468 
Center frequency 
Polarization 
Resolution 
Swath Width 
Bandwidth 
Integration Time 
1.4 GHz 
Horizontal 
+/- 4 degrees at nadir 
+/- 45 degrees 
25 MHz 
0.25 seconds 
Aperture synthesis is an interferometric technique in which the product (complex correlation) of the 
output voltage from pairs of antennas is measured at many different baselines. Each baseline produces a sample 
point in the Fourier transform of the scene, and a map of the scene is obtained after all measurements have been 
made by inverting the transform. ESTAR is a hybrid real and synthetic aperture radiometer which uses real 
antennas (stick antennas) to obtain resolution along-track and aperture synthesis (between pairs of sticks) to 
obtain resolution across-track (Le Vine et al., 1990). This hybrid configuration could be implemented on a 
spacebome platform (Le Vine et al., 1989). 
ESTAR employs five stick antennas, each consisting of eight horizontally polarized dipoles. The 
minimum spacing between sticks is A/2 (A = wavelength) and the maximum spacing is 7A/2 permitting 
measurements to be made of baselines which are integer multiples of A/2 between 0 and 7. For the non-zero 
spacings, ESTAR uses a sideband correlator in which the signal from each antenna is mixed to two intermediate 
frequencies (113.5 MHz and 143.5 MHz) and the multiplication is accomplished by summing the signals from 
the desired pair of antennas using channels with different intermediate frequencies and passing the sum through 
a square law detector and filter. This produces a signal at the difference frequency, 30 MHz, which is 
coherently mixed with a reference oscillator at 30 MHz to produce the in-phase and quadrature product. The 
products are averaged for 0.25 seconds in hardware. The measurement at zero spacing is done slightly 
differently to minimize system noise which would be correlated in this case. Additional details can be found 
in Le Vine et al. (1990). 
The effective beam created in the ESTAR image reconstruction (essentially an inverse Fourier 
transformation) is about +/- 45 degrees wide at the half power points. The image reconstruction algorithm in 
effect scans this beam across the field of view in 2 degree steps. The field of view is restricted to +/- 45 
degrees to avoid distortion of the beam but could be extended to wider angles if necessary. 
3 - ARIZONA 1991 
This was the first successful soil moisture deployment of the ESTAR radiometer. The primary reasons for 
selecting this particular site were that an intensive mapping effort had been conducted the previous year at this 
location using the L band push broom microwave radiometer (PBMR) and that meteorological conditions made 
it likely that a wide variety of moisture conditions could be encountered in a short duration field deployment. 
3.1. Walnut Gulch Watershed Experiment Description 
Walnut Gulch is a semiarid rangeland watershed located in southeastern Arizona and operated by the 
USDA-ARS Southwest Watershed Research Center. This watershed has a relatively long history of detailed 
hydrologic measurements and associated analysis (Renard, 1970) and was the focus of a major interdisciplinary 
experiment in the summer of 1990 (Kustas et al., 1990). As part of that experiment multitemporal L band 
radiometer data were collected using the push broom microwave radiometer (PBMR) (Schmugge et al., 1991). 
Vegetation cover of this watershed consists of sparse grass and desert shrub in the eastern and western 
portions of the study area, respectively. Wet biomass values in 1990 were typically 200-300 g/m 2 which should 
have minimal effects on the interpretation of the microwave brightness temperatures. Surface soils are mostly 
sandy loams with varying rock fractions ranging from 10 to 60% at sample locations. The uniformity of these 
surface conditions was a benefit in an initial evaluation of the ESTAR for soil moisture estimation. 
All ESTAR flightlines flown using the NASA C-130 aircraft during 1991 were also used in the 1990 
experiment involving the PBMR. This pattern resulted in contiguous coverage of an area approximately 5 km 
by 10 km at a ground resolution of 200 m. The 1991 flights were conducted on August 1 and August 3. 
Ground data collection during this experiment consisted of gravimetric sampling of the surface 5 cm 
of the soil and the measurement of the 5 cm soil temperature within one hour of the aircraft overflights. These 
data were collected at ten locations distributed over the area.