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Introduction
SADS : Generic Web Access to Space Data
Thierry Levoir,
CNES, CT/TI/PS/TD
18, avenue Edouard Belin
31055 Toulouse Cedex, FRANCE
E-mail : levoir@cst.cnes.fr
Rémi Bastide
L.I.S., Université Toulouse I,
Place Anatole-France,
31042 Toulouse Cedex, FRANCE
E-mail: bastide@cict.fr
Abstract : This paper is an experience report from the SADS project which was conducted at
the French Space Agency (CNES) in Toulouse, France. SADS is the acronym for "Système
d'Accès aux Données Spatiales", Space Data Access System. The project was aimed at
offering Web access to the huge volume of scientific data coming from earth observation
satellites that the CNES maintains. The project was also meant to serve as a testbed for Web
development environments and techniques that were new at that time at CNES, and to study
how those techniques could be integrated in the Software Engineering practices and rules
currently in use. The SADS project presents several interesting aspects and problems that
were to be solved : the enormous volume of data that are to be accessed and eventually
transferred to the user's site, the special structure of this data, which may vary according to the
various experiments, and the fact that further evolution of the system had to be planned in this
first prototype.
This paper introduces the SADS project, which aims at offering to the scientific community a Web access to the
data gathered in space experiments, stored and maintained by the CNES. This projects uses some common Web
development techniques (e.g. CGI gateways to relational databases), and other not-so-common ones (e.g. access
to large data storage and retrieval systems). The paper also focuses on organizational and software engineering
issues : We report our experience in deploying Web development techniques, which are still new and rapidly
evolving, in conventional software developments procedures, and also detail the solutions we have chosen to
ensure long term maintenance and evolution of the project.
Goals of the SADS
Any large scale scientific project (such as solar or earth observation satellites) usually develops a data access
system, to allow research teams to access to data stemming from their own instrument. These systems are
conceived on a per-project basis, and are designed for users who know perfectly the structure of the data in
question.
The continued used of these specific systems after the end of the mission is hindered by two major difficulties :
• after the exclusive utilization phase of data by the Principal Investigator (PI), the data has to be made
available to a larger scientific community that does not know the detail of the embarked instruments. However,
the specific data access systems were not conceived in the first place for those non-initiate users.
• the necessity, today recognized, to preserve space data in the long term, can in no case have for consequence
the duty to maintain in functioning state all systems developed by projects, and this for several decades.