Remote Determination of Soil Moisture by Microwave Radiometry Methods from Space and from Unmanned Aerial Vehicles

Язык труда и переводы:
Дата публикации:
16 декабря 2021, 00:09
Секция 11. Наукоемкие технологии в ракетно-космической технике
Gudkov Alexander Grigorievich
BMSTU; SPI firm “Hyperion”
Novichikhin Evgenii Pavlovich
Kotelnikov institute of radio engineering and electronics of RAS
Khokhlov Nikolai Fedorovich
Russian State Agrarian University – Moscow Timiryazev Agricultural Academy
The report considers the possibility of remote determination of soil moisture when sensing from a spacecraft and from an unmanned aerial vehicle. The relevance of remote measurement of soil moisture for the needs of agriculture, in particular for precision farming systems, is substantiated. The characteristics of digital soil moisture maps obtained from space and from an unmanned aerial vehicle are compared. The prospects of integrating digital soil moisture maps into the precision farming system are evaluated. The possibility of reducing the mass and dimensions of the sensor equipment is described. The results of flight tests of a microwave radiometer for creating digital maps of soil moisture made in various climatic zones are presented. Conclusions are drawn about the advantages and prospects of the proposed method.
Ключевые слова:
microwave radiometry, remote sensing, soil moisture, spacecraft, quadcopter
Основной текст труда

The modern agriculture technology for intensification of production and the development of modern digital farming technologies require prompt, actual, accurate and complete information about the state of soils during the cultivation of crops. Therefore, the task of remote determination of humidity and surface temperature of soils is urgent. One of the ways to solve this problem is to use of remote sensing methods, in particular, microwave radiometric sensing. At the current moment, various systems of space monitoring of the Earth's surface are known. However, the size of the resolved element of the known space systems is more than fifty by fifty kilometers, which is not enough to solve most of the tasks of the precision farming system. Aviation monitoring systems are too expensive to use. An alternative is the use of unmanned aerial vehicles (UAVs) [1–3]. Since the operation of a small-class unmanned aerial vehicle with a payload of up to five kilograms is significantly cheaper than using heavy unmanned aerial vehicles, the task of reducing the weight and size parameters of the onboard radiometer is also urgent.

The purpose of the report is to give a comparative characteristic of the obtained maps of soil moisture content during remote sensing of the Earth's surface from a spacecraft and from an unmanned aerial vehicle. And also to evaluate the prospects of integrating the results of remotely obtained data on soil moisture into the digital farming system for guaranteed sustainable crop yield growth and to demonstrate the possibility of reducing the weight and size characteristics of the onboard microwave radiometer by simplifying the antenna system and improving the elements of the microwave tract. Due to the rejection of direct measurement of radio brightness temperature by the nadir channel, that is, when viewing strictly vertically, with its replacement by calculation based on measurement data on vertical and horizontal polarizations when viewing at an angle to the vertical.

The proposed method of sounding the underlying surface from an unmanned aerial vehicle can also be used to solve such tasks as monitoring the hydrological situation along highways and railways [4], along pipeline routes [5], searching for places of underground leaks in earthen dams [6], assessing the fire hazard of forests [7], detecting oil films on the water surface [8]. The report presents the results of flight experiments on remote sensing of agricultural lands and earthen dams with a new bipolarization radiometer in the L-band, has made in various climatic zones of the Russian Federation, namely in the Smolensk, Moscow, Voronezh regions and in the Krasnodar Territory. In the results of flight experiments, digital terrain maps of the most important soil parameters were obtained remotely — moisture content in the productive layer, surface temperature, radio brightness temperatures for vertical and horizontal polarizations, polarization index. As a result of moving the carrier along pre-planned parallel lines over the studied field, information about radio brightness temperatures accumulates simultaneously at two polarizations. According to the measured values of radio brightness temperatures at vertical and horizontal polarizations, the polarization index is calculated as the temperature difference between vertical and horizontal polarizations divided by their sum. The value of the polarization index weakly depends on the temperature of the soil layer and is mainly determined by the value of the dielectric permittivity of the soil, which depends on the moisture content. The dependence of the polarization index on soil moisture varies under different conditions and depends on the composition of the soil, the presence of vegetation, etc. To accurately determine the radiation-humidity dependence, a special calibration of the radiometric system is required, in relation to specific conditions. All digital maps are obtained with the size of the resolved element not less than ten meters.

As a result of the conducted research, the following conclusions can be drawn:

  • the resolution of the track radiometers depends on the altitude of the flight and can range from meters to hundreds of meters, depending on the required detail of the moisture content map and the shooting performance;
  • sensing from an unmanned aerial vehicle can be performed at any time of the day or night in favorable weather, unlike space shooting, which is possible only when the spacecraft flies over a given area and in the absence of precipitation;
  • the obtained data of soil moisture measurement by a microwave radiometer from an unmanned aerial vehicle, together with data from ground weather stations and field samples, can be used to calibrate and refine data obtained by space systems on a global scale.
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