Scientists Develop Reference Target for Orbital Satellites Measures Calibrating

13:30, February 19, 2019

The panel developed in cooperation with a Russian scientist will help improve the quality of the Earth’s surface remote sensing

For the first time, an international team of scientists from Russia, Estonia and Finland analyzed the operation of a “target” panel built in the south-east of Estonia to calibrate orbital satellites and to ensure the remote sensing of the planet’s surface with maximum accuracy (scanning the conditions of soil, crops, harvests, pastures, forests; urban, transport, industrial infrastructure, natural disasters, emergencies).

The scientists have published the research results in the “International Journal of Applied Observation of the Earth and Geoinformation".

The panel, being one of only a few similar structures on the planet, was constructed on the territory of the Järvselja Nature Reserve, between Tartu and the Estonian-Russian border, and is a horizontal, perfectly smooth, plain gray concrete platform 10×10m in size, protected from environmental exposure by a removable roof.

‘There are several methods for calibrating the sensors of artificial Earth satellites. The first is laboratory tests. This is when data on how light of a particular wavelength is scattered at a particular scattering angle and in a certain medium (for example, volcanic sand) are detected in “sterile” conditions by comparing the reflection spectra with special designs of the diffuser (material with high and stable reflectance). The second method of satellite sensors calibration is using the deserts, since their composition is homogeneous, and the state is relatively stable, or airfields. However, neither method can be compared in accuracy with the one we use,’ the co-author of the study, senior research fellow at UrFU Extra Terra Consortium, Candidate of Physical and Mathematical Sciences, associate professor Maria Gritsevich explains.

For example, the airfield grass changes its color throughout the year, but the platform developed by the scientists is “of the same color in winter and summer”. This feature provides high quality measurements, which is fundamentally important: satellite sensing is very expensive and the cost of a mistake is rather high. In the period of time when the satellite "sees" the platform, it receives data. They serve as a reference target in the "tuning" of the measures that the satellite outputs in the course of further Earth's surface monitoring. Without calibration, without comparison with the reference target, the data obtained during the sensing are irrelevant; they cannot be interpreted and applied.

The calibration process is the following: the panel reflects a stream of light coming from the sun or from a satellite, and polarizes it. That is, it converts natural, "unordered" light into the "organized" one – a bundle of oriented rays, which is received and processed by satellite sensors. Thus, it is not only the intensity of the reflected light that is measured, but also the polarization – how the reflecting surface scatters light. Moreover, using the platform, it is possible to achieve a detailed picture of how a certain wave of a particular frequency and length “behaves” in a given light scattering geometry. It is worth noting that the panel in Järvselja gives a stable signal throughout the year, which ensures steady reproducibility of the processes and results of satellite measurements and points at the reliability of the system.

Besides that, the panel can be used for processing measurements not only of visible light (waves ranging from 380 to 760 nanometers in length). The scientists have investigated how the panel interacts with waves in the range of up to 2,500 nanometers, that is, in the field of infrared and microwave radiation.

‘Some satellites are designed so that they capture only waves of a certain length and at a certain angle with which these waves are reflected from the Earth’s surface. The large wavelength range in which the panel that we studied and described operates allows us to “tune” a wide range of instruments, including artificial Earth satellites,’ Prof. Gritsevich adds.

Moreover, the use of the Estonian panel as a reference target allows to improve new models of artificial satellites, choosing the optimal wavelength, observation geometry (angle of light reflection) and image resolution.