1223
PASSIVE MICROWAVES
T. Schmugge (1) and Y. Kerr (2)
(1) USDA, Hydrology Lab. Beltsville (U.S.A.)
(2) LERTS, UMR CNES/CNRS, Toulouse (France)
The session was devoted to the use of passive microwaves in remote sensing. The 3 oral presentations and 11
posters dealt mainly with applications over land and covered field experiments, modelling activities and new
instruments.
1 - EARLY RESULTS
Results from two new instruments were presented.
The first one is the Electronically Steered T hinn ed Array radiometer (ESTAR) which uses the antenna
synthesis technique. With such systems, low frequency radiometers (L band) could be space borne and give
reasonable ground resolutions (10 to 50 km), at considerably lower weights than with conventional systems. The
ESTAR presented is an airborne version developed to test the concept. The instrument has been flown on a
number of occasions in the US and the paper presented the first results. They showed that not only the concept is
proved and the instrument worked as expected but also that the retrieved brightness temperatures are at least as
good as those obtained with conventional radiometers (Push Broom Microwave Radiometer - PBMR).
Subsequent studies related changes in brightness temperature to rainfall patterns in semi-arid environments and
showed that brightness temperature were well correlated to ground measurements of soil moisture.
The second instrument presented is PORTOS which is a conventional radiometer (5-90 GHz) which can
be used either on an airplane or at ground level (with additional 1.4 GHz channels). The instrument is
performing very well and several posters were based on PORTOS data. The PORTOS radiometer can now be
considered as operational and should prove an important tool for both modelling activities and preparation of the
M1MR to be launched on EOS pm (NASA) and Metop (ESA).
2 - EXISTING SENSORS
Several presentations dealt with the use of existing sensors, either on satellite or aircraft and/or ground.
2.L Satellites
Most of the studies performed with satellite data were related to relatively high frequencies (i.e. above 20 GHz)
since those are the only ones currently available. The sensors considered were the SSM/I and the ATSR/MW.
Considering the relatively high frequencies, studies were devoted to specific applications. Two studies aimed at
the retrieval of surface temperature, either with use of satellite data only (giving accuracies of the order of
2.6°C) or coupled with models. Another study presented results on the determination of rainfall rates in Southern
Europe during very severe storms using a cloud radiance model. The comparison with conventional
measurements (Met radar, rainfall gauges) was satisfactory. The major problem with such algorithms is that of
having satellite acquisitions synchronous to the rain event. The only study related to the ATSR/MW was dealing
with the retrieval of water vapor over the oceans and a comparison with the results gained from the ATSR/IR.
The method relied on the two microwave channels (22.8 & 36.5 GHz) and proved to be efficient.
22. Field experiments with A/C data
Two presentations dealt with the extraction of soil moisture. The first one was related to an intercomparison of
brightness temperature maps obtained during the Hapex-Sahel experiment with the L band PBMR and PORTOS'
C band. The two sensors agree very well qualitatively. The relationships between rain events and brightness
temperatures, soil maps patterns related to soil hydraulic properties, were also presented.
