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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Decorrelation in interferometric radar echoes

A radar interferometric technique for topographic mapping of surfaces promises a high resolution, globally consistent approach to generation of digital elevation models. One implementation approach, that of utilizing a single SAR system in a nearly repeating orbit, is attractive not only for cost and complexity reasons but also in that it permits inference of changes in the surface over the orbit repeat cycle from the correlation properties of the radar echoes. The various sources contributing to the echo correlation statistics are characterized, and the term which most closely describes surficial change is isolated. There is decorrelation increasing with time, but digital terrain model generation remains feasible.

Zebker, Howard A.↗
NASA NTRS1989

A model for depolarized radar echoes from Mars

The depolarized radar echoes from Mars are modeled using a combination of remote-sensing observations. The model reproduces the variations of the total radar cross-sections with longitude observed by Goldstone (1986) along 7 S, yields larger magnitudes of total radar cross-sections along 22 N than those along 7 S, and produces depolarized echo spectra that broadly match those observed by the Arecibo radar in 1980 and 1982. The model indicates that volcanoes and lava plains of the Tharsis-Alba Patera, Elysium, and Amazonia regions have the strongest depolarized echoes from the entire planet. Rock populations for the moon and Mars are estimated assuming depolarized radar echoes result from rocks with radii between 1.3 and three times the wavelength.

Thompson, T. W.↗
NASA NTRS1991

A radar-echo model for Mars

A radar-echo model for Mars based on 12.6 continuous-wave radio transmissions backscattered from the planet is developed. Mars' surface is divided into radar map units that are based on generalized geologic map units, and the radar map units are further subdivided using thermal inertias because the geologic units are insufficient to account for the quasi-specular echoes. The most important of the model's 118 radar-scattering units are characterized. Analyses of the observations and the model show that there are at least two populations of martian surfaces with distinct radar properties: the cratered uplands and northern low plains, and volcanoes and fields of lava flows. It is concluded that there are two dominant populations of surfaces that have distinct echo properties and that there is general agreement between the quasi-specular echo reflectivities of the present model and those of many other radar observations of Mars.

Moore, H. J.↗
NASA NTRS1984

Radar echoes at 2.66 and 40.92 MHz from the mesosphere, part 2.6A

During recent decades, the ionospheric D region has been scanned extensively by radar in the frequency range from 1 to 60 MHz. Progress has been made in understanding the reflection/scattering of radio waves in that area. Rocket measurement of ion density irregularities were compared to radar echo observations at 2.75 MHz with the conclusion that the received radar signal was due to scattering from isotropic and homogeneous turbulence in the altitude region between 70 and 95 km. However, scattering cross sections at 2 and 6 mHz suggest that the radar echo from the region below 80 km is in part due to partial reflection from stratified layers. The VHF scattering cross section is aspect sensitive in the D region below about 75 km and tends to be isotropic at higher altitudes. Positive correlations between scattered signal power and signal correlation time (P/C) have been observed by VHF radars in the lower mesosphere, with the conclusion that it might be an additional indication of partial reflection from stratified layers. In the upper mesosphere where the P/C correlation is negative, it is generally believed that the scattering is caused by isotropic turbulence. Radar echoes at the 2.16 and 40.92 MHz ranges are compared, assuming that both result from turbulent scatter. Adjusting the radar Bragg wavelength, it was found that both sets are due to scattering from the same layer of turbulence-generated irregularities.

Royrvik, O.↗
NASA NTRS1990

A radar-echo model for Mars

Researchers developed a radar-echo model for Mars based on 12.6 cm continuous wave radio transmissions backscattered from the planet. The model broadly matches the variations in depolarized and polarized total radar cross sections with longitude observed by Goldstone in 1986 along 7 degrees S. and yields echo spectra that are generally similiar to the observed spectra. Radar map units in the model include an extensive cratered uplands unit with weak depolarized echo cross sections, average thermal inertias, moderate normal refelectivities, and moderate rms slopes; the volcanic units of Tharsis, Elysium, and Amazonis regions with strong depolarized echo cross sections, low thermal inertia, low normal reflectivities, and large rms slopes; and the northern planes units with moderate to strong depolarized echo cross sections, moderate to very high thermal inertias, moderate to large normal reflectivities, and moderate rms slopes. The relevance of the model to the interpretation of radar echoes from Mars is discussed.

Thompson, T. W.↗
NASA NTRS1984

Ion cyclotron waves as a possible source of resonant auroral radar echoes

Some auroral radar Doppler data which show the presence of spectra not previously discussed are described. The data were obtained during highly distributed magnetic conditions with a relatively small 50 MHz radar pointed northeast of Ithaca, New York. The data are characterized by strong discrete echoes and display spectral peaks which are even narrower than type 1 and are centered at a considerably smaller Doppler shift. These echoes were present simultaneously with and adjacent in range to the commonly observed auroral spectra, and were probably obtained from a height of about 140 km or higher, well above the center of the auroral electrojet. The results are compared with existing electrojet instability theories, and other plasma instabilities which might be important are discussed. The Doppler shift suggests that the narrow spectra might be caused by ion cyclotron waves generated by field-aligned currents, but the observations cannot be fully explained by any of the theories.

Fejer, B. G.↗