Estudio de la medida de la humedad del suelo mediante reflectometría GPS

Abstract

Following the advance in climate change on our planet, appears the need to study accurately the weather of Earth. In this PFC, Soil Moisture (SM) has been the focus of study , which is one of the fundamental parameters to understand the water flow and to make more specific weather forecasts, to increase agricultural productivity, to reduce the risks of climate disasters, etc. Taking reference of SMOS and SMIGOL projects, obtaining soil moisture is carried out by reflectometry GPS, in other words, based algorithms used in the calculation of specular reflection points of the GPS signals on the ground. While the area in which it has developed the PFC can be considered as a flat ground, the necessary equations for rough terrain has been defined as well. They define three mathematical conditions between the vectors formed between the point of the ground and the antenna, the point and the satellite and the normal to the ground. As a result we obtain all points of the ground where, for a given satellite position (azimuth and elevation), we can consider that there is a specular reflection. To capture satellite signals GPS, direct and reflected by the ground, Down and Up Observa- tions (DUO) instrument was used, which is a two-channel reflectometer that is based on a very high directivity antenna (D=26,25 dB, a parabolic reflector with a diameter of 1.5 m). This reflector is feed by a dual-polarized feeder (Vertical and Horizontal). Furthermore, using a GPS receiver which allows to obtain the map of visible satellites. The receiver antenna has been fixed to a mo- tor that moves it from a PC controller in azimuth and elevation in order to point to the sky or ground. The data capture process consists of 3 steps, the first point directly to the satellite and get one minute of data. The second point to the specular reflection point and get samples during 8 minutes. In that time the position of the satellite and the reflection point is updated every 2 minu- tes. Finally, the data is saved in a text file and the process is repeated with the next satellite visible. As we do not have internet access on the ground where the study has been conducted, the DUO data has been stored in order to study it later. Using these data the reflectivity values es- timated for each of the polarizations are obtained. To simplify the effects of the roughness of the terrain, both reflectivity values are divided and estimated is obtained. On the other hand, from the theoretical formulas the values of theoretical reflectivity are obtained for all soil moisture va- lue, then a division between polarizations is made to obtain atheory. Finally, a comparison between theory atheory and aestimated is made in order to extract the estimated value of Soil Moisture (SM). To PFC operation have been developed Matlab and Visual Basic the scripts for controlling the movements of the antenna, capturing data in the field, obtaining the soil moisture and the representation on Google Earth (GE). To conclude, a web side has been created to show the results obtained. This would be useful in the case of having internet access on study area, because that data could be sent in real time.