Prisma is defined as an orbital demonstration technology mission for formation flying and rendezvous technologies, involving in-flight testing of sensors and actuators.
The project’s primary objectives were to demonstrate technologies and test manoeuvres, involving both GNC (Guidance, Navigation and Control) and sensors, in preparation for future missions which may require rendezvous and/or formation flying. The main demonstrations were:
- GNC manoeuvring experiments with high autonomy, including Autonomous Formation Flying, Rendezvous and Autonomous Guidance, and Tri-dimensional Proximity Operations including Final Approach and Separation. These experiments were conducted by the Swedish Space Corporation (SSC) with significant contributions from the German Space Agency (DLR);
- testing the DLR’s GPS-based guidance system, which will give a real-time assessment of the differential GPS’s performances as an autonomous flight sensor;
- Vision Based Sensor (VBS) evaluation as a multi-distance pursuit and rendezvous sensor;
- In-flight demonstration of the formation flying radio-frequency sensor (FFRF) in open-loop and closed-loop configurations.
The Prisma mission also included secondary objectives from SNSB programmes focusing on various developments in platform techniques:
- The 1-N HPGP (high-performance green propellant) propulsion system developed in an SSC, Volvo Aero and Swedish Defence Research Agency (FOI) partnership, with the support of ESA. This non-toxic propulsion system’s performance is equivalent to Hydrazine systems.
- Improvements on the SSC’s onboard management unit (which flew on the Smart-1 mission) and its primary functions; additional improvements on Smart-1 energy distribution and battery management equipment (developed by Omnisys in Gothenburg).
- New onboard software developments, in which Matlab/Simulink and code generation (successfully tested on Smart-1’s central application) incorporate the Data Management layer.
- Developing ground equipment compatible with the Consultative Committee on Space Data Systems (CCSDS)’s Packet Utilisation Standard (PUS), used for tests and operations, with a flexible infrastructure compatible with multi-satellite operations.
- A cold gas micro-propulsion system developed by Nanospace AB.
CNES was both a partner on the Prisma mission and a passenger, with its Formation Flying In-Orbit Ranging Demonstration (fiord). Ffiord’s main goal was to complete the first in-orbit demonstration of the FFRF sub-system. This demonstration had a double objective:
In-orbit validation of the FFRF subsystem
The FFRF subsystem validation consisted of various operational tests and an assessment of the FFRF sensor in order to ensure the component complies with specifications. Operational tests focused on the following aspects:
- RF signal acquisition
- Integer Ambiguity Resolution (IAR) procedure
- Power management
- Antenna management (manual or automatic)
- Communication data rate
- Inter-Satellite Link (ISL)
These tests were performed at different relative distances and behaviours, in particular for RF signal acquisition and IAR which can be affected by these parameters. Performance assessment included data collection in different geometrical and dynamic conditions covering as much of the FFRF sensor’s range as possible (distance under 30 km, speed under 0.5 m/s, angular rate under 5°/s).
The FFRF subsystem validation process entailed work sessions all along the duration of the mission. These sessions were separated into two categories depending on resource availability:
- The FFRF sensor is activated as a secondary experiment in order to collect extensive data in open-loop configuration
- The FFRF sensor is activated as a primary experiment and the satellite must follow a relative orbit and behaviour profile specific to Ffiord
|The Tango target satellite follows its initial orbit.|
The Mango chaser satellite revolves around Tango to simulate different stages of a formation flight.
The first stage of the experiment was to complete a series of separation and approach movements controlled from the ground, with different relative speeds in order to save fuel.
Conducting formation flight experiments using the FFRF sensor and the CNES onboard software
RF-based formation flight experiments used CNES’s dedicated FSW (Flight Software) and included the following demonstrations, all of which will prove useful for future formation flight missions:
- proximity operations:
- station keeping at different positions relative to the orbit track (VBAR),
- low-speed movements on and off the orbital plane;
- collision avoidance (autonomous transfer to the “Football” orbit in 1 or 2 manoeuvres)
- standby on a relative orbit
- recovery after system anomaly
These formation flight experiments were conducted in a closed-loop configuration. They lasted approximately 17 days.