Mass, Power and Telemetry

Unusually for a space mission, tight mass and power budgets were not the main drivers for Double Star.  The mass and power consumption of the instrument are moderately higher than those of Cluster.  The higher mass is due to an increase in outer wall thickness of the electronics box (an average value of 0.6mm on Cluster to 3.2mm on Double Star) to provide shielding from the increased radiation seen by the instrument in the lower orbits, and additional time spent in the radiation belts, together with a limited availability of radiation hardened electronic components due to export restrictions.  The increased power is due mainly to the incorporation of two non-identical sense electronics including separate drive circuits - the power consumption of each sensor and sense electronics alone is of the order of 1W. 

 Telemetry Formats

The Double Star magnetometers have two telemetry modes, 'Normal' and 'Gradiometer' as detailed below.  The data stream from each sensor is controlled by selecting one of the sensors to be primary (high data rate), the other secondary (lower data rate).

Normal mode, with a resolution of 22 vectors per second from the primary sensor, was the pre-flight default operating mode of the instrument (as is the case on all four Cluster spacecraft). However, both Double Star instruments have operated for considerable periods in a so-called 'gradiometer' mode due to spacecraft perturbations of the ambient field. This is a new mode specific to Double Star which results in equal numbers of vectors from each sensor. We use the descriptive name 'gradiometer' for this mode since it is used to measure the magnetic field difference (gradient) between the location of the outboard sensor on the end of the boom and the inboard sensor which is 0.5 m from the end of the boom. This gradient is entirely due to localised magnetic sources on the spacecraft, since the ambient field (which we desire to measure) is identical at both sensors. Therefore, the field gradient observed can be used to calculate and remove the effect of the spacecraft magnetic sources. Currently the default mode on the equatorial spacecraft (TC-1) is gradiometer mode but on the polar spacecraft (TC-2), it is normal mode with the OB sensor selected as primary.