This project has been developed for the European Space Agency (ESA Contract No. 4000132509/20/NL/FF/ab) by the joint venture of ARS group of Commsenslab and the Department of Agri Food Engineering and Biotechnology (DEAB) of Barcelona School of Agri-Food and Biosystems Engineering (EEABB).
The main objective of this activity is to demonstrate the retrieval of soil moisture and vegetation parameters in an agricultural field under controlled conditions using a ground based fully-polarimetric SAR instrument (GB-PolSAR) to simulate the frequent acquisitions of GeoSAR missions. The data are being used to investigate GeoSAR's capability to retrieve soil moisture and to measure vegetation parameters.
This has been done with the GB-PolSAR instrument, designed and built by members of ARS group of CommSensLab. The incidence angles are similar to those of geosynchronous satellite observations over Europe. The measurement campaign has been designed to provide a continuous monitoring of an agricultural field with a temporal resolution of 10 minutes and a spatial resolution in the order of a square meter providing calibrated quad-pol Single Look Complex (SLC) images with a large number of resolution cells.
Two different crops have been sown and monitored. In the spring phase of the campaign, barley, a typical cereal cultivated in southern Europe, was sown and monitored until beginning of summer. Immediately after barley harvest, the field was ploughed, and corn was sown supported by an irrigation system. The corn crop was monitored through its life cycle until harvesting time in November. The different densities, sizes and water content of both crops will allow to assess GeoSAR capabilities to provide soil moisture and crop water content data.
The field is in front of the North North-West facade of the EEABB building. The cultivated area is 25 m far from the façade, its depth is approx. 22 m, and its width is around 60 m. The GB-SAR is mounted on the roof of the building, centred respect the cultivated area in order to minimize the squint angles
The main instrument is a Full-Polarimetric Ground Based Synthetic Aperture Radar, with two independent Zero-IF receivers and one transmitter chain. A Solid State Microwave Switch is connected among the transmitter and two orthogonal polarized antennas. Each of the two receiver chains is connected respectively to one of the two orthogonally polarized receiving antennas. The frequency generation unit is a DDS that generates a stepped linear frequency modulated continuous wave (SLFM-CW) chirp signal.The RADAR is mounted on a linear motion unit, capable of performing up to 2m long apertures. The aperture length and rail speed are fully controllable, with a speed limit of 1m/s. The linear motion unit is attached to a rigidized steel frame .
Quad-Pol Ground Based SAR data with incidence angles between 50◦ and 65◦ with a temporal resolution of 10 minutes has been provided. Two Single Look Complex (SLC) products have been generated: calibrated data (level 1b) and data with atmospheric refraction index change correction (level 1c). There are several techniques for atmospheric phase screen compensation in GB-SAR systems. Considering the size of the test site, refraction index changes are evaluated and compensated from the atmospheric ancillary data provided by the local Weather Station (installed at the test field). Other ancillary data have been collected as,
- Soil moisture
- Soil roughness
- Vegetation sampling: plant density, planting row direction, size and orientation of main plant elements, biomass and crop phenology, water cycle, Leaf Area Index (LAI), crop height and vegetation water content.
- Meteorological parameters: barometric pressure, precipitation, temperature, humidity and wind speed and direction
- Detailed irrigation information for corn crop.
A Corner Reflector was placed along the scenario. It facilitated the identification of the field extension and provided information of the polarimetric image calibration.
The set of four pyramidal horns were characterized in the Anechoic chamber
Soil moisture ground-truth was based on gravimetric measurements from samples taken at different positions of the field
Soil roughness was measured by means of a laser profiler
Different corner reflectors were permanently deployed for radiometric calibration of radar data
Long-term backscattering evolution of the barley crop. Different ancillary data has been included as rain, leaf area index (LAI), plant water content, plant height and soil moisture.
Long-term backscattering evolution of copolar and cross-polar channels of the corn crop. Different ancillary data has been included as rain, LAI and plant water content. Important peaks in the evolution of the backscattering can be clearly seen at the beginning of the corn campaign due to some rain events and the irrigation periods.
Time series of 10 min, 1h, 4h, 1 day, 3 days and 6 days temporal baseline coherence of the whole corn crop campaign for HH, VV, HV and VH channels for Mid Range: incidence angles (59.1 to 63 ). The evolution can be compared with ancillary data (plant height, rain and irrigation)