Ion drift meter calibration and photoemission correction for the C/NOFS satellite

Abstract

The Ion Drift Meter (IDM) onboard the Communication/Navigation Outage Forecasting System (C/NOFS) measures ion arrival angles transverse to the satellite path and requires precise attitude determination for proper conversion to ambient ion drift. Low ionospheric densities during the 2008/9 solar minimum restricted quality measurements to altitudes near perigee, limiting the daily local time coverage of the satellite. As the primary ionospheric boundary condition available to calibrate the IDM requires that the integral of meridional (vertical) ion drifts be zero over all local times and longitudes, the limited daily local time coverage necessitated a new calibration procedure. A procedure is presented that utilizes the median meridional drift over all local times determined over a 67 day perigee precession period to calibrate the instrument. Offsets are isolated for both measurement directions by exploiting the different symmetry relationships that the spacecraft orientation has with respect to the meridional direction as a function of magnetic latitude. The low ionospheric densities along with the equatorial orbit of C/NOFS also allowed photoemission currents within the instrument to be detected. A numerical model of photoemission within the drift meter is presented in detail and used to generate a first order software correction to remove a large portion of this error from measurements. Both the photoemission correction and the drift meter calibration procedure are supported by a comparison to vertical ion drifts measured by the Jicamarca Radar Observatory.