Modular Concept and Adaptation to Different Missions

As described above the CODIF, TEAMS and ESIC sensors consist of several well separated subsystems that can be separately designed built and tested. This modular design allows a natural split of the work load and exchange of modules without significant impact on the other subsystems. The modular package is sketched in a block diagram in Fig. 7. The subsystems are the electrostatic analyzer, the time-of-flight system that can even be separated into a sensor- and an electronic-module, and the data processing system. Each subsystem can be adapted individually to reflect the requirements of a specific space mission. The instrument capabilities are compiled in Table 1, including the individual differences of the instruments. For all 3 sensors the responsibility for the central TOF subsystem rests with the University of New Hampshire (sensor) and the Max-Planck-Institut für extraterrestrische Physik (electronics). This key system, which has been the centerpiece in the previous chapters, remains identical for all 3 instruments. The ESA is being provided by CESR, Toulouse, for CODIF and ESIC and by the Lockheed Palo Alto Research Lab for TEAMS. CLUSTER and Equator-S will scan regions with a wide dynamic range of fluxes and encounter occasionally very cold plasmas. Therefore, the CODIF and ESIC are equipped with two sections that are different in geometric factor by approximately a factor of 100. This flexibility comes at the price that one full spin is needed to complete a 3D distribution. For FAST the emphasis is on very high spatial and thus temporal resolution of auroral structures and therefore a single geometric factor is chosen for TEAMS allowing twice the time resolution. CODIF and ESIC also carry an additional retarding potential analyzer (RPA) which extends the energy range to practically 0 eV (spacecraft potential). The RPA is implemented with a separate entrance aperture that is adjacent to the regular ESA aperture. The ions are guided into the ESA by a sequence of deflection plates and grids.

The other profound difference is found in the data processing systems of these instruments. In the case of CODIF and ESIC the data processing system consists of a microprocessor capable of compressing the raw data stream very efficiently by the accumulation of mass, angle and energy bin distributions and computation of the first 3 to 4 moments of the distribution function for 4 different ion species. The first step allows a compression by a factor of 120. Reduction of the distribution elements by taking into account the oversampling of the sphere at the poles allows another factor of 2.5 in reduction. The moments need only 3% of the original bit stream. These data compression schemes provide for a great variety of possible telemetry modes for different environments and spacecraft data rates. The only differences between CODIF and ESIC are the spacecraft interface circuitry and the on board software which has to cope with a 1 sec spin rate for Equator-S. Because FAST has a large memory capability and will be run in a campaign mode during the aurora passages, a much higher data rate can be offered. Therefore, TEAMS possesses only a data processing interface with storage capability for the mass, angle, energy distributions, which are buffered, before the next voltage sweep starts, and then read out completely during this sweep cycle. FAST TEAMS will allow a tracking of distribution functions with the highest time resolution.

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