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The increase in accuracy of soil moisture retrieval from radar data requires a complete and accurate ground truth for calibration and verification. In order to support microwave backscattering models, one has to provide reliable permittivity and volumetric water content profiles of the soil. The conventional methods do not meet these requirements. The gravimetric sampling does not even deliver the volumetric water content. It has to be completed with an inaccurate measurement of the soil bulk density. Furthermore, the frequent probing of large areas is a tedious and expensive task. The electric conductivity is also a poor indicator due to its strong dependency on the salt content. Neutron probes have less and less acceptance and suffer from the shortcomings in measuring thin surface layers. The remaining possibility is the dielectric moisture determination by rf or microwaves, preferably in the optimal frequency range between 100 MHz and 1000 MHz. The measurement at single frequencies is better than the use of broadband pulses in the time domain, because of the capability to convert permittivities measured at one frequency to permittivities at the radar frequencies. Based on the measurement of the permittivity we have developed a combined sensor system for the ground truth. Our previously published frost-calibrated in-situ device evaluates the permittivity profile, the volumetric water content profile and the physical properties of the soil in regard to the permittivity of the solid component and in regard to the bound water. It uses a frequency of 250 MHz and it is installed at stationary locations. A second portable sensor extends the measuring area and completes the system. With frequencies of 500 MHz and 1000 MHz it is intermediate to the in-situ sensor and also to the radar frequencies in L-, C- and X-band. The scanning rate is about 100 m2 in 10 minutes, which is suited for extensive field campaigns. This portable sensor is based on a Goubau-type open-ended surface waveguide, where a simple dielectric coated stick touches the soil surface. Its sensing area is sufficiently large to integrate over soil inhomogeneities. The diameter of the area and the penetration depth of the electromagnetic wave are adjusted by the frequency and the dimensions of the stick. The relation between permittivity and water content is determined by the in-situ sensor, resulting in an overall system accuracy of better than 6% for the volumetric water content.
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