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Dai, Eryan ¹ ; Gasiewski, Albin J ² ; Stachura, Maciej ³

¹ Dept. of ECEE, University of Colorado
² Dept. of ECEE, University of Colorado
³ Black Swift Technologies LLC, Boulder, CO, United States

Soil moisture is of fundamental importance to many hydrological, biological and biogeochemical processes, plays an important role in the development and evolution of convective weather and precipitation, and impacts water resource management, agriculture, and flood runoff prediction. The launch of NASA?s Soil Moisture Active/Passive (SMAP) mission in 2015 promises to provide global measurements of soil moisture and surface freeze/thaw state at fixed crossing times and spatial resolutions as low as 5 km for some products. However, there exists a need for measurements of soil moisture on smaller spatial scales and arbitrary diurnal times for SMAP validation, precision agriculture and evaporation and transpiration studies of boundary layer heat transport. The LDCR provides a means of mapping soil moisture on spatial scales as small as several meters (i.e., the height of the platform). Compared with various other proposed methods of validation based on either situ measurements [1,2] or existing airborne sensors suitable for manned aircraft deployment [3], the integrated design of LDCR on a lightweight UAS thus is capable of providing sub-watershed (~km scale) coverage at very high spatial resolution (~15 m) suitable for scaling scale studies, and at comparatively low operator cost.

Based on the requirements of LDCR as a lightweight broad-beam radiometer with lobes that view both nadir and zenith, a low-cost thin lobe-differencing 2x2 microstrip colinear (MiCo) antenna array using MiCo radiators was designed and implemented[4,5]. The allocated EESS frequency band from 1400-1427 MHz was selected to provide a compact antenna design that probes soil moisture to ~5-10 cm under most conditions. Each of the four elements consists of a 5-segment microstrip colinear patch antenna with an omnidirectional radiation pattern. Measured return losses are compared to an HFSS model of these antennas and are in good agreement with simulations. The separation and orientation of the 2x1 subarrays to form the complete 2x2 array is optimized, and the array center frequency sensitivity is determined based on both simulations and laboratory measurements (Figure 1). The performance of a 2x2 antenna array integrated into the Tempest fuselage is presented, and a mutual coupling analysis for antenna subarray pairs with respect to vertical distance is investigated. A method of tuning the resonant frequency is also presented (Figure 2).

LDCR build01 is an experimental portable soil moisture sensor using the LDCR technique that can be operated by single person directly or held stable by an experimental fixture using a long pole with two fulcrums at the midpoint. (Figure 3). LDCR build01 had output DC voltage which is in ratio of the brightness temperature difference in nadir and zenith direction along with NDVI information. Compared with analytically calculated value, the measured ΔTRMS is 44% higher for man held unit and 15% higher for experimental fixture held unit.

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Holub, A.and M. Polivka, ?Colinear Microstrip Patch Antenna,? Passive Microwave Component and Antennas, April2010.