Figure 1. REM installed on the micro-satellite STRV-1V
Contractors:
Paul Scherrer Institute (CH)
Defense Research Agency (UK)
Funding:
Basic Technology Research Programme (CACH) and Technology Demonstration Programme.
The space technology research vehicle, STRV 1B, isone of a pair of small (50 kg)spacecraft designed and built by the Defence Research Agency (UK)to demonstrate advanced concepts in spacecraft engineering, structural materials and electronic components. Both spacecraft were put in the geostationary transfer orbit by an Ariane V64 launcher on 17 June, 1994.
The geostationary transfer orbit is an eccentric orbit whose altitude ranges from 300 km altitude to about 36 000 km. This orbit is extremely interesting for measuring the Earth's radiation environment, since it provides full exposure to the trapped radiation belts and well as cosmic rays and solar energetic particles.
STRV 1B was originally configured to carry a number of experiments connected with space radiation environment effects. Thus ESA's proposal to add its radiation environment monitor, REM, to the payload offered the means to gather valuable data on the radiation environment, needed to assess the performance of the other experiments. The development of REM was initiated by ESA as a forerunner to providing radiation monitors for routine use on ESA spacecraft.
Monitoring of the space environment becomes more important as spacecraft become more sensitive to radiation. A monitor allows protection of on-board systems, aids interpretation of spacecraft performance and provides data for environment models.
REM has two shielded silicon detectors which monitor the environment by counting the pulses generated by energetic charged particles. A domed shield prevents particles below certain energies from being counted. The magnitude of a pulse, which depends to some extent on particle energy, is used to select which one of the 16 channels belonging to a given detector will be incremented.
With a mass of 3 kg, a power dissipation of 5 W and a data output of 20 kilobytes per orbit, the REM was not easily accommodated in a micro-satellite. A further complication was the requirement for the REM detectors to have as clear a view into space as possible, preferably perpendicular to the spin axis of the spacecraft.
Figure 1 shows the final arrangement. The detectors, housed under the two domes of the white detector head, have a clear view into space via a cut-out in the solar panel. The main REM electronics is painted black and is connected to the detector head by the shielded cable.
Figure 1. REM installed on the micro-satellite STRV-1V
The STRV control centre is at Lasham (UK). For the early orbits it was supported by the NASA Deep Space Network. Command sequences can be uplinked from Lasham and there have been several command uplinks to the REM, initially for verification orbits and subsequently to optimise routine operations. Thepresent operating mode accumulates data over 100-second periods while the spacecraft moves rapidly through the low altitude regions, and over 300-second periods at higher altitudes.
A built-in test circuit determines the dead time, a measure of how many events have been missed due to the response time of the detector electronics and an important parameter for subsequent data processing on ground.
Raw data are disseminated by using a modem to download data from a personal computer at the control centre.
Spacecraft and experiment housekeeping data are also routinely accessible. REM data are retrieved by the Paul Scherrer Institute (PSI) and by ESTEC within 24 hours of their acquisition. Data then undergo quick-look processing to ensure that the instrument is operating correctly and that housekeeping is within limits, and to check for anomalous environment events such as particle events.
PSI are responsible for further detailed data evaluation to build a radiation environment data base and to provide inputs for updating the environment models. Figure 2 shows uncorrected data taken with the REM. Several corrections have to be applied to the data during detailed evaluation.
The type and thickness of the dome material and the setting of pulse height thresholds in the REM processing electronics allow one detector to be optimised for electrons and the other for protons. But this does not fully prevent a few protons from being counted by the electron detector, and to a lesser extent, a few electrons counted by the proton detector, errors which fortunately can be measured and a corrected.
Further corrections are needed for the the solid angle viewed by the detector (which is energy dependent) and high energy protons penetrating the detector from the rear.
Prior to launch, the REM was extensively calibrated at PSI using high energy protons and electrons, and the angular dependence was measured as part of this calibration.
The detectors, detector unit and surrounding spacecraft mass have been extensively modelled using the GEANT Monte-Carlo code. Using the calibration data, in orbit data and modelling, it is possible to derive detector response functions which should allow the extraction of data on particle type, energy and flux.
At the time of writing the REM has been in operation for 38 full orbits and the data already show that the STRV REM will be able to provide very high quality environment data with a great amount of detail. In addition, a second model of REM has been placed on the exterior of the Russian space station MIR, providing an excellent complementary spatial location.
Figure 2. Full orbit data (11 hours) for a number of orbits which show the passage through the radiation belts twice per orbit and the variability of the environment on a short term basis
Preparing for the Future Vol. 4 No. 4.