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At least 109 records · Page 6
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NASA NTRS1991

The AIRSAR image browse system

The volume of data acquired by synthetic aperture radar (SAR) missions motivates the development of a technique that will provide investigators with timely access to image data, and the capability to visually inspect images to determine various characteristics of the data collected, as well as the extent of the data coverage. A system was designed for accessing and browsing an on-line archive of NASA Airborne SAR (AIRSAR) imagery and is being developed as an extension to the SAR Data Catalog System (SDCS). The AIRSAR browse system consists of three elements: (1) an on-line, compressed imagery archive; (2) a user interface for selecting and accessing the on-line images; and (3) a software application program for reconstructuring and displaying the images. Details of these elements are described. The goals are to maximize the utilization of the AIRSAR data set, and to demonstrate the image browse concept for SAR data so that baseline catalog systems supporting SIR-C and future SAR missions will feature this capability.

Pang, A.↗
NASA NTRS1985

The NASA/JPL multipolarization SAR aircraft program

The NASA/JPL aircrft program has been in existence for more than a decade. However, during the past two years, there has been a dramatic increase in the capability and scientific utility of this system as an L-band, 4-polarization synthetic aperture radar (SAR) system has become operational. The system is capable of simultaneously and coherently acquiring four independent complex polarization measurements for every pixel in the image. This capability has been exploited by a number of experimenters to generate several new data types that appear to contain significantly more information than was previously available from single or dual polarized SAR systems. The system will ultimately be capable of estimating the entire complex scattering matrix for every pixel on the ground. Within the next year the system will be extended to C-band so that it will be possible to simultaneously acquire 4-polarization imagery at L-band and C-band. This system will be the prototype for the SIR-C experiment that will have similar capabilities and be flown on the Space Shuttle in 1989.

Held, D. N.↗
NASA NTRS1985

Spaceborne Imaging Radar (SIR) project

An overview is given of elements of the SIR (Shuttle Imaging Radar) project, covering the coming decade. The project is intended to develop the scientific basis and the required technology of the Earth Orbiting Satellite (EOS) Synthetic Aperture Radar (SAR). Plans are ongoing for a SIR-B reflight in 1987, a dual SIR-C flight in 1989 and a SIR-D flight in 1992 leading to an advanced multispectral multipolarization imaging sensor for flight on EOS in 1994. The conventional and distributed radar approaches are compared.

Elachi, C.↗
NASA NTRS1986

Spaceborne Imaging Radar Project

In June of 1985 the Project Initiation Agreement was signed by the Jet Propulsion Laboratory and the NASA Office of Space Science and Applications for the Spaceborne Imaging Radar Project (SIR). The thrust of the Spaceborne Imaging Radar Project is to continue the evolution of synthetic aperture radar (SAR) science and technology developed during SEASAT, SIR-A and SIR-B missions to meet the needs of the Earth Observing System (EOS) in the mid 1990's. As originally formulated, the Project plans were for a reflight of the SIR-B in 1987, the development of a new SAR, SIR-C, for missions in mid 1989 and early 1990, and the upgrade of SIR-C to EOS configuration with a qualification flight aboard the shuttle in the 1993 time frame (SIR-D). However, the loss of the shuttle Challenger has delayed the first manifest for SIR to early 1990. This delay prompted the decision to drop SIR-B reflight plans and move ahead with SIR-C to more effectively utilize this first mission opportunity. The planning for this project is discussed.

Herman, Neil↗
NASA NTRS1986

Advances in spaceborne synthetic aperture radar sensor technology

The evolution of space SARs for NASA projects since Seasat (1978) is surveyed, with an emphasis on hardware development. The fundamental principles of SAR are reviewed; the SIR-A and SIR-B instruments flown as Shuttle payloads are characterized; their antennas, transmitters, receivers, and data subsystems are described; the advantages offered by the SIR-C dual-frequency (L and C band) dual-polarization distributed SAR (being developed for a future Shuttle flight and as the basis of an SAR for the Earth Observing System) are explained; and a number of technical challenges are identified (including RF elements, structural fidelity, pointing accuracy, data handling, and dc power). Drawings, diagrams, sample images, photographs, and tables are provided.

Caro, E. R.↗
NASA NTRS1986

Shuttle Imaging Radar-B/C instruments

The successfully flown Shuttle Imaging Radar-B (SIR-B) instrument is described, giving details of the hardware, the subsystem functions, and the Shuttle interfaces. The preliminary design of the SIR-C instrument is described, giving an overview of the preliminary hardware design, the subsystem functional design, and the Shuttle interfaces.

Huneycutt, Bryan L.↗
NASA NTRS1987

Spaceborne Imaging Radar-C instrument

The present discussion of the Spaceborne Imaging Radar-C (SIR-C) hardware design, subsystem functional design, and interfaces with the NASA Space Shuttle, gives attention to antenna characteristics and to instrument performance parameter characteristics in the C- and L-bands. The SIR-C antenna is a dual-frequency, dual-polarization distributed array antenna whose distribution of transmit/receive modules improves the system noise figure and eliminates the need for a single, high-power RF source. Phase shifters for individual subarrays allow electronic beam steering in elevation and azimuth.

Huneycutt, Bryan L.↗
NASA NTRS1987

The measurement of precipitation with synthetic aperture radar

The radar equation for the measurement of precipitation by SAR is identical to that for a conventional radar. The achievable synthetic beamwidth, beta(s), is proportional to sigma(v)/U, the ratio of the spread of the precipitation Doppler spectrum to the platform velocity. Thus, a small beta(s) can be achieved only with small sigma(v) or from a fast-moving vehicle such as a spacecraft. Also, the along-track resolution is variable with sigma(v) and is not known. Nevertheless, the reflectivity is measured correctly. A possible approach to the measurement of sigma(v) is noted. The C-band SAR proposed for the SIR-C mission is capable of detecting a rain rate as small as 0.5 mm/h at nadir when the beam is filled. Because the cross-track beam dimension is about 20 km wide, the use of a high-resolution microwave radiometer to correct for the unfilled beam and the variation of gain across it is suggested. Alternatively, the cross-track dimension should be decreased to no more than about 5 km by increasing the antenna width and/or decreasing the wavelength.

Atlas, David↗
NASA NTRS1991

Airborne SAR determination of relative ages of Walker Valley moraines, eastern Sierra Nevada

A regional study of the distribution and elevations of Pleistocene moraines in the Andes requires a method of determining relative age from space. One of our primary objectives is to establish the relative chronology of major climatic events responsible for glaciation in the Andes and other regions that are difficult to access on the ground and where suitable material for absolute age determination is lacking. The sensitivity of radar to surface roughness makes it possible to develop a remotely-based relative dating technique for landforms for which surface age and roughness can be correlated. We are developing such a technique with Airborne Synthetic Aperture Radar (AIRSAR) imagery of the eastern Sierra Nevada where independent evidence is available for the ages and physical characteristics of moraines. The Sierra Nevada moraines are similar in form and environmental setting to Andean moraines that we have targeted for study during the pending Shuttle Imaging Radar-C (SIR-C) mission. SAR imagery is used to differentiate the ages of five moraine sequences of Walker Valley in the eastern Sierra Nevada. Other aspects of this investigation are briefly discussed.

Fox, A.↗
NASA NTRS1991

The evolution of synthetic aperture radar systems and their progression to the EOS SAR

The authors describe the evolution of the spaceborne imaging radar starting with the Seasat SAR, through the SIR-A, SIR-B, and SIR-C/X-SAR missions, to the Earth Observing System (EOS) SAR which is scheduled for launch as part of EOS in the late 1990s. A summary of the planned international missions, which may produce a permanent active microwave capability in space starting as early as 1991, is also presented, along with a description of the airborne systems which will be essential to the algorithm development and long-term calibration of the spaceborne data. A brief summary of the planetary missions utilizing SAR and a comparison of their imaging capabilities with those available on Earth are presented.

Way, Jobea↗
NASA NTRS1992

Application of the multiple PRF technique to resolve Doppler centroid estimation ambiguity for spaceborne SAR

Estimation of the Doppler centroid ambiguity is a necessary element of the signal processing for SAR systems with large antenna pointing errors. Without proper resolution of the Doppler centroid estimation (DCE) ambiguity, the image quality will be degraded in the system impulse response function and the geometric fidelity. Two techniques for resolution of DCE ambiguity for the spaceborne SAR are presented; they include a brief review of the range cross-correlation technique and presentation of a new technique using multiple pulse repetition frequencies (PRFs). For SAR systems, where other performance factors control selection of the PRF's, an algorithm is devised to resolve the ambiguity that uses PRF's of arbitrary numerical values. The performance of this multiple PRF technique is analyzed based on a statistical error model. An example is presented that demonstrates for the Shuttle Imaging Radar-C (SIR-C) C-band SAR, the probability of correct ambiguity resolution is higher than 95 percent for antenna attitude errors as large as 3 deg.

Chang, C. Y.↗
NASA NTRS1992

Performance analysis of multiple PRF technique for ambiguity resolution

For short wavelength spaceborne synthetic aperture radar (SAR), ambiguity in Doppler centroid estimation occurs when the azimuth squint angle uncertainty is larger than the azimuth antenna beamwidth. Multiple pulse recurrence frequency (PRF) hopping is a technique developed to resolve the ambiguity by operating the radar in different PRF's in the pre-imaging sequence. Performance analysis results of the multiple PRF technique are presented, given the constraints of the attitude bound, the drift rate uncertainty, and the arbitrary numerical values of PRF's. The algorithm performance is derived in terms of the probability of correct ambiguity resolution. Examples, using the Shuttle Imaging Radar-C (SIR-C) and X-SAR parameters, demonstrate that the probability of correct ambiguity resolution obtained by the multiple PRF technique is greater than 95 percent and 80 percent for the SIR-C and X-SAR applications, respectively. The success rate is significantly higher than that achieved by the range cross correlation technique.

Chang, C. Y.↗
NASA NTRS1993

Forest investigations by polarimetric AIRSAR data in the Harz mountains

The Harz Mountains in the North of Germany have been a study site for several remote sensing investigations since 1985, as the mountainous area is one of the forest regions in Germany heavily affected by forest decline, especially in the high altitudes above 800 m. In a research program at the University of Berlin, methods are developed for improving remote sensing assessment of forest structure and forest state by additional GIS information, using several datasets for establishing a forest information system. The Harz has been defined as a test site for the SIR-C/X-SAR mission which is going to deliver multifrequency and multipolarizational SAR data from orbit. In a pilot project let by DLR-DFD, these data are to be investigated for forestry and ecology purposes. In preparing a flight campaign to the SIR-C / X-SAR mission, 'MAC EUROPE 1991', performed by NASA/JPL, an area of about 12 km in the Northern Harz was covered with multipolarizational AIRSAR data in the C-, L- and P-band, including the Brocken, the highest mountain of the Harz, with an altitude of 1142 m. The multiparameter AIRSAR data are investigated for their information content on the forest state, regarding the following questions: (1) information on forest stand parameters like forest types, age classes and crown density, especially for the separation of deciduous and coniferous forest; (2) information on the storm damages (since 1972) and the status of regeneration; (3) information on the status of forest destruction because of forest decline; and (4) influence of topography, local incidence angle and soil moisture on the SAR data. Within the project various methods and tools have been developed for the investigation of multipolarimetric radar backscatter responses and for discrimination purposes, in order to use the multipolarization information of the compressed Stokes matrix delivered by JPL.

Keil, M.↗
NASA NTRS1994

Mission 59 to survey earth with multiband radar

The preparations for Space Shuttle Mission 59 are described, as are the planned mapping operations of the Space Imaging Radar C (SIR-C). Crew tasks and shuttle maneuvers during the 18 million square mile Earth mapping mission are also outlined.

Mckenna, James T.↗
NASA NTRS1996

Dual-Frequency Interferometric SAR Observations of a Tropical Rain-Forest

Repeat-pass, interferometric, radar observations of tropical rain-forest collected by the Shuttle Imaging Radar C (SIR-C) in the state of Rondonia, Brazil, reveal signal coherence is destroyed at C-band (5.6-cm) in the forest, whereas L-band (24-cm) radar signals remain strongly coherent over the entire landscape. At L-band, the rms difference in inferred topographic height between the forest and adjacent clearings is 5 m, equivalent to the height noise. Atmospheric delays are large, however, forming kilometer-sized anomalies with a 1.2-cm rms one way. Radar interferometric studies of the humid tropics must therefore be conducted at long radar wavelengths, with kilometric baselines or with two antennas operating simultaneously.

Rignot, E.↗
NASA NTRS1996

Dual-Frequency Interferometric SAR Observations of a Tropical Rain-Forest

Repeat-pass, interferometric, radar observations of tropical rain-forest collected by the Shuttle Imaging Radar C (SIR-C) in the state of Rondonia, Brazil, reveal signal coherence is destroyed at C-band (5.6-cm) in the forest, whereas L-band (24-cm) radar signals remain strongly coherent over the entire landscape. At L-band, the rms difference in inferred topographic height between the forest and adjacent clearings is 5 m, equivalent to the height noise. Atmospheric delays are large, however, forming kilometer-sized anomalies with a 1.2-cm rms one way. Radar interferometric studies of the humid tropics must therefore be conducted at long radar wavelengths, with kilometric base-lines or with two antennas operating simultaneously.

Rigot, E.↗
NASA NTRS1994

Overview of Results from the SIR-C/X-SAR Mission

The US Space Transportation System, or Shuttle, has shown itself to be a useful platform from which to evaluate remote sensing instrumentation for later use in a free-flying satellite.

Spaceborne Imaging Radar (SIR) Synthetic Aperture ↗