Scientists produce orbital sensor calibration guidebook

The many applications of satellite data include investigating climate change, monitoring deforestation and estimating crop yields.

By José Tadeu Arantes

Agência FAPESP – A guidebook explaining the procedures required to guarantee the quality and accuracy of the information supplied by satellites has just been published by a team of Brazilian researchers.

Entitled Calibração de sensores orbitais, the book was written by Flávio Jorge Ponzoni (National Space Research Institute, INPE), Cibele Teixeira Pinto (INPE and the Brazilian Air Force Command’s Institute for Advanced Studies, IEAv), Rubens Augusto Camargo Lamparelli (University of Campinas, UNICAMP), Jurandir Zullo Junior (UNICAMP), and Mauro Antonio Homem Antunes (Federal Rural University of Rio de Janeiro, UFRRJ).

Distilling insights from years of study, the book comes at a time of growing demand for remote sensing to investigate climate change, monitor forests and estimate crop yields, among many other applications.

Several of the book’s authors are members of the CEOS Working Group on Calibration & Validation (WGCV). CEOS is the Committee on Earth Observation Satellites, which Brazil joined in 2009.

The WGCV’s mission is to standardize the methodologies and procedures used by countries that possess earth-observing systems to ensure their interoperability and the comparability of data obtained from different parts of the globe.

The first calibration performed by Ponzoni, the coordinator of the group, and the first in the southern hemisphere, occurred at Bolivia’s Uyuni Salt Flats as part of the project “Evaluation of changes in the radiometric sensitivity of the TM/Landsat sensor from 1988 to 1997 and spectral characterization of the test area,” supported by FAPESP.

“Uyuni was chosen because we needed to perform the calibration on a very bright, smooth surface,” Ponzoni told Agência FAPESP. “In addition to these two features, the Bolivian salt desert offered the advantage of low atmospheric disturbance thanks to its high altitude at 3,600 m.”

The first step of calibration consists of ground measurements of the radiance reflected by the surface at the exact moment of the satellite’s passage over the area. The reflected radiance is the intensity of the electromagnetic energy reflected by the surface, measured at different wavelengths.

Ideally, the satellite should detect the same value measured on the ground, but the atmosphere produces distortions. Thus, the next step entails modifying the data collected in the field using information about the atmosphere to obtain a theoretical estimate of the radiance measured by the sensor on board the satellite.

In the final step, this theoretical estimate is compared with the orbital sensor reading to determine a calibration coefficient, which can then be used to convert the digital counts from satellite images into physical values.

“Once the calibration has been performed, we can determine real ground reflectance in each band of the electromagnetic spectrum for all satellite sensor data,” Ponzoni said.

“And when we know that, we can estimate the physicochemical properties of the object observed, such as the soil’s iron content, or the amount of biomass in the plant cover and even leaf inclination angles.”

Radiometric values

The many applications of satellite sensor data include monitoring deforestation, appraising reservoir water quality, estimating crop yields and investigating the phytosanitary characteristics of plant cover, among countless other qualitative and quantitative parameters.

According to Ponzoni, Brazil was not especially interested in orbital sensor calibration until around the mid-1990s because there was a culture of buying data produced abroad.

“We would buy the rights to satellite images produced by other space programs and use them basically as if they were photographs,” he said.

As demand became more sophisticated, however, users learned that satellite images are much more than photographs; they provide a wealth of quantitative data, such as plant biomass, chlorophyll stocks, suspended sediment loads in water supply reservoirs, and many other variables in a wide variety of contexts.

“This led to the realization that we had to develop our calibration capabilities,” Ponzoni said. “By acquiring sensor calibration data, we can use satellite images not just as photographs but also as a source of radiometric data. This, in turn, enables us to characterize targets spectrally and hence arrive at quantitative estimates of the variables described.”

INPE maintains a database of all satellite data collected in Brazil.

Calibração de sensores orbitais
Authors: Flávio Jorge Ponzoni, Cibele Teixeira Pinto, Rubens Augusto Camargo Lamparelli, Jurandir Zullo Junior, Mauro Antonio Homem Antunes
Publisher: Oficina de Textos
No. of pages: 96
Price: R$45.00 (e-book R$38.25)
More information: www.ofitexto.com.br/calibracao-de-sensores-orbitais/p (in Portuguese)