Engineering topics

Walsh, E. J.

Publications and source records attributed to Walsh, E. J..

At least 37 records · Page 2

NASA NTRS1989

Wide swath ocean topography mapping with interferometric altimeters

An interferometric radar altimeter is proposed to provide wide-swath high-resolution ocean topography. Several system design issues of such an interferometric altimeter are presented. Tradeoffs between processing of the interferometric signal using the so-called amplitude approach and the so-called phase approach are shown. The systematic errors associated with uncertainties in the interferometer baseline and the attitude of interferometer orientation are also discussed. Described is an approach using the measurements at orbit cross-over regions, together with the topography measurements from a traditional nadir-looking altimeter that are not contaminated by the baseline and attitude noises. Preliminary simulation results show that such an approach can generate an acceptable error level if the ocean surface does not change appreciably between the observations.

Rodriguez, E.↗

NASA NTRS1988

Earth Observing System (EOS) advanced altimetry

In the post-TOPEX era, satellite radar altimeters will be developed with the capability of measuring the earth's surface topography over a wide swath of coverage, rather than just at the satellite's nadir. The identification of potential spacecraft flight missions in the future was studied. The best opportunity was found to be the Earth Observing System (EOS). It is felt that an instrument system that has a broad appeal to the earth sciences community stands a much better chance of being selected as an EOS instrument. Consequently, the Topography and Rain Radar Imager (TARRI) will be proposed as a system that has the capability to profile the Earth's topography regardless of the surface type. The horizontal and height resolutions of interest are obviously significantly different over land, ice, and water; but, the use of radar to provide an all-weather observation capability is applicable to the whole earth. The scientific guidance for the design and development of this instrument and the eventual scientific utilization of the data produced by the TARRI will be provided by seven science teams. The teams are formed around scientific disciplines and are titled: Geology/Geophysics, Hydrology/Rain, Oceanography, Ice/Snow, Geodesy/Orbit/Attitude, Cartography, and Surface Properties/Techniques.

Parsons, C. L.↗

NASA NTRS1988

Surface Contour Radar (SCR) contributions to FASINEX

The SCR was asked to participate in the Frontal Air-Sea Interaction Experiment (FASINEX) to provide directional wave spectra. The NASA P-3 carrying the SCR, the Radar Ocean Wave Spectrometer, and the Airborne Oceanographic Lidar was one of five aircrafts and two ocean research ships participating in this coordinated study of the air sea interaction in the vicinity of a sea surface temperature front near 28 deg N, 70 deg W. Analysis of data from the February 1986 experiment is still ongoing, but results already submitted for publication strengthen the hypothesis that off-nadir radar backscatter is closely correlated to wind stress. The SCR provided valuable information on the directional wave spectrum and its spatial variation.

Walsh, E. J.↗

NASA NTRS1988

Surface contour radar observations of the directional wave spectrum during Fasinex

The surface control radar (SCR), a 36-GHz computer-controlled airborne radar which generates a false-color coded elevation map of the sea surface below the aircraft in real time, is described. The SCR turned out to be ideal for documenting the wave spectra during Fasinex (the Frontal Air-Sea Interaction Experiment) due to its high spatial resolution and rapid mapping capability over extensive areas. Synoptic weather maps for February 15-18, 1986 are presented.

Walsh, E. J.↗

NASA NTRS1987

Observations of and a new model for fetch-limited wave growth

The Surface Contour Radar (SCR) is a 36-GHz computer-controlled airborne radar which generates a false-color coded elevation map of the sea surface below the aircraft in real time. In the present paper, SCR observations are discussed which demonstrate the existence of a full developed sea state. These observations are used to judge the validity of growth rates for fetch-limited wave spectrum development and lead to new refinements in the modeling of wave generation by wind. It is noted that the observations have resolved an apparent paradox in the JONSWAP and Donelan et al. (1985) fetch-limited algorithms.

Walsh, E. J.↗

NASA NTRS1986

Oceanographic measurement capabilities of the NASA P-3 aircraft

Instrumentation on NASA P3 aircraft available to provide ground truth for ERS-1 is described. The wave sensors include the 36 GHz Surface Contour Radar (SCR), the Ku-band Radar Ocean Wave Spectrometer (ROWS), and the Airborne Oceanographic Lidar. The other sensors include a C-band scatterometer, video camera, radiation thermometer, and AXRTs. The SCR and ROWS directional spectrum measurements are discussed. When planning for an underflight mission, the limited endurance of the aircraft (6 hr) and flight cost (2.7 K$/hr) must be considered. The advantage of the redundancy afforded by the several wave instruments is another important consideration.

Mollo-Christensen, Erik↗

NASA NTRS1986

A comparison of SIR-B directional ocean wave spectra with aircraft scanning radar spectra

Directional ocean wave spectra derived from Shuttle Imaging Radar-B (SIR-B) L-band imagery collected off the coast of Southern Chile on 11 and 12 October 1984 were compared with independent spectral estimates from two airborne scanning radars. In sea states with significant wave heights ranging from 3 to 5 meters, the SIR-B-derived sspectra at 18 deg and 25 deg off nadir yielded reasonable estimates of wavelengths, directions, and spectral shapes for all wave systems encountered, including a purely azimuth-traveling system. A SIR-B image intensity variance spectrum containing predominantly range-traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum. The prediction of a U.S. Navy global spectral ocean wave model on 11 October 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30 deg with respect to the mean of the aircraft and spacecraft estimates.

Beal, R. C.↗

NASA NTRS1986

Altimeter height measurement error introduced by the presence of variable cloud and rain attenuation

It has recently been recognized that spatially inhomogeneous clouds and rain can substantially affect the height precision obtainable from a spaceborne radar altimeter system. Through computer simulation, it has been found that typical levels of cloud and rain intensities and associated spatial variabilities may degrade altimeter precision at 13.5 GHz and, in particular, cause severe degradation at 35 GHz. This degradation in precision is a result of radar signature distortion caused by variable attenuation over the beam limited altimeter footprint. Because attenuation effects increase with frequency, imprecision caused by them will significantly impact on the frequency selection of future altimeters. In this paper the degradation of altimeter precision introduced by idealized cloud and rain configurations as well as for a realistic rain configuration as measured with a ground based radar is examined.

Monaldo, F. M.↗

NASA NTRS1985

Elimination of directional wave spectrum contamination from noise in elevation measurements

The Surface Contour Radar (SCR) is a 36-GHz computer-controlled airborne radar which generates a false-color-coded elevation map of the sea surface below the aircraft in real time, and can routinely produce ocean directional wave spectra with post-flight data processing which have much higher angular resolution than pitch-and-roll buoys. The SCR range measurements are not error-free and the resulting errors in the elevations corrupt the directional wave spectrum. This paper presents a technique for eliminating that contamination.

Walsh, E. J.↗

NASA NTRS1985

Remote sensing of directional wave spectra using the surface contour radar

A unique radio-oceanographic remote sensing instrument was developed. The 36 GHz airborne Surface Contour Radar (SCR) remotely produces a real-time topographical map of the sea surface beneath the aircraft. It can routinely produce ocean directional wave spectra with off-line data processing. The transmitter is a coherent dual-frequency device that uses pulse compression to compensate for the limited available power at Ka band. The radar has selectable pulse widths of 1, 2, 4, and 10 nanoseconds. The transmitting antenna is a 58 lambda horn fed dielectric lens whose axis is parallel to the longitudinal axis of the aircraft. It illuminates an elliptical mirror which is oriented 45 deg to the lens' longitudinal axis to deflect the beam towards the region beneath the aircraft. The mirror is oscillated in a sinusoidal fashion through mechanical linkages driven to a variable speed motor to scan the transmitter beam (1.2 deg X 1.2 deg) with + or - 16 deg of the perpendicular to the aircraft wings in the plane perpendicular to the aircraft flight direction.

Walsh, E. J.↗

NASA NTRS1985

Contamination of the fetch-limited directional wave spectrum by waves emanating from an embayment

The Surface Contour Radar (SCR) was used to map the evolution of the fetch-limited directional wave spectrum (DWS) off the eastern seaboard. Flight lines were displaced both north and south of the Delaware Bay following the passing of a weather front. The near shore DWS was found to be dominated by waves emanating from the Delaware Bay for distances of at least 85 km up the coastline and out to sea for 200 km from the mouth of the bay.

Walsh, E. J.↗

NASA NTRS1985

Measurement of sea ice backscatter characteristics at 36 GHz using the surface contour radar

Scattering studies of sea ice off the coast of Greenland were performed in January 1984 using the 36-GHz Surface Contour Radar (SCR) aboard the NASA P-3 aircraft. An oscillating mirror scans an actual half-power width of 0.96 degrees laterally to measure the surface at 51 evenly spaced points. By banking the aircraft, real-time topographical mapping and relative backscattered power are obtained at incidence angles between 0 and 30 degrees off-nadar, achieving at 175 m altitude a 2.9 by 4.4 m spatial resolution at nadir. With an aircraft ground speed of 100 m/s, 5-m successive scan line spacing and 1.8-m cross-track direction spacing is provided. By circling the aircraft in the 15 degree bank, the azimuthal anisotropy of the scattering is investigated along with the incidence angle dependence.

Fedor, L. S.↗

NASA NTRS1985

Design and benefits of a multibeam Earth Observing Radar

The oceanographic rationale is described for continuing to advance the state of the art in satellite radar altimetry, and the expected capabilities of a multibeam Earth Observing Radar are noted. At the end of this decade, there is the possibility that altimeters may be in orbit aboard the American TOPEX, the ESA ERS-1, and the French SPOT satellites at the same time. The TOPEX version will be the most precise altimeter yet built. Global ocean circulation will be measured by using TOPEX to monitor the elevation changes across the ocean basins due to oceanic currents. It will then be possible to monitor the 'mean' circulation patterns in the oceans. The multiple beams of EOR might be used to measure the curvature of the topographic surface. The use of curvature is especially beneficial because it is directly related to ocean circulation, which is a function only of the Laplacian of the topographic height field in a given area. With the EOR, that height field will be known and ocean circulation can be immediately computed.

Parsons, C. L.↗

NASA NTRS1985

Spectral measurements in support of SIR-B using the Surface Contour Radar

The use of the Surface Contour Radar (SCR) from an aircraft to obtain spectral information on the seas off the tip of South America, in support of the SIR-B experiment in October 1984, is reported. The SCR is a computer-controlled 36-GHz radar that measures sea surface directional wave spectra and produces a real-time topographical map of the surface below the aircraft. Ground tracks and polar plots of the data obtained are illustrated.

Walsh, E. J.↗

NASA NTRS1984

Rain and cloud effects on a satellite dual-frequency radar altimeter system operating at 13.5 and 35 GHz

The influence of clouds and rain on the return waveform signatures from satellite borne radar altimeters operating at 13.5 and 35 GHz are examined. It is specifically demonstrated that spatial nonuniformity in the cloud liquid water content or variations of the rain rate may result in significant distortions of the altimeter signature. The distorted signal is produced as a result of nonuniform attenuation occurring at the different range bins associated with the reflected signal. Determination of the mean sea height by employing tracking algorithms on these distorted echoes may result in gross errors. Although the influence of clouds on the altimeter signature and hence tracking precision is minimal at 13.5 GHz (e.g., less than 4 cm for a 1-s average), it may produce unacceptable mean sea level uncertainties at 35 GHz (e.g., 20 cm for a 1-s average) assuming a significant waveheight of 4 m. On the other hand, the signatures at both 13.5 GHz and 35 GHz become grossly distorted for rain rates of 10 mm/h and higher resulting in mean sea height errors of 46 and 65 cm, respectively, for significant wave heights of 2 m.

Walsh, E. J.↗

NASA NTRS1984

Altimeter height measurement errors introduced by the presence of variable cloud and rain attenuation

Monaldo, F. M.↗

NASA NTRS1984

The evolutionary trend in airborne and satellite radar altimeters

The manner in which airborne and satellite radar altimeters developed and where the trend is leading was investigated. The airborne altimeters have progressed from a broad beamed, narrow pulsed, nadir looking instrument, to a pulse compressed system that is computer controlled, to a scanning pencil beamed system which produce a topographic map of the surface beneath the aircraft in real time. It is suggested that the airborne systems lie in the use of multiple frequencies. The satellite altimeters evolve towards multifrequency systems with narrower effective pulses and higher pulse compression ratios to reduce peak transmitted power while improving resolution. Applications indicate wide swath systems using interferometric techniques or beam limited systems using 100 m diameter antennas.

Fedor, L. S.↗

NASA NTRS1984

Electromagnetic bias of 36-GHz radar altimeter measurements of MSL

The data reduction techniques used to determine the magnitude of electromagnetic (EM) bias in radar altimeter measurements of mean sea level (MSL) area described. Particular attention is given to the bias reduction scheme developed specifically for the Surface Contour Radar (SCR) instrument of the Ocean Topography Experiment (TOPEX). The SCR makes it possible to determine the amount of the backscattered power due to EM reflectance per unit area by measuring both the return power and elevation. Variations of backscattered power for different sea states are determined as a function of displacement of the MSL. On the basis of the recent SCR observations from aircraft, a standard error due to EM bias is predicted for MSL measurements performed with a satellite altimeter radar operating at a frequency of 36 GHz. The obtained standard error was 1 percent for regions with waves 1.9-5.5 meters in height.

Walsh, E. J.↗