Noise features of the CHAMP vector magnetometer in the 1-25Â Hz frequency range

CHAMP (CHAllenging Minisatellite Payload), a German small satellite mission to study the Earth's gravity field, magnetic field and upper atmosphere, ended in space on 19 September 2010. Thanks to the good quality of the satellite and to several altitude maneuvers, the satellite provided continuous and reliable observations including house-keeping data at different processing levels for more than 10 years. Among them the high-resolution vector magnetic field data, provided by the FGM (FluxGate vector Magnetometer) at 50 Hz sample rate, are one of the most important data products. The 50 Hz data allows us to investigate all signals from DC to 25 Hz. Here we report on noise features found in the frequency range above 1 Hz. There are predominantly three types of disturbances found regularly in the vector magnetic field data. One is at ∼2.1 Hz and harmonic signals, an other is an 8 Hz single frequency signal. The features of these two signals are characterized and analyzed, which helps us to trace the signals to the source of disturbance. The 2.1 Hz harmonic signals mainly depend on the incident angle of sunlight, which points at the star tracker instrument ASC (Advanced Stellar Compass), as the signal depends on the level of blinding. The 8 Hz sinusoidal signal is continuously present and its amplitude is controlled by the temperature inside of the satellite. For higher temperatures the signal gets smaller, and it practically disappears above 20 °C. Only the FGM x-axis is affected by the 8 Hz signal. The third artificial signal affects the FGM y-axis. A frequency sweep from the Nyquist frequency down to zero and up again occurs when the readings of the FGM y-axis go through zero. This is probably related to a ringing of the analog-to-digital converter at the sign change. The amplitudes of all these noise features are generally below 50 pT.