Scientific and Technical Aerospace Reports Volume 38 July 28, 2000

temperature, and frequency. For wet snow, we use another mixing formula which gives the effective complex permittivity as a
function of snow wetness. With dry snow, the loss at low microwave frequencies is very small so the corresponding penetration
depths are very large (eg., 100 m). Clearly seasonal snow covers are far too thin to have a direct scattering or emission effect on
these low frequency bands. There are, however, indirect effects introduced because of altered reflection at the snow/soil boundary,
and an extra reflecting interface at the snow/air boundary. For C-band, layers of snow with different densities can have an impact
if the number of layers grows sufficiently large. (eg., many meters of snow pack with cm-scale density layering.) Wet snow poses
a more difficult problem. Liquid water is much more effective than ice at scattering and absorbing L-band radiation, so even a
small amount of wetness will greatly reduce penetration. For example, with a snow wetness of 1 percent (volume fraction), and
a density of 300 kg/cu. m, the penetration depths for L-band, S-band, and C-band are about 1.6 m, 0.3 m, and 0.15 m respectively.
Very wet snow has more than 10 percent liquid water, and the corresponding L-band penetration is less than 0.2 m. Because the
penetration distance is a strong function of frequency, it may be possible to identify a particular class of snow where the wetness
is a few percent, and the depth is around one meter by looking at the frequency gradient.
Author
Snow; Microwave Emission; Remote Sensing; Low Frequencies; Microwaves; Dielectric Properties; Ice
20000067687 NASA Glenn Research Center, Cleveland, OH USA
Using Airborne Laser Altimetry to Detect Topographic Change at Long Valley Caldera California
Hofton, M. A., Scripps Institution of Oceanography, USA; Minster, J.-B., Scripps Institution of Oceanography, USA; Ridgway,
J. R., Scripps Institution of Oceanography, USA; Williams, N. P., Scripps Institution of Oceanography, USA; Blair, J. B., NASA
Glenn Research Center, USA; Rabine, D. L., NASA Glenn Research Center, USA; Bufton, J. L., NASA Glenn Research Center,
USA; Remote Sensing of Active Volcanism; [2000], pp. 249-263; In English
Contract(s)/Grant(s): NAG5-3001; NAS5-33019; Copyright; Avail: Issuing Activity
The topography of the Long Valley caldera, California, was sampled using airborne laser altimetry in 1993, 1995, and 1997
to test the feasibility of using airborne laser altimetry for monitoring deformation of volcanic origin. Results show the laser altimeters
are able to resolve subtle topographic features such as a gradual slope and to detect small transient changes in lake elevation.
Crossover and repeat pass analyses of laser tracks indicate decimeter-level vertical precision is obtained over flat and low-sloped
terrain for altimeter systems performing waveform digitization. Comparisons with complementary, ground-based CPS data at a
site close to Bishop airport indicate that the laser and GPS-derived elevations agree to within the error inherent in the measurement
and that horizontal locations agree to within the radius of the laser footprint. A comparison of the data at two sites, one where no
change and the other where the maximum amount of vertical uplift is expected, indicates approximately 10 cm of relative uplift
occurred 1993-1997, in line with predictions from continuous CPS measurements in the region. Extensive terrain mapping flights
during the 1995 and 1997 missions demonstrate some of the unique abilities of laser altimetry; the straightforward creation of high
resolution, high accuracy digital elevation models of overflown terrain, and the ability to determine ground topography in the presence
of significant ground cover such as dense tree canopies. These capabilities make laser altimetry an attractive technique for
quantifying topographic change of volcanic origin, especially in forested regions of the world where other remote sensing instruments
have difficulty detecting the underlying topography.
Author
Airborne Lasers; Spaceborne Lasers; Altimetry; Laser Altimeters; Remote Sensing
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ENERGY PRODUCTION AND CONVERSION
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20000063364 Maryland Power Plant Research Program, Annapolis, MD USA
Bibliography of the Maryland Power Plant Research Program. 20th Edition Final Report
McLean, R. I.; Feb. 2000; 126p; In English
Report No.(s): PB2000-105792; PPRP-B-20; No Copyright; Avail: CASI; A07, Hardcopy; A02, Microfiche
This bibliography is a compilation of over 700 reports of electric power related studies conducted by the Power Plant Research
Program since 1971.
NTIS
Bibliographies; Electric Power Plants; Hydroelectric Power Stations; Maryland