QSAM - An Approach to Detect Low Frequency Acceleration in Spacelab

Most of the microgravity experiments show highest sensitivity to residual acceleration in the low frequency range, typically below 0.01 Hz where atmospheric drag, gravity gradient and centrifugal forces are pre-dominant. QSAM (Quasi-Steady Acceleration Measurement) is an instrument especially developed to detect this range where conventional methods are hampered by high amplitude noise problems entailing errors in measurement results. Noise is defined to include all disturbing contributions surrounding the signal, which the experimentalist wants to reject but has no control over the source because of its unpredictable nature. One of the most disturbing noise source covering the low frequency acceleration signal is the residual zero point drift (bias) of an accelerometer and its electronics, which is slowly varying due to unknown dependency on temperature, aging and other effects. The measurement system QSAM applies signal modulation by turning the sensor's sensitive axis. This allows the detection of accelerations in a frequency range between 0 and 0.02 Hz with a resolution better than 10-⁷ g (g=9.81m/s²) and a huge improvement of the signal-to-noise ratio what allows to read the spacecraft's low frequency acceleration environment nearly free of any noise within an arbitrary vibration spectrum. Once the low frequency acceleration vector is known at one point the entire field can be calculated on the basis of rigid body kinematics. QSAM is selected to be part of the IML-2 payload (Second International Microgravity Laboratory). A second flight is planned for the Russian free flyer FOTON in fall 1995. In this paper the principle of QSAM is described together with its hardware design and data processing system.