In order to downlink data gathered by instruments in-flight payload must first transfer their data to the spacecraft data handling system via the spacecraft bus.The utilised bus and associated communication protocol will vary from mission to mission as will the nature of the data structure sent by the payload. Each instrument will include hardware and software in order to effect this communication. Our group has considerable experience in this crucially important area of payload data handling for both single and multiple instrument sensor chains.

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We may distinguish between two models of payload data handling

Traditional Model – Dedicated payload processing

Analog to digital conversion and data processing takes place inside individual payload Interface Contoller Units (ICU). The ICU delivers to the spacecraft bus a downlink ready data product - eg compressed, filtered and decimated digital stream of magnetic field vectors.

Future Model – A more distributed System

Processing is transferred to the spacecraft side of the interface – the payload delivers unprocessed raw sensor data – all instrument related processing is performed by the spacecraft

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Mass and power are large drivers on the design of spacecraft especially payload. Technological developments leading to reduction in payload resources are essential to ensure we continue to fly our sensors on the next generation of science missions. Our laboratory is involved with a project investigating how such optimisation can be achieved by investigating systems based on distributed processing.

PRISM is a project led by EADS Astrium UK to build a technology demonstrator illustrating how a distributed processing system could work in practice through provision of a real world example.

Its main goals will include

• verifying that magnetometer and UV imager instrument processing code can be successfully developed using a common Application Programmable Interface (API) within a RTEMS environment
• migrating the assembled code to a centralised LEON3 target platform
• verifying centralised operation and processing of payload using data simulators connected to the target via SpaceWire. A key part of the verification stage is segregation of processes. i.e. to ensuring smooth running of simultaneous payload control and data processing within a single processor.

Successful demonstration could lead to a significant reductions for mass, power and processing resources on platforms implementing such a scheme through the elimination of payload specific ICUs. The target datasets to be implemented within the PRISM payload simulators will be those expected of Solar Orbiter – a mission where the mass and power resource is extremely limited. The project partners in this collaboration are: