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researchers used a radiometer, operating at 0.61 GHz and 4.7 GHz for 
sea ice studies (Kunzi, 1975). A 1.4 GHz (21 cm) radiometer is being 
used by the USGS to map relative soil moisture, and the Swiss have been 
conducting experiments with a realtime passive microwave imaging radi- 
ometer that operates in the millimeter range (3.3 mm) (Schaerer and 
Schanda, 1974; Schanda and Hofer, 1974). 
Platform status 
  
Skylab, Nimbus, Cosmos, and GEOS-3 satellites have carried 
microwave radiometers which have successfully performed in space. 
Interpretation status 
  
Collection and analysis of earth science data by passive microwave 
systems have been undertaken or sponsored by research organizations of 
several countries (Kunzi and others, 1974). Theoretical studies and 
research activities include modeling of microwave emissions to improve 
understanding of internal scattering, temperature, dielectric and other 
properties (England, 1974), determination of dielectric properties of 
soils under a variety of conditions (Hoekstra and Delaney, 1974), utili- 
zation of oscillatory interference as a microwave radiometric technique 
for determining bulk electrical properties of geological materials, 
snow, ice and other material conductive to layering experiments (Blinn 
and others, 1972; and Blinn and Quade, 1972) and soil moisture (Newton 
and others, 1974). : 
The data from the Skylab S194 experiment showed its usefulness for 
determining soil moisture over large areas (Moore and others, 1975). 
Good correlation coefficients were obtained from various soil layers to 
15 cm (Eagleman, 1974). Data for Cosmos-243 also supports these con- 
clusions. Agricultural crops and cultural features also showed terrain 
features at all seasons with details of fields and streams seen through 
accumulated snow (Thaman and others, 1974). 
Application status 
  
Passive microwave mapping of polar areas from the Nimbus-5 satellite 
at a frequency of 19.4 GHz (1.55 cm) revealed a number of significant 
differences from anticipated results: ice coverage on existing charts 
were at considerable variance with synoptic microwave imagery; multiyear 
ice in the Arctic was not distributed as predicted by models of ice 
dynamics; boundaries of the Antarctic icepack were more irregular than 
expected and areas of open water were more prevalent than shown on con- 
ventionally made maps; and the brightness temperatures over continental 
ice in both polar regions (lower than any except open water) show con- 
tours which are probably controlled by structural rather than 
temperature variations in the ice (Gloersen, Wilheit and others, 1973). 
The Nimbus-5 microwave system is an effective mapper of sea ice distri- 
bution on a timely basis and can be used for navigation as well as for 
studying regional dynamics.