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NASA NTRS1985

Analyses of Radar Images of Small Craters

Clouds hide the surface of Venus from all but radar imaging systems, supplemented by limited views from land spacecraft. Among the surfaces features likely to be observed by radar are craters that have formed by a variety of processes. In order to assess the radar characteristics of craters, volcanic craters and impact structures on Earth are described as imaged by the Shuttle Imaging Radar (SIR-A) experiment. Although most of the craters are small, this analysis provides insight into the ability to discriminate craters of various origins and provides some basis for interpreting radar images returned from Venus.

Greeley, R.↗

NASA NTRS1984

Frequency and site selection criteria for MST radars, part 5.1A

The majority of mesosphere-stratosphere-troposphere (MST) and ST radars are located in or near mountainous terrain. When measuring horizontal velocities, the terrain is a small factor, but when measuring vertical velocities, the meteorological noise induced by rough terrain can severely limit the usefulness of the observations. When the variance of the vertical velocity is too large, it is not possible to suitably filter the data to detect the small synoptic-scale signal with reasonable statistical confidence. The variance of vertical velocity at all tropospheric levels is directly related to the low level wind speed during flow over rough terrain. It is suggested that the synoptic-scale vertical velocity can be measured by ST radars where the terrain is smooth. The large-scale vertical velocity cannot always be reliably determined from MST radar data when the underlying terrain is rough. The vertical velocity is potentially on of future radar site selections, taking into account the desired meteorological applications of the data and engineering design factors. If the synoptic-scale vertical velocity is a desired variable, the radar should not be located near mountains.

Nastrom, G. D.↗

NASA NTRS1984

The MU radar now partly in operation

The MU radar (middle- and upper-atmosphere radar) of RASC (Radio Atmospheric Science Center, Kyoto University) is now partly in operation, although the facility will be completed in 1985. The active array system of the radar makes it possible to steer the radar beam as fast as in each interpulse period. Various sophisticated experiments are expected to be performed by the system. A preliminary observation was successful to elucidate atmospheric motions during Typhoon No. 5 which approached the radar site in August, 1983.

Kato, S.↗

NASA NTRS1984

The Ponape ST radar

In May, 1984, a 50-MHz ST radar was installed on the island of Ponape in the western equatorial Pacific (7 deg N, 158 deg E) by the Aeronomy Laboratory of NOAA. The radar consists of a 100 m x 100 m array with a single, vertically directed, beam and is initially transmitting micro sec. (2.25 km) pulses. The radar is operating continuously, with Doppler spectra being recorded at approximately 1 1/2 minute intervals and sent to Boulder for later analysis. One of the principal goals of the radar is to measure vertical motions in the troposphere and lower stratosphere at a location which is within the intertropical convergence zone during part of the year. First results, during generally fair weather conditions, show detectable echoes up to about 21 km with the tropopause at 17-18 km. Once daily balloon soundings are available locally from a NOAA Weather Service Office on the island, it is planned that this radar will be joined in the coming year by two others with oblique as well as vertical beams on two yet-to-be-selected equatorial islands as part of the TOGA (Tropical Oceans Global Atmosphere) program.

Carter, D. A.↗

NASA NTRS1984

Electromagnetic bias of 10-GHz radar altimeter measurements of MSL

Electromagnetic bias, the small difference that exists between the radar measured mean sea level and the geometric mean sea level is an important issue in high precision satellite altimetry. Present day satellite altimetry has achieved, with SEASAT-1, a precision of 5 cm rms in the range measurement. Future altimeter designs are expected to improve the range measurement precision to cm rms. In order to exploit the capability of these precise radar altimeters are marine geodesy and oceanography, it is necessary to understand and account for all of the known biases in the range measurement. The electromagnetic bias or the EM bias, which has been attributed to the observed fact that ocean wave troughs tend to be better reflectors of nadir viewing microwave radar energy than ocean wave crests, can be observed with high resolution airborne radar. This report presents the results of the EM bias measurements made by NRL using an airborne radar altimeter operating at 10 GHz with a 1 ns range resolution. Data were taken for various sea states and wind conditions. The experimental results are compared with current theories.

Choy, L. W.↗

NASA NTRS1985

Mapping diverse forest cover with multipolarization airborne radar

Imaging radar backscatter in continuously forested areas contains information about the forest canopy; it also contains data about topography, landforms, and terrain texture. For purposes of radar image interpretation and geologic mapping researchers were interested in identifying and separating forest canopy effects from geologic or geomorphic effects on radar images. The objectives of this investigation was to evaluate forest canopy variables in multipolarization radar images under conditions where geologic and topographic variables are at a minimum. A subsidiary objective was to compare the discriminatory capabilities of the radar images with corresponding optical images of similar spatial resolution. It appears that the multipolarization images discriminate variation in tree density, but no evidence was found for discrimination between evergreen and deciduous forest types.

Ford, J. P.↗

NASA NTRS1986

A study of 35-ghz radar-assisted orbital maneuvering vehicle/space telescope docking

An experiment was conducted to study the effects of measuring range and range rate information from a complex radar target (a one-third scale model of the Edwin P. Hubble Space Telescope). The radar ranging system was a 35-GHz frequency-modulated continuous wave unit developed in the Communication Systems Branch of the Information and Electronic Systems Laboratory at Marshall Space Flight Cneter. Measurements were made over radar-to-target distances of 5 meters to 15 meters to simulate the close distance realized in the final stages of space vehicle docking. The Space Telescope model target was driven by an antenna positioner through a range of azimuth and elevation (pitch) angles to present a variety of visual aspects of the aft end to the radar. Measurements were obtained with and without a cube corner reflector mounted in the center of the aft end of the model. The results indicate that range and range rate measurements are performed significantly more accurately with the cooperative radar reflector affixed. The results further reveal that range rate (velocity) can be measured accurately enough to support the required soft docking with the Space Telescope.

Mcdonald, M. W.↗

NASA NTRS1985

Preliminary geologic analyses of SIR-B radar data for Hawaii

A number of distinct topographic and textural units were discerned on SIR-B radar images of the Kilauea/Kau Desert area. The SIR-B enhanced large-scale topographic features, such as the Kahuku Pali and the Koae Faults (both oriented towards the radar antenna), while it was unable to detect features facing away from the radar antenna such as the Hilina and Kaoiki Pali. Textural units which were identified on the SIR-B images include aa flows of various ages, and several pyroclastic units. The extreme surface roughness of aa flows produced high radar returns on both the 28 deg and 48 deg SIR-B data sets. In contrast, pahoehoe flows produced smooth, low return units which were difficult to distinguish from surrounding radar-dark materials. It is suggested that multilook direction data, acquired at steeper incidence angles during a reflight of SIR-B, may aid in the further identification of pahoehoe flows and subtle topographic features.

Gaddis, L. R.↗

NASA NTRS1986

Paleodrainages of the Eastern Sahara - The radar rivers revisited (SIR-A/B implications for a mid-tertiary Trans-African drainage system)

The images obtained by the Shuttle Imaging Radar (SIR)-A and -B systems over the southwestern Egypt and northwestern Sudan were coregistered with the Landsat images and the existing maps to aid in extrapolations of the buried paleodrainages ('radar rivers'), first discovered by SIR-A. Field observations explain the radar responses of three types of radar rivers, RR-1 (broad, aggraded valleys filled with alluvium), RR-2 (braided channels inset in the RR-1 valleys), and RR-3 (narrow, long, bedrock-incised channels). A generalized model of the radar rivers, based on field studies and regional geologic relations, shows inferred changes in river regimen since the large valleys were established during the later Paleogene-early Neogene. It is suggested that a former Trans-African master stream system may have flowed from headwaters in the Red Sea Hills southwestward across North Africa, discharging into the Atlantic at the Paleo-Niger delta, prior to the Neogene domal uplifts and building of volcanic edifices across the paths of these ancient watercourses.

Mccauley, J. F.↗

NASA NTRS1986

Measurements of precipitating atmosphere by the MU radar

Sensitive VHF Doppler radars have the capability to detect echoes from precipitation particles as well as refractive index irregularities. Researchers used the middle and upper (MU) atmosphere radar at Shigaraki, Japan for tropospheric observations of precipitating atmosphere. They detected precipitation motions simultaneously with the ambient air motion and showed the capabilities of the MU radar in investigating mesoscale structures of meteorological phenomena such as air precipitation motions within a cold frontal system. More recently, a direct method for deducing the drop size distribution of precipitation particles was developed using Doppler spectra of the MU radar. This method is free from errors inherent in conventional measurements using microwave Doppler spectra. Meteorological microwave radar data are also utilized for monitoring vertical and horizontal structures of precipitation.

Wakasugi, K.↗

NASA NTRS1986

Possibility of measuring gravity-wave momentum flux by single beam observation of MST radar

Vincent and Reid (1983) proposed a technique to measure gravity-wave momentum fluxes in the atmosphere by mesosphere-stratosphere-troposphere (MST) radars using two or more radar beams. Since the vertical momentum fluxes are assumed to be due to gravity waves, it appears possible to make use of the dispersion and polarization relations for gravity waves in extracting useful information from the radar data. In particular, for an oblique radar beam, information about both the vertical and the horizontal velocities associated with the waves are contained in the measured Doppler data. Therefore, it should be possible to extract both V sub Z and V sub h from a single beam observational configuration. A procedure is proposed to perform such an analysis. The basic assumptions are: the measured velocity fluctuations are due to gravity waves and a separable model gravity-wave spectrum of the Garrett-Munk type that is statistically homogeneous in the horizontal plane. Analytical expressions can be derived that relate the observed velocity fluctuations to the wave momentum flux at each range gate. In practice, the uncertainties related to the model parameters and measurement accuracy will affect the results. A MST radar configuration is considered.

Liu, C. H.↗

NASA NTRS1986

Investigations of the lower and middle atmosphere at the Arecibo Observatory and a description of the new VHF radar project

The atmospheric science research at the Arecibo Observatory is performed by means of (active) radar methods and (passive) optical methods. The active methods utilize the 430 NHz radar, the S-band radar on 2380 MHz, and a recently constructed Very High Frequency (VHF) radar. The passive methods include measurements of the mesopause temperature by observing the rotational emissions from OH-bands. The VHF radar design is discussed.

Rottger, J.↗

NASA NTRS1986

Determination of U, V, and W from single station Doppler radar radial velocities

The ST/MST (stratosphere troposphere/mesosphere stratosphere troposphere) clear air Doppler radar, or wind profiler, is an important tool in observational meteorology because of its capability to remote observe dynamic parameters of the atmosphere. There are difficulties in transforming the observed radial velocities into meteorological wind components. How this problem has been treated in the past is reviewed, and some of the analysis is recast to a form more suited to the high diagnostic abilities of a number of fixed beam configurations with reference to a linear wind field. The results, in conjunction with other works which treats problems such as the effects of finite sample volumes in the presence of nonhomogeneous atmospheric reflectivity, have implications important to the design of both individual MST/ST radars and MST/ST radar networks. The key parameters to uncoupling terms in the scaling equations are w sub x and w sub y. Whenever the stratiform condition, which states that these two parameters are negligible, is satisfied, a five beam ST radar may determine unbiased values of u, v, and w for sample volumes directly above the radar. The divergence and partial deformation of the flow may also be determined. Three beam systems can determine w and w sub z, but are unable to obtain u and v wind components uncontaminated by vertical sheer terms, even when the stratiform condition is satisfied.

Clark, W. L.↗

NASA NTRS1986

A study of radar cross section measurement techniques

Past, present, and proposed future technologies for the measurement of radar cross section were studied. The purpose was to determine which method(s) could most advantageously be implemented in the large microwave anechoic chamber facility which is operated at the antenna test range site. The progression toward performing radar cross section measurements of space vehicles with which the Orbital Maneuvering Vehicle will be called upon to rendezvous and dock is a natural outgrowth of previous work conducted in recent years of developing a high accuracy range and velocity sensing radar system. The radar system was designed to support the rendezvous and docking of the Orbital Maneuvering Vehicle with various other space vehicles. The measurement of radar cross sections of space vehicles will be necessary in order to plan properly for Orbital Maneuvering Vehicle rendezvous and docking assignments. The methods which were studied include: standard far-field measurements; reflector-type compact range measurements; lens-type compact range measurement; near field/far field transformations; and computer predictive modeling. The feasibility of each approach is examined.

Mcdonald, Malcolm W.↗

NASA NTRS1986

Spaceborne imaging radar research in the 90's

The imaging radar experiments on SEASAT and on the space shuttle (SIR-A and SIR-B) have led to a wide interest in the use of spaceborne imaging radars in Earth and planetary sciences. The radar sensors provide unique and complimentary information to what is acquired with visible and infrared imagers. This includes subsurface imaging in arid regions, all weather observation of ocean surface dynamic phenomena, structural mapping, soil moisture mapping, stereo imaging and resulting topographic mapping. However, experiments up to now have exploited only a very limited range of the generic capability of radar sensors. With planned sensor developments in the late 80's and early 90's, a quantum jump will be made in our ability to fully exploit the potential of these sensors. These developments include: multiparameter research sensors such as SIR-C and X-SAR, long-term and global monitoring sensors such as ERS-1, JERS-1, EOS, Radarsat, GLORI and the spaceborne sounder, planetary mapping sensors such as the Magellan and Cassini/Titan mappers, topographic three-dimensional imagers such as the scanning radar altimeter and three-dimensional rain mapping. These sensors and their associated research are briefly described.

Elachi, Charles↗

NASA NTRS1987

Airborne Doppler radar for wind shear detection

There has been extensive discussion concerning the use of ground based Doppler radars for the detection and measurement of microburst features and the mapping of associated wind shears. Recent and planned research at Langley into technology and techniques useful for the future development of airborne Doppler weather radar systems for both turbulence and wind shear detection are addressed. Such systems, if successfully developed, would represent a marked increase in performance over airborne weather radars currently available. A principal difficulty in extending to airborne radars the capabilities of current ground based Doppler radars is emphasized.

Staton, Leo↗

NASA NTRS1987

Landform identification in lunar radar images

Lunar radar images have been investigated in order to understand how various radar parameters affect landform identification. The high-resolution (1-2 km cell size) 3.8-cm images of Zisk et al. (1974) were used along with the low and high resolution 70-cm mosaics of Thompson (1974 and 1987). The results indicate that radar cell size is the single most important radar parameter and that lunar features are likely to be correctly identified if they are longer than an 'identification resolution' of five times the radar cell size. The moon is considered here as an analog of Venus, since the two planets have similar mean scattering behavior.

Thompson, T. W.↗

NASA NTRS1987

Meteor observation by the Kyoto meteor radar

The Kyoto Meteor Radar is a monostatic coherent pulsed Doppler radar operating on the frequency of 31.57 MH. The system is computer controlled and uses radio interferometry for echo height determination. The antenna, an improvement, can be directed either to the north or the east. The system has been continuously collecting data on winds at meteor heights by radar observation. The meteor echo rate was also measured, the echo rate distribution with height and the daily variation in height integrated echo rate are discussed. Investigations of atmospheric tides are being pursued by cooperative observations. A novel approach to the study of gravity waves was attempted using the meteor radar which is able to detect the horizontal propagation of the waves by observing the changing phase through the region illuminated by the radar.

Kato, Susumu↗