In preparation for SPOT 4's operational exploitation, CNES has extensively upgraded the SPOT acquisition programming system. Acquisition programming covers all tasks from enquiries concerning programming requests to imagery acquisition by one or more SPOT satellites.

By making better use of the system's acquisition capabilities, the new programming centre, commissioned in April 1995, allows Spot Image to offer its customers improved acquisition programming services, including :

The programming centre is operated by Spot Image, the CNES subsidiary handling the distribution and marketing of SPOT data and products.

[Programming requests] [Main functions] [Selecting the best scenes] [An example of optimal scene selection]

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Programming requests

Whenever customers fail to find the imagery they require in the Dali catalogue of scenes held in the SPOT archive, they can fill out a programming request giving a detailed description of their imagery needs. Each programming request gives all the technical details needed to program the satellite, including :

Programming requests are stored in a customer-accessible database.

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Main functions

The new programming centre :

Selecting the best scenes

Given the cyclical variations in the number of programming requests, which tend to focus on certain regions at certain times (typically crops and vegetation during the spring and summer), the programming centre must select, each day, the scenes offering the best chance of successful acquisition.

This selection process must also take into account such technical considerations as :

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An example of optimal scene selection

Let us consider a typical day's acquisition over France. As can be seen from the diagram below, the satellite could acquire some 30 scenes on this particular day.

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At the latitude of Paris, five scenes are within the satellite's observable corridor, but the imaging instruments can only acquire two at a time. It is the programmer's job to choose which solution is the best.

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Using a sophisticated algorithm based on operations research, the programming centre prepares a daily schedule of all theoretically accessible scenes. This algorithm takes account of :

The programming centre's scheduling software can thus analyse, depending on the volume of input data, several thousand, perhaps even several million, acquisition sequences and combinations and identify the optimal sequence or combination.

In our example, there are at least four acquisition possibilities as SPOT flies over the Paris latitude :

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Various pairs of acquisition solutions

Suppose the programmers choose to acquire the three-scene data strip extending across northern France. This immediately eliminates solutions 2 and 4, since the satellite will not have the time to move its strip-selection mirrors from the previous positions to the new ones. Solution 1 remains technically feasible, but the prospects for clear skies are not good. The programming centre therefore chooses scheduling solution 3 since it combines technical feasibility and the most favourable cloud forecast.

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The scheduling algorithm analyses every possible acquisition sequence and path along each portion of each orbital revolution to identify that representing the best overall compromise.

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page updated on the 00-06-06