Figure 1. Comparison of in-orbit radiation measurements with predictions based upon current static radiation environment models.
Résumé
Les rayonnements ionisants présents
dans l'espace peuvent causer des dommages aux appareils
sensibles. La surveillance in situ de leur intensité et de
la dose accumulée est donc d'une utilité pratique
indéniable. Suite aux bons résultats d'un modèle
de vol expérimental, l'Agence a décidé de mettre
au point un système normalisé de surveillance du
rayonnement ambiant, qu'elle entend utiliser de manière
systématique dans ses futurs projets. L'instrument de base
mesure 95x22x217 mm, pèse 2,5 kg et consomme moins de 2 W.
Il permet de mesurer le spectre incident et la dose
accumulée. Il est prévu d'embarquer un modèle
"protovol" sur le microsatellite britannique STRV1-d, dont le
lancement doit avoir lieu en 1998.
Contractors: Oerlikon Contraves Space A.G. (CH), Paul Scherrer Institute (CH).
Funding: General Budget, Technical Assistance.
Figure 1. Comparison of in-orbit radiation measurements with
predictions based upon current static radiation environment
models.
The aims of the SREM are:
Development of SREM was started in January 1996, after pre- development in ESTEC of a number of critical items. The performance of the detector system is crucial to the overall performance of the instrument. Considerable effort was therefore expended by ESTEC and the Paul Scherrer Institute (PSI) to simulate candidate designs, characterise breadboard systems using the proton irradiation facility at PSI and subsequently to optimise the design of SREM.
Although the basic concepts of the earlier REM were adopted (one electron detector system, one proton detector system) it was decided to upgrade detection capability by replacing the shielding domes by telescopes, and to improve discrimination properties through the use of multiple detectors with coincidence and anti-coincidence logic for event detection.
To provide flexibility in operation, with the possibility of autonomous operation, the electronics was equipped with a type MAS31750 computer from GEC-Plessey Semiconductors (UK), a radiation-hard processor of European origin.
The routine application of Radfet dosimeters for the monitoring of total dose damage in orbit ² led to the incorporation of one Radfet in the SREM itself plus additional circuitry to support a further seven Radfets, which could be remotely mounted in radiation-sensitive units throughout the spacecraft. The Radfet data would be part of the telemetry data from the SREM.
The basic instrument (Figure 2) measures 95x122x217 mm and weighs 2.5 kg. Its power consumption is less than 2 W with an operating input voltage range of 22 to 55 V. The instrument is fully compatible with ESA's on board data handling standards.
Figure 2. The proto-flight Standard Radiation Effects Monitor
(SREM).
The electron detector is a telescope containing a single silicon detector and the proton detector consists of a telescope with two silicon detectors in tandem. Events are sorted, according to their energy, into bins'; there are two energy bins for electrons and twelve for protons and cosmic rays. The electron bins count the number of events with energies greater than 0.5 MeV and greater than 1.0 MeV; the proton bins cover the range 20 MeV to 400 MeV. SREM can count up to rate of 105 events per second and the acceptance angles of the telescopes are designed to ensure that they cover the highest fluxes likely to be encountered in the peaks of the radiation belts which surround the Earth. Programmable counting thresholds can be set to give warning of count rates which are higher or lower than a preset level for a given energy range.
An engineering model detector system and front end electronics is currently being calibrated at PSI using the ESA proton irradiation facility and formal acceptance of this model is scheduled for December 1996. A proto-flight model (PFM) is being manufactured in parallel, and is planned for acceptance in March 1997. The PFM is intended to fly on the UK micro-satellite STRV1-d, planned to be launched into an elliptical geostationary transfer orbit in 1998. The first ESA project to use SREM will be Integral satellite, in which SREM will be part of the payload and its operations will be specifically tailored to the requirements of Integral instrument package. SREM has also been incorporated in the model payload of PROBA and, in addition, it is intended for inclusion in early Space Station payloads.
References
1. Preparing for the Future,
4, 4 (December 1994)
2. Ibid, 3 4 (December 1983)
In the face of stiff international competition Dornier's bid to supply the solid state recorder for the European meteorological satellite Metop has been selected by the European Space Agency (ESA). The first meeting of the contract took place in August 1996.
The bid assumes delivery of an engineering model in July 1998, one flight model for the Metop-1 mission in July 1999 and two further flight models, in March 2000, for the Metop-2 and -3 missions. Dornier held the cost of the solid state recorder down by exploiting their experience gained through previous developments of solid state mass memories for programmes such as Mars '94/96, HRSC and Cluster.
Some missing elements were, and will be, supported by additional ESA-funded technology research programmes. One example is a contract for the Solid State Recorder Technology Demonstrator for the Envisat and Metop satellites, awarded to Dornier in 1995. These latter developments have closed the gap between earlier research and the performance required by Metop and other future missions. Early integration and validation demonstrations should be carried out with the help of this technology demonstrator, whose integration into Envisat is planned for early 1997.
Preparing for the Future Vol. 6 No. 4