitude balance between channels) are emphasized. Methods of detecting and correcting channel imbalances are discussed, and AIR test results are presented.

Klein, J. D.↗
NASA NTRS1988

Calibration of multipolarisation imaging radar

An experiment was designed to calibrate airborne imaging radar (AIR) images, in terms of relative and absolute backscatter, and relative phase between polarization channels. The calibration uses measurements made within the radar system itself (internal calibration) and using ground-based corner reflectors and transponders (external calibration). The techniques developed for the AIR calibration campaign will form the basis of the calibration approach for SIR-C.

Freeman, A.↗
NASA NTRS1988

The NASA/JPL multifrequency, multipolarisation airborne SAR system

Polarimetric synthetic aperture radars, operating at L-, C- and P-band, were designed to replace and upgrade a system destroyed in an aircraft accident. Ground and flight tests were conducted, and the radar was flown over a calibration site in a sequence of experiments designed to calibrate the system. The radar also took part in science campaigns.

Held, D. N.↗
NASA NTRS1966

Radial coolant channels fabricated by simplified method

Radial coolant channels for distributing a coolant over the inner wall of a circular section are fabricated by cold-rolling indentations on the inside circumference of the base section and covering the indentations with a rolled flange.

Freeman, A.↗