The paper discusses a new approach to software algorithm for a microwave radiometer-based measurement system to determine the soil moisture content of the observed surface. The approach provides rapid processing of radio brightness temperature readings to calculate the surface dielectric permittivity corresponding to the soil moisture content. The paper also discusses the design and development details of a portable instrumentation device for measuring and processing the soil moisture data using the proposed approach. The radiometer prototype was built and employed to conduct a practical experiment to determine the soil moisture of a test field. The results showed an effective operation of the algorithm to measure the moisture data and to produce the geo-referential map. The radiometer has a weight of 3.9 kg, a portable form factor and is designed for various proximal measurement applications including deployment on the board of small-class unmanned aerial vehicles (drones).