STRV 1A: EXPERIMENTS

       The experiment employs a low energy plasma generator which emits xenon ions and electrons into the vicinity of the spacecraft. If a surface electrostaticpotential exists, the experiment would demonstrate whether is canbe reduced by the presence of the low energy plasma. In order to guarantee the presence of a high surface potential and have the ability to monitor the level of the potential, and electrostatic charge detector is also aboard STRV-1a. The Langmuir probe and Cold Ion Detectors on the spacecraft serve as plasma diagnostics equipment.

       The device that produces the plasma is a hollow cathode derived from the DRA UK-10 ion engine. Xenon is used as the engine propellent and is therefore used in the charge alleviation experiment.

       Ground-based testing of the ability of different structural and thermal materials and coatings to withstand erosion is extremely useful; however such tests do not reproduce the simultaneous conditions of vacuum, oxygen flux, temperature and energy distribution that occurs in space. STRV-1a provides a valuable opportunity to perform such tests since the initial perigee altitude is only 300 km.

       The resistance of the thin silver films (1850 Å) started to increase immediately after launch and increased to the maximum of the measurement range after 280 and 330 orbits respectively. The form of the raw data of resistance as a function of fluence in the laboratory experiment and resistance as a function of orbit for the flight experiment is similar. The resistance change was converted to silver loss to facilitate analysis. The initial stage of the oxidation process was linear until approximately 300 Å of silver had been oxidised. The rate of oxidation then decayed to a low rate parabolic growth. The linear stage is thought to be surface reaction-controlled and the parabolic stage diffusion- limited.

       The resistance of both thick silver films remains unchanged. This suggests either a contamination problem or component failure in the electronic circuit. Analysis of future results should resolve this issue.

       No increase in the resistance of the carbon, silica or polyethylene sensors has vet been observed. On the basis of flux estimates (ESABASE), reported erosion yields for these materials and the thickness of the overlay coatings, these results are as expected.

       It should be noted that the coating thicknesses were selected on the basis of an earlier launch date and a lower perigee which would have provided a more concentrated AO environment.

       After 350 orbits, the resistance of the PTFE-coated sensors had increased to the upper bound of the measurement range. On the basis of a preliminary analysis, it is estimated that the erosion rate is 0.77 x 10^-24 cm³ atom^-1. This is of the same order as that achieved from the flight spare in the ESTEC facility and higher than that reported for flight experiments. This suggests that synergistic effects between the ultraviolet environment and AO may be influencing the erosion process.

       In conclusion, the experiment has enabled the value of the silver resistance technique as a monitor for the AO environment and the resistance of materials to AO degradation to be evaluated. It is concluded that whilst there is considerable merit in the technique for AO measurement, in common with other thin film techniques the question as to whether a thin film is truly representative of a bulk material must be seriously considered. Further ground-based tests are required to resolve this issue.

       A concentrated programme has recently commenced using the ESTEC facility. This will address some of he issues raised by the flight experiment.

       Flying a second experiment on a satellite such as STRV-1c or - 1d is a goal for future research activities.

       CREDO measures both the energy and arrival rate of cosmic rays as the particles pass through a 3 cm square array 6f 300 µm thick diodes. The total charge generated in nine energy bands (over 300s intervals) is recorded by the instrument thereby providing a clear picture of the cosmic ray environment of the GTO.

       ParticleS; trapped in the geomagnetic field and other naturally occurring ionising radiation are also measured by CREDO. The instrument includes a network of Radiation Field Effect transistors (RadFET's) located at different positions within STRV-1a. Each RadFET has a 100 nm thick gate oxide layer which traps positive charges generated as particles pass through, thereby altering the threshold voltage of the device. The total received radiation dose is therefore measured by monitoring the threshold voltage of each transistor.

       Due to battery. anomaly on STRV-1a during August 1994, the BRE was not operated until September 1994. The experiment had to contend with two difficult conditions: a) the operating temperature of the battery. was around 30° C, and b) the battery terminal voltage was below nominal. In the first case there was concern that the dynamic range of the temperature detector would be compromised, since circuit saturation was predicted to commence at 35° C, while the second case dictated that the BRE end-of-charge (EOC) logic was held permanently reset and that the BRE could only be used in monitoring mode with the data being routed to the ground for assessment before any on-board response could be initiated. Thus although the BRE could not directly control the STRV-1a battery' chargers, it was extensively used during the mission as a batteD' thermal dissipation monitor to determine the duration of applied battery, charge. This came about since the depressed battery, terminal voltage also resulted in the operation of the conventional EOC control being compromised.

       The BRE displayed a high degree of sensitivity to battery EOC dissipation whilst at the same time showed itself to be basically insensitive to the dynamic temperature environment of the battery and spacecraft around the orbit. In the course of the STRV-1a mission it successfully detected EOC conditions on a battery whose temperature could vary over the range 20-35° C.

       The BRE also incorporates 6 hybrid power MOSFET's ("HyFETs"). These devices are being calibrated in the radiation environment. The gate voltage of each device is measured and based on ground results, a drift rate of 200 mV/krad was expected. The results to date suggest approximately an order of magnitude less than predicted by software models.

       Several hundred discharge/charge cycles to date on STRV-1a have shown the temperature derivative (Tdt) charge control technique of the BRE to be both a reliable and thermally stable battery management concept. It has proved that the applied charge rates of C/14 and C/7 in conjunction with the pre-selected circuit sensitivity has produced sufficient thermal impulse to the battery when approaching end-of-charge, to allow accurate detection.