The technique to control the spacecraft's potential, used in ASPOC, is based on the emission of positive ions at an energy of several thousand electron volts in a beam along the rotational axis of the spacecraft. These ions do not return to the spacecraft due to their large gyro radii. As a result the equilibrium potential of the spacecraft gets shifted to lower positive values. This allows a large portion of the photo electrons to leave its vicinity. The current balance remains stable. The measurements by the particle spectrometers (both electrons and ions) benefit from the spacecraft potential, as it is maintained below +10V. The energy and direction distribution of the oncoming particles is less disturbed in this case. Without spacecraft potential control, the greater part of the photo electrons will return to the positively charged spacecraft and thus also to the sensors. They will partially blanket the flow of externally inflowing electrons, and contribute to the saturation and prematurely change of the micro-channel plates. Positive ions that cannot cross the potential hill can never be measured this way.