Introduction
PRISMA is a technology mission primarily aiming at the demonstration of different sensor technologies and guidance/navigation strategies for Rendezvous and Formation Flying in space.
The project consists of two spacecraft, one advanced and highly maneuverable, earlier called MAIN, and one simplified, earlier called TARGET. On September 17, 2008 the two spacecraft were officially renamed to Mango and Tango. Tango is a couple dance (two in formation) and Mango is thought to be connected with manouvering. These two names have also the same first capital letter as the old ones. Mango is equipped with several sensor systems for Formation flying (FF) and Rendezvous purposes such as GPS, a vision based camera (VBS) and a radio frequency based navigation instrument (FFRF). Together with advanced Guidance, navigation and Control algorithms, several high level demonstrations shall be performed.

The satellites were launched from Yasnu in Russia on 15 June, 2010 to a sun-synchronous orbit at 700 km altitude by a Russian Dnepr rocket. The mission duration is approximately 10 months.

BACKGROUND

Several advanced future missions are planned in Europe (Darwin, missions within Aurora such as Mars Sample and Return, In Orbit Servicing type mission etc.) will need advancements within Guidance, Navigation and Control (GNC) and associated sensor technology. However, no precursor or other technical demonstrator mission is currently planned for this purpose. The Swedish National Space Board (SNSB) and SSC have therefore decided on the PRISMA mission, a RVD and FF technology test bed which, in combination with national interests in demonstrating platform technology developments, shall fill parts of the need for flight demonstrations.

European companies with sensors and instruments with a need for flight demonstration have been invited to contribute to the mission with contributions of flight instruments and software developments. The major partners in the PRISMA project are:

• German Aerospace Centre (DLR)
• French Space Agency (CNES) in cooperation with Centre for the Development of Industrial Technology (CDTI), Spain
• Danish Technical University (DTU).

Other contributing organizations and companies are:

• ECAPS (subsidiary to SSC)
• Nanospace (subsidiary to SSC)
• Techno Systems, Italy
• Institute of Space Physics, Kiruna. 

MISSION GOALS

Primary goals
The primary goals are to perform a series of GNC manoeuvring experiments with a high level of autonomy containing:

• Autonomous formation flying
• Homing and Rendezvous
• Proximity Operations
• Final Approach and Recede Operation.

Within these experiments, the performance of the following sensor systems can be evaluated:

• GPS
• Vision based sensor (VBS) based on star camera technology
• Radio frequency metrology (FFRF).

Secondary goals
The mission also has a set of drivers originating from the Swedish National Space Programme where different developments in platform technology have been undertaken. These are in brief:

• A Hydrazine system will be used but the High Performance Green Propellant (HPGP) 1-N motor system developed by ECAPS (company owned by SSC) will be included as an experiment.
• Micro System Technology cold gas thrusters under development by Nanospace.
  Further development of the core functions of the Data Handling System and the Power distribution and battery handling units.
• New Onboard Software development, with extensive utilization of Matlab/Simulink and autocode generation.
• To test a new ground system, RAMSES, developed by the Swedish Space Corporation. RAMSES will be used both for tests of the satellites on ground and for operations when the spacecraft are in orbit.

EXPERIMENTS

GNC experiments
The GNC manoeuvring experiments will be run by SSC. These can be divided in the following categories:

Autonomous Formation Flying	This experimental series is divided in 3 sets, each run by SSC, DLR, and CNES in cooperation with CDTI. Each organization will have dedicated GNC algorithms implemented in the GNC software for this purpose. The aim of the SSC part is to demonstrate initialization, reconfiguration and autonomous control of T-periodic motions. This may be used in, for example, future passive aperture missions, orbit servicing or for coarse formation control. The GPS system is the cornerstone for this experiment. The DLR, CNES and CDTI´s parts of the Autonomous Formation Flying are briefly described below.
Homing and Rendez-Vous	An experiment aimed at demonstrating long range tracking and rendezvous capability in missions where GPS systems are not available. Areas to investigate are among others the degree of cooperativeness needed of the Tango spacecraft, the division of functionality between on-board software and ground control centre etc. The VBS sensor is the key sensor systems for this experiment
Proximity Operations	This experimental set is developed and run by SSC. It consists of experiments aimed at demonstrating the capability to circum-travel Tango, i.e. moving from and to different holding points, typically requested in a potential Servicing mission. The experimental set is run in two different branches utilizing GPS or VBS sensor data.
Final Approach/Recede manoeuvres	This experimental set is developed and run by SSC. It consists of experiments aimed at demonstrating final approach and recede manoeuvres with only optical systems, typically the situation in HEO and beyond situations.

A high level of onboard autonomy will be implemented.

The experiments will be developed further in close collaboration with the sensor experiments and demonstrations. In the practical mission they will be mixed with the other experiments and conducted in a sequence optimized for fuel consumption, time and operational complexity.

GPS-based navigation and SAFE

DLR contributes to the mission with a navigation system residing in the PRISMA Mango spacecraft computer, capable of absolute and relative orbit determination and prediction. Each spacecraft will be equipped with a Pheonix GPS receiver. The position and raw data from the Tango receiver will be transmitted to the Mango spacecraft via an Intersatellite Link (ISL) and processed in the Mango Computer together with the Mango GPS data. The processing utilizes a dynamic filter with an onboard orbit model, which ideally gives real-time accuracies of the relative positions in the decimetre.

The DLR experiment also comprises the Spaceborne Autonomous Formation Flying Experiment (SAFE) which will demonstrate a fully autonomous, robust and precise formation flying of the spacecraft. The SAFE operation comprises both open-loop and closed-loop phases with spacecraft separation of typically 100 m.

The GPS is not only a sensor and navigation experiment, but it also provides fundamental navigation system functionality for the PRISMA formation as a whole. It also can provide the baseline for validation of the other sensor accuracies after detailed post-processing on ground.
DLR also contributes with Precision Orbit Determination on ground, in order to serve all experiments with reconstructed trajectory parameters with centimetre precision.

Vision Based Sensor (VBS)

The VBS sensor is developed by the Danish Technical University (DTU). The DTU star camera, used in several space missions, has the capability to track non-stellar objects. Software will be developed in order to use the image information to determine range to and relative attitude of Tango when the distance between the spacecraft is in the sub-kilometer range, given geometry information of Tango.

In the PRISMA mission, a dedicated VBS star camera head, identical to the standard camera head, will be fitted, beside the 2 redundant camera heads used for ordinary star camera functions. A 4th head will be installed for covering the short range (0-10 m) to Tango where the standard optics cannot focus. This sensor set-up shall be used in such a way so that the Mango spacecraft can find the Tango spacecraft from several hundred kilometres and deliver sufficient data in order to precisely determine the Tango spacecraft orbit (either on-board or on ground), and to perform manoeuvres in order to approach closer. On short range, the image information will be used to perform the Proximity operations, and finally to perform the linear Approach and Recede manoeuvres.
All 4 camera heads will be connected to the micro-ASC (Advanced Stellar Compass) newly developed by DTU. The micro-ASC is fully redundant and can handle 4 camera heads in any configuration.

The GNC algorithms to control this experiment series will be developed by SSC.

Formation Flying RF sensor (FFRF)

The FFRF instrument is developed by Alcatel Alenia Space on contract from CNES. The sensor development is currently driven by the formation flying mission Darwin, however several other multi-spacecraft missions have this sensor system specified as a baseline sensor. Each spacecraft in the formation has a FFRF unit and a set of antennas. Each FFRF unit communicates with the other units on dual S-band frequencies, and can with time and phase measurements determine the other units’ positions in both range and angle.

The sensor has been initially developed as an ESA contract. PRISMA will now take the system into space for the first time in order to test the functionality and performance in real space flight conditions, especially with respect to multipath, RF acquisition stability, accuracy etc.

Additionally, CNES and CDTI will run a series of GNC experiments based on the FFRF sensor. These experiments are aimed at testing both coarse formation flying and anti-collision algorithms with the FFRF instrument in closed loop with the GNC system.

MISSION DESIGN AND MISSION TIMELINE

The mission consists of 2 spacecraft orbiting in Low Earth Orbit (LEO) at 700 km altitude. The time of the local ascending node is at 18.00, giving very short eclipse periods and durations. A launch opportunity as piggyback with the Dnepr launch vehicle in late 2008 / early 2009 to a sun-synchronous orbit has been identified as a baseline option, but other options are investigated.

Preliminary accommodation on DNEPR

The two spacecraft are launched clamped together, and remain clamped during the first weeks of commissioning. During this period, all platform equipment will be checked and the first motor experiments are performed.

After separation of the Tango satellite, the relative navigation where the GPS system is the backbone, and the GNC experiments will begin. The GNC experiments will be performed with an “early harvest” principle, ensuring that all participating experiments will get a minimum output before going to more advanced manoeuvring.

The Mission control centre will be located at the SSC technical centre in Solna, while the practical operations will be performed from the Esrange TT&C station in Kiruna in Northern Sweden.

Spacecraft DESIGN

The spacecraft design has been under development since 2005. Only a very brief description of the Mango and Tango spacecraft is given below.

The Mango and Tango satellites are fundamentally different. The Mango spacecraft is about 140 kg and contains 3 propulsion systems; a standard hydrazine system and the 2 experiment systems (HPGP and micro propulsion). Mango is 3 axis control stabilized using reaction wheels and star cameras, and also contains the VBS system with it’s 2 additional camera heads. Mango also communicates with the ground.

The Tango spacecraft is about 40 kg and shall act mainly as a target object for the Mango spacecraft and contains only simplified 3 axis attitude control based on magnetic control, with no translational capacity. Tango is equipped with a GPS receiver which can transmit the GPS data to the Mango spacecraft via an Intersatellite Link (ISL). It also contains one of the FFRF units in order to perform the FFRF experiments.

Although Mango and Tango are very different, the avionics is to a large extent identical. Both spacecraft have the same processor board and internal communication architecture, and many of the interface electronics boards are identical.

The general layout of the two spacecraft can be seen in the figures below.

Separated configuration Mango

Separated Configuration, Tango

Prisma Mango in cross-section

PROJECT SCHEDULE

The project has the following major milestones:

Milestone / Review / Meeting	Planned
Mission Requirements Review (MRR)	End of April 2005
System Requirements Review (SRR)	Mid June 2005
Preliminary Design Review      (PDR)	November 2005
Bench tests of engineering models in SSC’s laboratories	September 2006
Guidance, navigatiaon and control system simulations started in SSC’s satellite laboratory	October 2006
Vibration tests of the two structural test models	December 2006
Critical Design Review             (CDR)	January 2007
Delivery of flight hardware and experiments	June – December 2007
FM Mech / Prop FM integration start	August 2007
Assembly, Integration and Test (AIT) of the vehicles and their subsystems started	September 2007 – January 2008
Advanced system tests of the satellite	March – October 2008
DHS Mango and Tango delivery to AIT	March – April 2008
FM electrical system integration start	April 2008
Start and finish of SFPT-1	April – September 2008
Environmental tests (vibration, therman, cycling, EMC, etc.)	October 2008 – February 2009
Environmental AIT	October 2008 – February 2009
FFRT integration	January 2009
Flight Acceptance Review          (FAR)	May 2009
Spacecraft ready for transportation to launch site	June 2009
Launch from Russia	15 June 2010

The status during early spring 2010

The two satellites, named Mango and Tango, were returned to SSC’s Solna facility after a few months in a test facility in France that specializes in environmental tests of spacecraft. In order to verify that the satellites will endure the launch and all specific conditions in space, they were put through a a number of tests in a simulated space-like environment. In a large vacuum chamber, with an artificial sun and chilled walls, thermal cycling and temperature control were performed. Also, using the artificial sun as energy source, the entire power supply system could be verified.The verification also included measurements of mass properties, vibration tests and electrical and electromagnetic tests.

Parallell to the physical tests of the satellites, the very complex onboard software has been completed and verified in SSC’s simulators. In the end of March, the final software version was implemented on the satellites. After so called “hardware-in-the-loop” tests during spring, the satellites were ready for launch in mid June.

The technologies onboard Prisma are developed in Sweden, with contributions from Germany, Denmark and France. The project is financed by the Swedish National Space Board with support from the French and German space agencies.


Read new PRISMA document, published on 2006-10-01

See also PRISMA website (in Swedish and English).