SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Three decades after the close encounter of 1566 Icarus, which marked the beginning of asteroid radar astronomy, we had a chance to observe the asteroid afresh by using a more developed radar system, and here we present the results of the study.
The polarization dependence of lightning echoes received with a 23-cm wavelength radar is estimated theoretically and experimentally. The reduction in the lightning radar cross-section due to the transmission of circular polarization instead of linear is measured to be about 4 dB and is much smaller than the 15-30 dB reduction in reflectivity of weak precipitation regions. The 8.7 dB average reduction in reflectivity maxima has been observed. This leads to an increase in the detected lightning flash rate by as much as 40%. The radar cross-section ratio of circular to linear polarization obtained experimentally suggests that lightning elements tend to be horizontally oriented.
RIG is a modeling system which creates synthetic aperture radar (SAR) and inverse SAR images from 3-D faceted data bases. RIG is based on a physical optics model and includes the effects of multiple reflections. Both conducting and dielectric surfaces can be modeled; each surface is labeled with a material code which is an index into a data base of electromagnetic properties. The inputs to the program include the radar processing parameters, the target orientation, the sensor velocity, and (for inverse SAR) the target angle rates. The current version of RIG can be run on any workstation, however, it is not a real-time model. We are considering several approaches to enable the program to generate realtime radar imagery. In addition to its image generation function, RIG can also generate radar cross-section (RCS) plots as well as range and doppler radar return profiles.
Imaging and altimeter-type radar employed in orbiting spacecraft for high resolution mapping, scattering cross section studies, climatology, etc
From the mid 1960s to the present, the normalized radar cross-section (NRCS) of the ocean has been measured using airborne radars operating over a frequency range of 0.4 to 14 GHz. Analyses of these data have shown that the NRCS was proportional to the ocean surface wind speed raised to some power, but the values of the exponent remained in dispute. This paper extends previous work and uses these NRCS measurements to demonstrate that to the first order, the NRCS is a function of only the friction velocity at the ocean's surface. Further analyses characterize the dependence of the NRCS on radar variables such as frequency, incidence angle, polarization, etc. Finally, recommendations are made for using Ku-band radars at large incidence angles for remote sensing of the wind friction velocity vector.
The design, construction, and data collection procedures of the mobile agricultural radar sensor (MARS) x band scatterometer are described. This system is an inexpensive, highly mobile, truck mounted FM-CW radar operating at a center frequency of 10.2 GHz. The antennas, which allow for VV and VH polarizations, are configured in a side looking mode that allows for drive by data collection. This configuration shortens fieldwork time considerably while increasing statistical confidence in the data. Both internal calibration, via a delay line, and external calibration with a Luneberg lens are used to calibrate the instrument in terms of sigma(o). The radar scattering cross section per unit area, sigma(o), is found using the radar equation.
The Meteoroid Input Function (MIF) model has been developed with the purpose of understanding the temporal and spatial variability of the meteoroid impact in the atmosphere. This model includes the assessment of potential observational biases, namely through the use of empirical measurements to characterize the minimum detectable radar cross-section (RCS) for the particular High Power Large Aperture (HPLA) radar utilized. This RCS sensitivity threshold allows for the characterization of the radar system s ability to detect particles at a given mass and velocity. The MIF has been shown to accurately predict the meteor detection rate of several HPLA radar systems, including the Arecibo Observatory (AO) and the Poker Flat Incoherent Scatter Radar (PFISR), as well as the seasonal and diurnal variations of the meteor flux at various geographic locations. In this paper, the MIF model is used to predict several properties of the meteors observed by the Middle and Upper atmosphere (MU) radar, including the distributions of meteor areal density, speed, and radiant location. This study offers new insight into the accuracy of the MIF, as it addresses the ability of the model to predict meteor observations at middle geographic latitudes and for a radar operating frequency in the low VHF band. Furthermore, the interferometry capability of the MU radar allows for the assessment of the model s ability to capture information about the fundamental input parameters of meteoroid source and speed. This paper demonstrates that the MIF is applicable to a wide range of HPLA radar instruments and increases the confidence of using the MIF as a global model, and it shows that the model accurately considers the speed and sporadic source distributions for the portion of the meteoroid population observable by MU.
An active radar calibrator (ARC), consisting of a receive antenna and a transmit antenna with an RF amplifier in between, is proposed as a tool for conducting high-precision calibration measurements of radar systems. The ARC can be designed to have a large radar cross-section with a broad pattern. Its major advantages over passive reflectors are its small physical size and its suitability for calibrating radars operating in a cross-polarized antenna configuration.
SCANSCAT conceptual scanning radar scatterometer placed in nearly polar orbit around Earth at altitude of 705 km aboard Spacecraft B of NASA's Earth Observing System. Measures radar backscattering from surface of ocean. Data processed on ground into normalized radar-backscattering cross sections, then processed into velocities of winds near surface of ocean by use of empirical mathematical model of relationship between normalized backscattering cross section, wind vector at scanned spot, and angle of incidence and azimuth angle of radar beam. Accuracy and coverage exceeds those of fan-beam scatterometer. Modified versions of scanning plan useful in laser inspection of surface finishes on machined parts.
Although designed specifically for the measurement of precipitation in the atmosphere, the Precipitation Radar (PR) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite alsomeasures the normalized radar cross-section at the Earth's surface. As such, this instrument providesan interesting opportunity to explore an alternative radar configuration in the satellite remote sensingnf ocean winds. In particular, it can measure the strength of the ocean winds in relatively higherspatial resolution as compared to the conventional scatterometer systems presently in use. The addition of wind sensing capability to precipitation radar also complements its rain profiling capabilitynicely and allows coincident wind and rain measurements. In this paper, we discuss the new ocean wind algorithm developed specifically for TRMM PR using maximum likelihood estimation. A uniquefeature of this wind algorithm is its capability to incorporate measurement sensitivity and noiseinformation consistently in both the along-track and cross-track directions. This PR wind algorithm hasbeen tested using data from the TRMM Microwave Imager (TMI) and NASA QuikSCAT scatterometer.Excellent agreement on the retrieved wind strengths is achieved among three sensors.
Orbital debris objects with sizes of 6 mm and larger were detected when they passed through the main beam of the high-power, 12.6-cm wavelength radar at the Arecibo Observatory. The number of objects detected at altitudes below 1000 km in the size range of 6 mm to 2 cm agreed with orbital debris model predictions to better than a factor of two. Radar detections of larger orbital debris (10-cm diameter and above) were also compared with the objects in the U.S. Space Command catalog (the cataloged objects have UHF radar cross-sections larger than 0.01 sq m). This experiment demonstrated that the large, earth-based radar telescopes normally used for planetary studies can provide useful data about the near-earth orbital debris populations.
Conceptual robot-vision system processes data from both microwave and visible-wavelength sensors. Concept, called "fusion of sensors," yields better information on shapes of some objects than obtained from one type of sensor alone. Proposed system fuses data from television images with polarized low-resolution radar-scattering cross sections. Using iterative procedure, generates successive approximations to shape of target by minimizing differences between observed radar returns and radar returns observed if object characterized by computed scattering transfer matrix.
Methods for the recovery of winds near the surface of the ocean from measurements of the normalized radar backscattering cross section must recognize and make use of the statistics (i.e., the sampling variability) of the backscatter measurements. Radar backscatter values from a scatterometer are random variables with expected values given by a model. A model relates backscatter to properties of the waves on the ocean, which are in turn generated by the winds in the atmospheric marine boundary layer. The effective wind speed and direction at a known height for a neutrally stratified atmosphere are the values to be recovered from the model. The probability density function for the backscatter values is a normal probability distribution with the notable feature that the variance is a known function of the expected value. The sources of signal variability, the effects of this variability on the wind speed estimation, and criteria for the acceptance or rejection of models are discussed. A modified maximum likelihood method for estimating wind vectors is described. Ways to make corrections for the kinds of errors found for the Seasat SASS model function are described, and applications to a new scatterometer are given.
For an airborne or spaceborne radar, the precipitation-induced path attenuation can be estimated from the measurements of the normalized surface cross section, sigma 0, in the presence and absence of precipitation. In one implementation, the mean rain-free estimate and its variability are found from a lookup table (LUT) derived from previously measured data. For the dual-frequency precipitation radar aboard the global precipitation measurement satellite, the nominal table consists of the statistics of the rain-free 0 over a 0.5 deg x 0.5 deg latitude-longitude grid using a three-month set of input data. However, a problem with the LUT is an insufficient number of samples in many cells. An alternative table is constructed by a stepwise procedure that begins with the statistics over a 0.25 deg x 0.25 deg grid. If the number of samples at a cell is too few, the area is expanded, cell by cell, choosing at each step that cell that minimizes the variance of the data. The question arises, however, as to whether the selected region corresponds to the smallest variance. To address this question, a second type of variable-averaging grid is constructed using all possible spatial configurations and computing the variance of the data within each region. Comparisons of the standard deviations for the fixed and variable-averaged grids are given as a function of incidence angle and surface type using a three-month set of data. The advantage of variable spatial averaging is that the average standard deviation can be reduced relative to the fixed grid while satisfying the minimum sample requirement.
Sea surface wind stress measurements recorded by a sonic anemometer are correlated with airborne scatterometer measurements of ocean roughness (cross section of radar backscatter) to establish the accuracy of remotely sensed data and assist in the definition of geophysical algorithms for the scatterometer sensor aboard Seasat A. Results of this investigation are as follows: Comparison of scatterometer and sonic anemometer wind stress measurements are good for the majority of cases; however, a tendency exists for scatterometer wind stress to be somewhat high for higher wind conditions experienced in this experiment (6-9 m/s). The scatterometer wind speed algorithm tends to overcompute the higher wind speeds by approximately 0.5 m/s. This is a direct result of the scatterometer overestimate of wind stress from which wind speeds are derived. Algorithmic derivations of wind speed and direction are, in most comparisons, within accuracies defined by Seasat A scatterometer sensor specifications.
We have investigated the small-scale topside ionospheric plasma structures first observed at Millstone Hill, Massachusetts with the 440 MHz incoherent scatter radar. These small-scale obliquely propagating plasma modes occurring in the vicinity of the midlatitude ionospheric trough, have large radar cross-sections and narrow spectral widths. They have, until recently, been dismissed solely as hard target contamination of the incoherent scatter radar. The geophysical conditions associated with the ionospheric trough, such as the field-aligned current activity and steep plasma density gradients, suggest that these recently discovered small-scale topside ionospheric plasmas may also appear in the auroral and polar ionosphere. In fact, this speculation has been corroborated by the preliminary experiments and data analyses at Tromso, Norway and Sondrestromfjord, Greenland. The primary research results are highlighted. Described in Section 3 are the experiments conducted at Arecibo, Puerto Rico in the past summer for simulating the geophysical conditions of generating these topside ionospheric plasma structures. Recommendation for the future research is finally given. Attached as the appendix of this report are several chapters which present the detailed results of research in the concerned topside ionospheric clutter. Highlights of the research results include: (1) causes of the enhanced radar backscatter (ERB) phenomenon; (2) occurrence of the ERB phenomenon; (3) altitudes of the ERB phenomenon; (4) strength of the ERB returns; (5) range of altitudes of the ERB returns; (6) occurrence frequency of the ERB phenomenon; (7) Doppler effect of the ERB phenomenon; (8) persistency of the ERB; and (9) distinction between ERB phenomenon and space object signatures.
Measurements of ocean, sea-ice and terrain geophysical parameters can be remotely sensed using an active microwave sensor known as the microwave scatterometer. Satellite scatterometers are spaceborne radars that measure the absolute backscattered power from the earth's surface; and from this measurement, the normalized radar cross-section (sigma(sup 0)) of that surface is determined. Scatterometers calculate sigma(sup 0) of a measurement cell by inverting the radar equation [1]: where: R is range to the cell center, P(sub t) is transmitted power, lambda is transmitted wavelength, L is total path loss, G(sub A) is peak antenna gain, A(sub c) is cell area, and c is a pre-computed factor due to the approximation of the radar equation for extended area targets.
A model is presented of the atmosphere and surface of Venus which best fits in the least-squares sense the available radio-brightness, radar cross-section, radio interferometric, and Mariner 5 and 10 radio occultation observations. The determinations of the radius of the planet obtained by others from radar time-delay measurements are included in the data set. The values of the adjusted parameters are: molar fraction of CO2 = 95 plus or minus 3%; fraction of combined nitrogen and argon = 5 plus or minus 3%; total atmospheric opacity at a wavelength of 1 cm = 19.4 plus or minus 1.3; mean radius of the surface = 6050.7 plus or minus 0.8 km; mean dielectric constant of the surface = 4.1 plus or minus 0.2, and percentage of total opacity due to chemical species other than CO2 = 45 plus or minus 12. The model temperature and pressure at the mean surface are 755 K and 91.4 atm, respectively.