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All remotely sensed soil moisture data have to be calibrated by reference measurements on ground. The gravimetric sampling (oven drying) is not suitable. It is time consuming, expensive, and destroys the site. Also the neutron probe has less and less acceptance. The remaining possibility is the dielectric moisture determination by rf or microwaves. The optimal measuring frequency for such a device is between 100 MHz and 1000 MHz. Our previously published frost-calibrated in-situ sensor uses 250 MHz. The difference to the used remote sensing frequencies, in L-C- and X-bands is too large. In addition the stationary in-situ device measures only its closest neighborhood. The remotely sensed pixel however is much larger. Therefore, we developed an intermediate sensor for a good compromise in these discrepancies. With frequencies of 540 MHz and 940 MHz it is intermediate to the in-situ sensor (250 MHz) and to the used L-C-X-bands. It is a Goubau-type open-ended surface waveguide, a simple dielectric coated stick, touching the soil surface. The reflected wave on the Goubau-line is a function of the moisture from a well-defined area under the line (0 0.4 m). So, it is intermediate concerning the measuring area, as well. We need approximately 10 minutes for scanning an area of 100 m2. This time is short enough. On the other side, this surface moisture sensor will be calibrated by a very accurate frost-calibrated in-situ sensor. An overall system accuracy of 6% can be achieved by this two step indirect ground truth.
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