A portable microwave radiometer for proximal measurements of soil permittivity (2023)


Precision agriculture (PA) has developed intensively over the last decade in response to the growing demand for food for the growing population. PA technologies ensure the efficient use and management of available resources to increase crop production, reduce freshwater consumption, maintain soil fertility, and protect the environment. Unlike conventional agricultural technologies, PA uses a variable application approach for cultivation practices (Shafi et al., 2019).

The effective implementation of AP is highly dependent on the availability and cost of advanced tools and technologies suitable for the agricultural industry, such as smart machinery and robotics, global positioning system (GPS), sensor networks, and monitoring systems, acquisition and information processing, etc. One of the key components of PA is a remote sensing/monitoring instrumentation responsible for measuring and processing data on plant and soil conditions. Remote monitoring works in conjunction with GPS to provide a georeferenced map of the characteristics of the field used for cultivation (Tsouros et al., 2019, Maes and Steppe, 2019, Kashyap and Kumar, 2021).

In the field of remote sensing, soil moisture monitoring is of particular interest due to global climate change in key agricultural regions and the extremely high proportion of fresh water used for irrigation. Campbell et al., 2017, reported that more than 70% of the world's fresh water is used for agricultural purposes. Soil water content primarily determines the biophysical processes that affect crop production and soil health. Therefore, a comprehensive and detailed knowledge of soil moisture is an important input parameter for a public address system to control water application and consumption. Further processing of georeferenced moisture data by advanced digitized farm management brings significant improvement in freshwater conservation and increase in crop production (Kashyap and Kumar, 2021).

In recent decades, various soil moisture measurement technologies based on different physical principles have been developed and are applicable to PAs. These technologies differ in terms of objective purpose, practical application, precision, cost, weight, etc. However, the most suitable soil moisture measurement solution for PA is a highly accurate, lightweight, mobile instrumentation system that allows for long/close distance measurement. -range measures. The demand for such a system is related to the growing group of drone-based surveillance applications integrated with public address infrastructure. Mobile sensor equipment is also suitable for integration into smart agricultural machines and robotics (Tsouros et al., 2019, Maes and Steppe, 2019, Inoue, 2020).

Traditional methods for determining soil moisture are generally based on stationary/well sensors or soil samples taken for analysis in a laboratory. These methods use a variety of physical principles, eg. B. Gamma ray laboratory analysis of a soil layer sample; the neutron scattering technique in wells (neutron probe); various electromagnetic sensor technologies, etc. (Kodikara et al., 2014, Balaghi et al., 2018, Babaeian et al., 2019) Conventional methods are actually point measurement techniques, making the development of a soil moisture map extremely difficult when a number of measurement points is not enough. Obviously, these methods cannot be used in applications where the mobility of the detection devices is essential.

Several advanced methods have been developed that provide remote measurement of soil moisture for aerial and satellite monitoring purposes. Soil moisture remote measurement technologies are generally represented by two methods: active measurement, where the instrument reads the reflected electromagnetic energy (radar), and passive measurement, where the energy emitted by the earth is recorded (radiometry). Remote sensing systems used in satellite Earth observation are designed for large-scale operation or for global mapping. Two projects have been launched to collect soil moisture data through satellite monitoring systems: (1) NASA's Soil Moisture Active Passive (SMAP) mission (Entekhabi et al., 2010, Chan et al., 2016) and (2) Soil Moisture and Ocean Salinity (SMOS) from the European Space Agency (ESA) (Kerr et al., 2001, Mecklenburg et al., 2012). However, the measurement accuracy of satellite monitoring systems is not suitable for AP and smart agriculture. The best precision of SMAP is 3 km in radar mode (Chan et al., 2016), while the spatial precision of the SMOS system is around 40 km (Mecklenburg et al., 2012).

Proximal methods developed in recent years include various ground-based radiometers and radar systems, as well as imaging and electromagnetic induction techniques. Proximal systems for soil moisture measurement are considered the best candidates for mobile/drone public address applications. The most promising technique is microwave radiometer technology. It is a passive microwave remote sensing instrument that measures the parameters of natural radiation emitted from the earth. The recorded emission data represents the brightness temperature of the Earth's surface (Ulaby and Long, 2014). Post-processing of the data converts it into the soil moisture value, which is linked to a specific geographic location. Microwave radiometer technology offers the opportunity to develop a soil moisture monitoring instrument with reduced size, weight, and cost.

The dielectric constant of the soil depends on many parameters. However, the dominant parameter that affects the dielectric constant is the biochemical composition of the soil. If the biochemical composition is defined, the main factor influencing the dielectric constant of the soil is the moisture content. Vegetation cover does not influence the dielectric constant of the soil, but it can lead to inaccuracies in the measurement process. This means that the best measurement accuracy can be obtained when the field has no vegetation cover or when the size of the vegetation is small.

This article proposes a new approach for a software algorithm for a microwave radiometer-based system to measure the soil moisture content of the observed earth. The proposed approach ensures rapid processing of the acquisition data to provide the surface dielectric permittivity, which is associated with the soil moisture content. The paper also discusses the details of design, development, and practical implementation of a low-cost, lightweight mobile instrumentation device for measuring and processing soil moisture data using the proposed approach. The instrument is designed to work in conjunction with GPS technologies and integrate with a public address management system. It is designed for use in drones, portable and mobile applications.

section cutouts

principle of operation

Microwave radiometry as a remote sensing method goes back to the work of Jansky and Dicke on the study of relict radiation (Tomiyasu, 1974). Originally, radio astronomers used ultrasensitive receivers of radio emissions from various objects and environments, called microwave radiometers. Subsequently, in the 1960s and 1970s, microwave radiometers for spacecraft installations were successfully developed and operated to provide observation and surveillance from space.

Proposed algorithmic approach

According to the proposed approach, two 3D temperature surfaces (Figs. 3 and 4) are intersected by a horizontal plane corresponding to a given radio brightness temperature measured by the radiometer in horizontal and vertical polarization. The cross section method produces two 2D curves in the plane with two axes: (x axis) real and (y axis) imaginary parts of the observed dielectric constant of the surface. Combined curves in the same plane intersect at a

Instrument prototypes

A prototype of the dual-polarization L-band portable radiometer was designed and built to measure soil temperature and moisture content using the proposed approach. The radiometer prototype was tested through a series of practical experiments to verify the performance of the software algorithm described in Fig. 6. Figure 7 shows a view of the prototype device. The main parameters of the radiometer are given in Table 1.

The radiometer circuit is based on the well-known diagram of

Field test results and discussion

For a practical field test, the microwave radiometer was mounted on a cart as shown in Fig. 12 and transported across the test field along pre-planned line tacks. Vertically and horizontally polarized radio brightness values ​​were recorded in the logger's memory simultaneously with data from the GPS navigation system and internal sensors. Two reference sources were calibrated at the end of each shift. Relic radiation was used as a "cold" source,


The document presents the development and construction details of the low-cost, lightweight mobile microwave radiometer prototype for measuring and processing soil moisture data. The instrument works according to the proposed software algorithm, which allows fast processing of the acquisition data. Using Earth radio brightness temperature readings, the software algorithm calculates the complex dielectric permittivity of the surface, which is related to the moisture content of the soil. The instrument

Conflict of Interest Statement

The authors declare that they are not aware of any competing financial interests or personal relationships that may have influenced the work described in this paper.


The research was funded by the Russian Scientific Foundation (Project No. 19-19-00349) and by a grant from the Federal State Budgetary Institution "Fund for Supporting the Development of Forms of Small Businesses in the Scientific and Technical Field" (Fund of Support for Innovation) contract n° 2886ГC1/ 45415. This document was supported by the Strategic Academic Leadership Program of the RUDN University.

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