Development of the operational ROSAS data processing chain for in-flight calibration of optical HR sensors

Telespazio VEGA was selected by CNES, the French National Space Agency, to ensure the development of the operational RObotic Station for Atmosphere and Surface (ROSAS) data processing chain and the estimation of the in-flight absolute calibration of a series of optical HR sensors.

For several years, CNES has developed alternative methods for the absolute calibration estimation of optical HR sensors. These methods are complementary to on-board calibration devices, and exploit in-situ measurements simultaneously acquired with satellite data. This project for the development of the operational ROSAS data processing chain is in line with the thematic dedicated to the operations for in-flight calibration.

Challenge

The idea to use in-situ measurements for the absolute calibration of optical HR sensors was initiated by CNES several years ago. However, the acquisition of ground measurements in the framework of experiment campaigns has many disadvantages:

• Long-time staff mobilisation
• High costs
• Bad representativity of the measurements in time
• Meteorological conditions hazards, etc

To overcome these issues, CNES has developed a more efficient alternative approach based on an automatic and perpetual data acquisition system named the ROSAS station. This station has been installed for several years on the experimental La Crau site in the south-east of France. It consists of a CIMEL photometer for the acquisition part which provides several types of measurements regularly acquired throughout the day: sun irradiance, atmospheric radiance and surface radiance. The absolute calibration coefficients for a sensor (one coefficient per spectral band) are then estimated by comparing the signal acquired by the sensor with the theoretical Top Of Atmophere signal estimated from the ROSAS data. 

Project description

Telespazio VEGA was selected by CNES to ensure the development of the operational ROSAS data processing chain and the estimation of the in-flight absolute calibration of a series of optical HR sensors such as SPOT and FORMOSAT. The main tasks performed in the framework of this contract are:

• The software specification has several processing modules:
  1. Management of the data acquired by the ROSAS station and by the sensors to be calibrated
  2. Photometer calibration
  3. Sensor calibration
  4. Two modules for the expertise of the results related to atmosphere and surface parameters.

All the requirements have been collected by Telespazio VEGA through a series of interviews with CNES staff. These requirements have been sorted into several categories which include functional requirements, operational requirements, performance, interface and resources requirements, security and confidentiality, software quality and reliability, maintenance, software reuse, installation, etc.

• The software design covers several aspects:
  1. Definition of the framework which contains five major components (Data management, Processing management, Users management, IHM and Visualisation)
  2. Definition of the detailed architecture both on a static and dynamic point of view
  3. Design of the architecture of the I/O data archive which contains ROSAS and sensor data for the inputs and photometer and sensor calibration results for the outputs.
• The coding of the scientific algorithms and the associated IHM development. The processing modules are coding in Python language and use scientific reused codes for the atmospheric radiative transfer modeling. Unit tests are systematically developed to check the integrity of the codes. The IHM is also developed in Python language.

At the end of this contract, the development of the software will not be totally finished. The main future tasks are related to the connections between the processing codes and the IHM including the visualisation of the results, the consultation of the data, the development of useful tools such as log, export functions, printer management functions, installation procedures, users management, deployment, the software validation which includes integration tests and results validation, and the writing of the main technical software documentation (user manual, installation manual, configuration management document).

Key factors of success

Telespazio VEGA's team, featuring engineers who are highly skilled in computer science techniques (architect and developer), and one scientific expert in the specific field of the Earth Observation sensor calibration, was one of the key factors of the project's success.