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Bracalente, E. M.

Publications and source records attributed to Bracalente, E. M..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Simulation of the 1994 Charlotte Microburst with Look-Ahead Windshear Radar

A severe microburst occurred on 2 July 1994 at Charlotte, NC, and was associated with the crash of USAir Flight 1016 (FL-1016) (Salottolo 1994; Phillips 1994). The inbound DC-9 unexpectedly encountered a rapidly intensifying rainshaft just seconds before it was to touchdown on runway 18R. The aircraft crashed after encountering strong windshear, killing 37 of the 57 souls on board. The pilots did not recognize the windshear condition in time to prevent the accident and received no warning from the aircraft's Honeywell in-situ windshear detection system or from ground-based systems (Charlotte maintains both an ASR-9 weather radar and a Phase-2 LLWAS). Also two other aircraft landed ahead of FL-1016 without incident and reported smooth approaches to 18R. Section-2 of this paper reports briefly on the reconstruction of the event based on numerical results generated by the Terminal Area Simulation System (TASS) as presented at the National Transportation Safety Board (NTSB) public hearing (Proctor 1994). Section-3 discusses the simulation of this event with a look-ahead windshear radar.

Proctor, F. H.↗
NASA NTRS1993

Integrated display of multi-sensor geophysical and electromagnetic flight data

Low-altitude wind shear has been identified by several aviation interests as a significant hazard to the safety of flying. The hazard may be greatest when the wind shear is due to an unpredicted and short-lived microburst occurring in the immediate flight path of an aircraft during takeoff or landing. Researchers from the NASA Langley Research Center have investigated wind shear by developing a suite of remote sensing instruments and then using these instruments in an airborne wind shear detection flight program. Among these instruments were a Doppler radar, a lidar, and an infrared sensor; these were supported by in situ measurements of aircraft and environmental parameters and by ground-based Doppler radars. The basic problem addressed was the airborne detection and measurement of meteorologically-induced wind shear sufficiently ahead of the aircraft to allow avoidance. If the remote measurements of the shear and its associated radar reflectivity did not exceed set limits, the airplane then continued through the wind shear so that in situ measurements could be made for comparison. Initial detection and vectoring to the wind shear was normally given from a ground-based Doppler weather radar and most of the wind shear events were due to microbursts, both wet and dry. This paper considers the problems of sampling the wind shear event, time and space registration among the various sensors, coordination of the various sensors' beams and sampling volumes, and also various techniques for portraying the data, both for research and for presentation. Examples are given based on data from the 1991 and 1992 NASA/FAA wind shear flights, and the results and conclusions are generalized to other flight experiments involving multi-sensor electromagnetic data sets.

Delnore, V. E.↗
NASA NTRS1993

Description and availability of airborne Doppler radar data

An airborne, forward-looking, pulse, Doppler radar has been developed in conjunction with the joint FAA/NASA Wind Shear Program. This radar represents a first in an emerging technology. The radar was developed to assess the applicability of an airborne radar to detect low altitude hazardous wind shears for civil aviation applications. Such a radar must be capable of looking down into the ground clutter environment and extracting wind estimates from relatively low reflectivity weather targets. These weather targets often have reflectivities several orders of magnitude lower than the surrounding ground clutter. The NASA radar design incorporates numerous technological and engineering achievements in order to accomplish this task. The basic R/T unit evolved from a standard Collins 708 weather radar, which supports specific pulse widths of 1-7 microns and Pulse Repetition Frequencies (PRF) of less than 1-10 kHz. It was modified to allow for the output of the first IF signal, which fed a NASA developed receiver/detector subsystem. The NASA receiver incorporated a distributed, high-speed digital attenuator, producing a range bin to range bin automatic gain control system with 65 dB of dynamic range. Using group speed information supplied by the aircraft's navigation system, the radar signal is frequency demodulated back to base band (zero Doppler relative to stationary ground). The In-phase & Quadrature-phase (I/Q) components of the measured voltage signal are then digitized by a 12-bit A-D converter (producing an additional 36 dB of dynamic range). The raw I/Q signal for each range bin is then recorded (along with the current radar & aircraft state parameters) by a high-speed Kodak tape recorder.

Harrah, S. D.↗
NASA NTRS1991

Airborne radar simulation studies of the Denver July 11, 1988 microburst

In the simulation program, a wind shear detection Doppler radar was placed in UAL 395 and 236 aircraft and flown along their landing flight paths. The microburst was placed at the appropriate location and intensity corresponding to each aircraft's landing approach time. A baseline set of radar design parameters were used in the simulation. Output display information and wind shear detection processing was produced as the aircraft approached the microburst. Information on the results of the simulation study are given in graphical form.

Britt, Charles L.↗
NASA NTRS1989

Airborne Doppler radar flight experiments for the detection of microbursts

In the interest of aviation safety, NASA and the FAA are jointly conducting research to determine the applicability of airborne, coherent Doppler radar techniques to detect early microburst in wind shear conditions during aircraft takeoff and landing. Researchers have developed a computer model of the radar which predicts its response when viewing a simulated microburst against the simulated clutter background of an airport, the so-called radar microburst ground clutter model. Studies employing this model revealed that Doppler radar can accurately detect microburst ahead of the aircraft in time for pilot evasive response, but flight experiments will be required for complete performance evaluation of the system. An experimental X band radar is being developed for future flight experiments to verify the simulation modeling results. A description of the experimental radar, recording equipment, and its installation on the NASA 515 aircraft is presented. The flight experiments to be conducted are also described.

Blume, Hans-J. C.↗
NASA NTRS1988

Airborne Doppler radar detection of low altitude windshear

NASA and the FAA, as part of a joint research effort aimed at the development of airborne sensor technology for low altitude windshear detection during aircraft takeoffs and landings, are giving attention to the potential usefulness of a microwave Doppler radar operating at X-band or above. A preliminary feasibility study was conducted with a microburst/clutter/radar simulation program. It is found that, using bin-to-bin automatic gain control, clutter filtering, limited detection range, and suitable antenna tilt, the windshear generated from a high-moisture microburst can be detected with 10-65 sec of warning time.

Bracalente, E. M.↗
NASA NTRS1987

Wind shear detection. Forward-looking sensor technology

A meeting took place at NASA Langley Research Center in February 1987 to discuss the development and eventual use of forward-looking remote sensors for the detection and avoidance of wind shear by aircraft. The participants represented industry, academia, and government. The meeting was structured to provide first a review of the current FAA and NASA wind shear programs, then to define what really happens to the airplane, and finally to give technology updates on the various types of forward-looking sensors. This document is intended to informally record the essence of the technology updates (represented here through unedited duplication of the vugraphs used), and the floor discussion following each presentation. Also given are key issues remaining unresolved.

Bracalente, E. M.↗
NASA NTRS1985

Electro-science Requirements for Shuttle-attached Antenna Flight Experiments

The activities of an in-house electro-science task group organized to conduct studies of shuttle-attached flight experiments using the 15-meter hoop-column antennas as a research tool for developing both improved sensor technology and LSA technology are described. Some experiments could provide significant amounts of scientific data such as radio star mapping and definition of ocean current eddies over limited geographic regions. The experiments originate from the microwave remote sensing community and other areas which require the inherently higher resolution and boresite gain of large space antennas. Technology experiments are also being studied which would use the 15-meter antenna experiments as a stepping stone to 50 to 100 meter class reflector technology in the future. An antenna technology experiment using the 15-meter antenna in a shuttle-attached mission is discussed. Electromagnetic modeling is correct for each major subsystem and also to verify the interrelations of the subsystems.

Grantham, W. L.↗
NASA NTRS1984

Analysis of normalized radar cross section (sigma-O) signature of Amazon rain forest using SEASAT scatterometer data

The normalized radar cross section (NRCS) signature of the Amazon rain forest was SEASAT scatterometer data. Statistics of the measured (NRCS) values were determined from multiple orbit passes for three local time periods. Plots of mean normalized radar cross section, dB against incidence angle as a function of beam and polarization show that less than 0.3 dB relative bias exists between all beams over a range of incidence angle from 30 deg to 53 deg. The backscattered measurements analyzed show the Amazon rain forest to be relatively homogeneous, azimuthally isotropic and insensitive to polarization. The return from the rain forest target appears relatively consistent and stable, except for the small diurnal variation (0.75 dB) that occurs at sunrise. Because of the relative stability of the rain forest target and the scatterometer instrument, the response of versus incidence angle was able to detect errors in the estimated yaw altitude angle. Also, small instrument gain biases in some of the processing channels were detected. This led to the development of an improved NRCS algorithm, which uses a more accurate method for estimating the system noise power.

Bracalente, E. M.↗
NASA NTRS1982

The Seasat-A satellite scatterometer - The geophysical evaluation of remotely sensed wind vectors over the ocean

A description is given of the algorithm used to convert Seasat-A satellite microwave scatterometer measurements of ocean normalized radar cross section to the neutral stability vector wind at 19.5 m height, as well as to compare these winds with high-quality surface observations. The wind vector algorithm used an empirical normalized radar cross section model function to describe the ocean normalized radar cross section's dependence on the 19.5-m neutral stability wind vector. In addition, two model functions were evaluated by means of an independent set of in situ surface wind observations from the Joint Air Sea Interaction experiment (JASIN). Better results were produced by these comparisons than the stipulated Seasat wind speed and direction accuracy specifications of + or - 2 m/sec and + or - 20 deg, respectively, over the 0-16 m/sec range of winds observed during JASIN.

Jones, W. L.↗
NASA NTRS1982

Evaluation of atmospheric attenuation from SMMR brightness temperature for the Seasat satellite scatterometer

The effect of attenuation in precipitation regions of the sea, which must be considered in order to measure the radar backscatter from the ocean with sufficient accuracy to allow determination of the wind vector, can be ascertained from a knowledge of the brightness temperature observed by a microwave radiometer such as the Seasat multifrequency scanning radiometer. Two algorithms relating radiometric measurements and attenuation, and thereby correcting measured scattering coefficient values, were compared with wind vectors reported by surface observers and those determined by the Seasat scatterometer measurements with and without correction for atmospheric attenuation. Although the attenuation correction yields some improvements, it is constrained by both radiometer footprint differences and different scan patterns.

Moore, R. K.↗
NASA NTRS1982

Sigma/deg/ signature of the Amazon rain forest obtained from the Seasat scatterometer

Scatterometer data from Seasat of the Amazon rain forest were examined to determine if the region is suitable to use as a reference for calibration of radars. The consistency of Skylab data viewing the Amazon region prompted the analyses, and the Seasat-A scatterometer system (SASS) gathered data of the same region at varying angles. The instrument employed a 100 W 14.6 GHz signal with the reflected power sampled 61 times during each 1.89 sec measurement period. Doppler filters were used in 15 parallel channels of reception, and represented areas 20 km by 50-70 km. Tests were made of regional and temporal stability of the Amazon area, with five measurement angles averaged at different incidence angles to find the mean deviation, which was found to be less than 0.5 dB. Diurnal effects were confined to early morning, and further tests are recommended to obtain results for an entire year, to develop screening methods for thick clouds and rain, and to determine the deviation more precisely.

Birrer, I. J.↗
NASA NTRS1981

Evaluation of the Seasat wind scatterometer

Surface wind velocities have been derived from backscatter measurements of the ocean surface made by a satellite-borne, microwave sensor. Comparisons with high-quality surface-based measurements obtained during the Joint Air-Sea Interaction experiment are described. The accuracy of the scatterometer winds at this mid-latitude site, + or 1.6 m/s in speed and + or - 18 deg in direction, for winds between 3 and 16 m/s is within the design specification.

Jones, W. L.↗
NASA NTRS1981

Off-nadir antenna bias correction using Amazon rain forest sigma deg data

The radar response from the Amazon rain forest was studied to determine the suitability of this region for use as a standard target to calibrate a scatterometer like that proposed for the National Ocean Satellite System (NOSS). Backscattering observations made by the SEASAT-1 scatterometer system show the Amazon rain forest to be a homogeneous, azimuthally-isotropic, radar target which is insensitive to polarization. The variation with angle of incidence may be adequately modeled as sigma deg (dB) = alpha theta + beta with typical values for the incidence-angle coefficient from 0.07 dB deg to 0.15 dB/deg. A small diurnal effect occurs, with measurements at sunrise being 0.5 dB to 1 dB higher than the rest of the day. Maximum likelihood estimation algorithms are presented which permit determination of relative bias and true pointing angle for each beam. Specific implementation of these algorithms for the proposed NOSS scatterometer system is also discussed.

Birrer, I. J.↗
NASA NTRS1981

Performance evaluation of a spaceborne scatterometer

Study results are presented showing performance capability of a spaceborne scatterometer to operationally measure ocean surface wind speed and direction. In addition, a research mode is described which will allow development of improved radar signatures for ocean, sea ice, and land targets. The study results show that a scatterometer can meet the operational requirements of + or - 2 m/s wind speed accuracy (or + or - 10%, whichever is greater) and + or - 20 deg wind direction accuracy over most of the expected ocean surface conditions. The six beam scatterometer design evaluated is shown to be skillful (greater than 90% correct) in specifying the correct wind vector solution (with a 180 deg ambiguity) from the multiple solutions derived; further improvement must rely on meteorological and pattern recognition techniques now under study.

Grantham, W. L.↗
NASA NTRS1981

The study of mesoscale ocean winds

SAR data gathered over the U.S. East Coast are used to infer near-surface wind speed, using a simplified model to relate the strength of the 30 to 40 cm SAR backscatter to surface wind speed. It is found that the SAR backscatter is nearly isotropic with a wind speed exponent of 0.4. Using this exponent, the model is tested against an independent data set. Subsequent agreement between wind speeds simultaneously determined by SAR and the Seasat-A Satellite Scatterometer (SASS) is within 0.7 m/s rms. A one-dimensional Fourier transform of the SAR-derived wind field shows an increasing energy content in rough agreement with the Van der Hoven spectrum.

Jones, W. L.↗