MESSENGER MAG Alternating Current Data Collection

To conserve telemetry the vector sampling will usually be less than the maximum rate, 20 sample/s. To provide a record of the amplitude of fluctuations for frequencies higher than 1 Hz, an AC amplitude is calculated from the internally sampled 20/s data for the sensor axis commanded as the AC_AXIS.

Thus, in addition to the vector samples data, a 1 Hz to 10 Hz bandpass average amplitude is evaluated for the commanded sensor axis, selected by command, and recorded as a log AC value with a 4-bit mantissa of the four most significant non-zero bits and a 4-bit power of two exponent of the 4th (least significant) bit of the mantissa. This is called the logAC value. It is created once per second and saved for telemetry at that rate or at the commanded science rate, whichever is less frequent.