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NASA NTRS1983

Application of the one-dimensional Fourier transform for tracking moving objects in noisy environments

In Riddle and Rajala (1981), an algorithm was presented which operates on an image sequence to identify all sets of pixels having the same velocity. The algorithm operates by performing a transformation in which all pixels with the same two-dimensional velocity map to a peak in a transform space. The transform can be decomposed into applications of the one-dimensional Fourier transform and therefore can gain from the computational advantages of the FFT. The aim of this paper is the concern with the fundamental limitations of that algorithm, particularly as relates to its sensitivity to image-disturbing parameters as noise, jitter, and clutter. A modification to the algorithm is then proposed which increases its robustness in the presence of these disturbances.

Rajala, S. A.↗

NASA NTRS1983

Investigation of image enhancement techniques for the development of a self-contained airborne radar navigation system

This study was devoted to an investigation of the feasibility of applying advanced image processing techniques to enhance radar image characteristics that are pertinent to the pilot's navigation and guidance task. Millimeter (95 GHz) wave radar images for the overwater (i.e., offshore oil rigs) and overland (Heliport) scenario were used as a data base. The purpose of the study was to determine the applicability of image enhancement and scene analysis algorithms to detect and improve target characteristics (i.e., manmade objects such as buildings, parking lots, cars, roads, helicopters, towers, landing pads, etc.) that would be helpful to the pilot in determining his own position/orientation with respect to the outside world and assist him in the navigation task. Results of this study show that significant improvements in the raw radar image may be obtained using two dimensional image processing algorithms. In the overwater case, it is possible to remove the ocean clutter by thresholding the image data, and furthermore to extract the target boundary as well as the tower and catwalk locations using noise cleaning (e.g., median filter) and edge detection (e.g., Sobel operator) algorithms.

Phatak, A. V.↗

NASA NTRS1983

Discrimination against interfering signals at the Poker Flat MST radar

Several on line and off line data processing techniques are used to remove interfering signals due to ground clutter, aircraft, instrumental effects, and external transmissions from the desired atmospheric echoes of Mesosphere Stratosphere, Troposphere (MST) radar. The on line, real time techniques are necessarily simple in order to minimize processing delays. This algorithm examines the individual Doppler spectra which are computed every two to four seconds (for oblique antenna beams). The total spectral power in each individual spectrum is computed by summing all the spectral points. If this integrated power increases from one spectrum to the next by a factor greater than a preselected threshold, then that spectrum is not added to the spectral sum. Succeeding spectra are compared to the last acceptable spectrum. Only a certain maximum number of spectra are allowed to be rejected in succession.

Carter, D. A.↗

NASA NTRS1983

Techniques for measurement of vertical and horizontal velocities: Monostatic versus bistatic measurements

Three techniques of direct measurement are presented from which velocities may be obtained: (1) Doppler frequency shift, (2) spaced antenna drift, (3) spatially modulated transverse beam measurements. Emphasis is placed on the Doppler frequency measurement approach. It will compare bistatic with monostatic configurations as regards received power (or sensitivity), spatial resolution, Doppler shift and avoidance of ground clutter.

Waterman, A. T.↗

NASA NTRS1983

Capabilities and limitations of the Jicamarca radar as an MST radar

The Jicamarca radar (Long. 76.52W, Lat. 11.56S), located at 20 km from Lima at approximately 500 meters over sea level, is surrounded by mountains which provide a good shield from man-made interference. The radio horizon goes from a few hundred meters, across the dry valley where it is located, to 15 km, along the valley in the direction of the continental divide. This limits the clutter to 15 km, except for one high peak at 21 km. It is the most equatorial of all existing MST radars. Its proximity to the Andes, makes its location unique for the study of lee waves and orographic-induced turbulence. Vertical as well as horizontal projections of MST velocities are obtained by simultaneously pointing with different sections of the antenna into three or four different directions. The transmitters, receivers, and systems for data acquisition, processing, and control are included.

Woodman, R. F.↗

NASA NTRS1983

Data processing techniques used with MST radars: A review

The data processing methods used in high power radar probing of the middle atmosphere are examined. The radar acts as a spatial filter on the small scale refractivity fluctuations in the medium. The characteristics of the received signals are related to the statistical properties of these fluctuations. A functional outline of the components of a radar system is given. Most computation intensive tasks are carried out by the processor. The processor computes a statistical function of the received signals, simultaneously for a large number of ranges. The slow fading of atmospheric signals is used to reduce the data input rate to the processor by coherent integration. The inherent range resolution of the radar experiments can be improved significant with the use of pseudonoise phase codes to modulate the transmitted pulses and a corresponding decoding operation on the received signals. Commutability of the decoding and coherent integration operations is used to obtain a significant reduction in computations. The limitations of the processors are outlined. At the next level of data reduction, the measured function is parameterized by a few spectral moments that can be related to physical processes in the medium. The problems encountered in estimating the spectral moments in the presence of strong ground clutter, external interference, and noise are discussed. The graphical and statistical analysis of the inferred parameters are outlined. The requirements for special purpose processors for MST radars are discussed.

Rastogi, P. K.↗

NASA NTRS1985

Microwave responses of the western North Atlantic

Features and objects in the Western North Atlantic Ocean - the Eastern Seaboard of the United States - are observed from Earth orbit by passive microwaves. The intensities of their radiated flux signatures are measured and displayed in color as a microwave flux image. The features of flux emitting objects such as the course of the Gulf Stream and the occurrence of cold eddies near the Gulf Stream are identified by contoured patterns of relative flux intensities. The flux signatures of ships and their wakes are displayed and discussed. Metal data buoys and aircraft are detected. Signal to clutter ratios and probabilities of detection are computed from their measured irradiances. Theoretical models and the range equations that explain passive microwave detection using the irradiances of natural sources are summarized.

Stacey, J. M.↗

NASA NTRS1984

Overview of on-line data processing for MST radars (keynote paper), part 7

The most important aspects of the processing of MST radar data are discussed. The important points of on-line data processing for MST radar are reviewed. The goals of the on-line and now almost exclusively digital processing, procedures are to achieve good altitude resolution and coverage, good frequency (Doppler shift) resolution, and good time resolution, while avoiding, the problems of range and frequency ambiguity (aliasing), ground clutter, and interference. Achieving optimum results requires pulse compression and some coherent integration. The first allows full utilization of the average power capability of the transmitter and the second reduces the computing requirements.

Farley, D. T.↗

NASA NTRS1984

Status of the Jicamarca radar

The capabilities of the large 50-MHz radar at Jicamarca for mesosphere-stratosphere-troposphere MST observations were discussed in some detail. Hence this description will be quite brief and will concentrate on recent improvements in the facility. The radar is located about 20 km from Lima, Peru. It is well shielded by surrounding mountains, and most of the ground clutter is restricted to ranges of 15 km or less. The antenna consists of 18,432 half-wave dipoles (9216 crossed pairs) covering an area of 290 m by 290 m and divided up into 64 independent modules which can be individually phased and/or used as separate antennas in any way desired. The whole array can be steered about 3 degrees from the on-axis position (the limit is the beam width of the individual modules, which cannot be steered), and any polarization can be arranged. Even with this limited steerability it is straightforward to determine vector wind velocities by pointing segments of the antenna in different directions. The radar can also be used as in interferometer.

Farley, T.↗

NASA NTRS1984

The Sondrestrom radar: Progress and proposed upgrades for ST work

The capabilities and limitations of the Sondrestrom Radar for stratosphere-troposphere ST observations were previously presented. Recent progress and proposed upgrades are summarized. During the first year of operation (April 1983 to April 1984) there was only one ST experiment, mainly with the objective of evaluating the site and determining that the hardare and software were still operational after the radar's move from Alaska. The Sondrestrom site has about the same degree of ground clutter as the former Chatanika site and the data appear to be similar. The radar's operation continues to be incoherent-scatter observations of the ionosphere. The main radar parameters are summarized.

Watkins, B. J.↗

NASA NTRS1984

Millstone Hill radar: Capabilities for S/T observations

During the past several years, the 440-MHz radar at Millstone Hill has been modified to detect coherent echoes from clear-air turbulence in the stratosphere/troposphere (S/T) over the altitude range 4-25 km. Two distinct modes of data acquisition have been developed, and data reduction programs have been completed for one of these modes. This mode (I-mode) transmits a 10 microsec (1.5 km) pulse on the fully steerable antenna. Typically, the antenna is set at a low elevation angle (e.g., 15 deg.) to reduce the altitude resolution to approximately 1 km., and power spectra are collected at some 40 range gates. The antenna may be scanned in azimuth to obtain the total wind vector, held fixed to monitor wave motion, or scanned in elevation to monitor the horizontal extent of the turbulent activity. This steerability gives Millstone a flexible system to focus on localized events, such as lee waves or convective storms. An additional advantage at low elevations is the relatively large Doppler shift of the signal, since the LOS velocity contains a large component of the horizontal velocity. This shift separates the turbulence signal sufficiently far from the ground clutter to allow the spectral moments to be readily inferred. Some 500 hours of S/T I-mode data have been reduced to geophysical parameters, and reside on a data base at Millstone Hill.

Loriot, G. B.↗

NASA NTRS1984

Pilot's Desk Flight Station

Aircraft flight station designs have generally evolved through the incorporation of improved or modernized controls and displays. In connection with a continuing increase in the amount of information displayed, this process has produced a complex and cluttered conglomeration of knobs, switches, and electromechanical displays. The result was often high crew workload, missed signals, and misinterpreted information. Advances in electronic technology have now, however, led to new concepts in flight station design. An American aerospace company in cooperation with NASA has utilized these concepts to develop a candidate conceptual design for a 1995 flight station. The obtained Pilot's Desk Flight Station is a unique design which resembles more an operator's console than today's cockpit. Attention is given to configuration, primary flight controllers, front panel displays, flight/navigation display, approach charts and weather display, head-up display, and voice command and response systems.

Sexton, G. A.↗

NASA NTRS1985

Birds and insects as radar targets - A review

A review of radar cross-section measurements of birds and insects is presented. A brief discussion of some possible theoretical models is also given and comparisons made with the measurements. The comparisons suggest that most targets are, at present, better modeled by a prolate spheroid having a length-to-width ratio between 3 and 10 than by the often used equivalent weight water sphere. In addition, many targets observed with linear horizontal polarization have maximum cross sections much better estimated by a resonant half-wave dipole than by a water sphere. Also considered are birds and insects in the aggregate as a local radar 'clutter' source. Order-of-magnitude estimates are given for many reasonable target number densities. These estimates are then used to predict X-band volume reflectivities. Other topics that are of interest to the radar engineer are discussed, including the doppler bandwidth due to the internal motions of a single bird, the radar cross-section probability densities of single birds and insects, the variability of the functional form of the probability density functions, and the Fourier spectra of single birds and insects.

Vaughn, C. R.↗

NASA NTRS1983

Evaluation of Doppler radar for airport wind shear detection

The utility of Doppler radar measurement techniques for detecting low-level wind shear at airports is examined. The Doppler radar data of the Joint Airport Weather Studies (JAWS) for windshear microburst features are analyzed, in order to determine the optimal resolution, sensitivity, and scanning strategy for a standard system. The performance of three separate systems for measuring wind shear at heights of less than 200 meters is compared. The three systems included a dual-Doppler system, a single off-airport Doppler system, and a single on-airport Doppler system. On the basis of the comparison the following recommendations are offered concerning the optimal performance of a Doppler radar system in the airport environment: (1) the system should be able to measure radial velocities over a range of reflectivity of -10-80 dBZ; (2) ground clutter should be reduced; and techniques for identifying and disseminating wind shear information should be automated.

Wilson, J.↗

NASA NTRS1986

Phased-array design for MST and ST radars

All of the existing radar systems fully dedicated to clear-air radar studies use some type of phased-array antennas. The effects of beam-steering techniques including feed networks and phase shifters; sidelobe control; ground-clutter suppression; low altitude coverage; arrays with integrated radiating elements and feed networks; analysis of coaxial-collinear antennas; use of arrays with multiple beams; and array testing and measure on structural design of the antenna are discussed.

Ecklund, W. L.↗

NASA NTRS1986

Remote sensing of Earth Terrain

The Mueller matrix and polarization covariance matrix for polarimetric radar systems were studied. The clutter is modelled by a layer of random permittivity, described by a three dimensional correlation function, with variance, and horizontal and vertical correlation lengths. This model is applied, using the wave theory with Born approximations carried to the second order, to find the backscattering elements of the polarimetric matrices. It is found that 8 out of 16 elements of the Mueller matrix are identically zero, corresponding to a covariance matrix with four zero elements. Theoretical predictions are matched with experimental data for vegetation fields. A general mixing formula is derived for discrete scatterers immersed in a host medium. The inclusion particles are assumed to be ellipsoidal. The electric field inside the scatterers is determined by quasistatic analysis, assuming the diameter of the inclusion particles to be much smaller than the wavelength. The results are applicable to general multiphase mixtures.

Kong, J. A.↗

NASA NTRS1986

Radar scene generation for tactical decision aids

The Mueller matrix and polarization covariance matrix for polarimetric radar systems was studied. The clutter was modeled by a layer of random permittivity, described by a three-dimensional correlation function, with variance, and horizontal and vertical correlation lengths. This model was applied, using the wave theory with Born approximations carried to the second order, to find the backscattering elements of the polarimetric matrices. Theoretical predictions are matched with experimental data for vegetation fields. The strong fluctuation theory was used to derive the backscattering cross sections. A two-layer anisotropic random medium model was developed for the active and passive microwave remote sensing of ice fields. A three-layer random medium model was adopted to study the volume scattering effects for the active and passive microwave remote sensing of snow-covered ice fields.

Kong, J. A.↗

NASA NTRS1987

Radar scene generation for tactical decision aids

The Mueller matrix and polarization covariance matrix for polarimetric radar systems was studied. The clutter is modeled by a layer of random permittivity, described by a three-dimensional correlation function, with variance, and horizontal and vertical correlation lengths. A general mixing formula was derived for discrete scatters immersed in a host medium. The results are applicable to general multiphase mixtures. The strong fluctuation theory was used to derive the backscattering cross sections, and was further extended to include higher order co-polarized and cross-polarized moments. A two-layer anisotropic random medium model was developed for the active and passive remote sensing of ice fields. A three-layer random medium model was adopted to study the volume scattering effects for the active and passive microwave remote sensing of snow-covered ice fields. The snow layer was simulated by an isotropic random medium and the ice layer by an anisotropic random medium. The vegetation canopy and snow-covered ice field were studied with a three-layer model, an isotropic random medium layer overlying an anisotropic random medium. The dyadic Green's function of the three-layer medium and the scattered electromagnetic intensities with Born approximation were calculated.

Kong, J. A.↗