A method for measuring large changes in the payload voltage of rockets and satellites

Abstract

We present a method for measuring large changes in the electrical potential of a spacecraft. It is known that a spacecraft in the ionosphere can obtain voltages as large as a few kilovolts. Spacecraft charging can occur naturally in the auroral regions due to high-energy streaming electrons or during the operation of active experiments, such as ion/electron beams or electromagnetic tethers. Charging and discharging events are often impulsive in nature and a method fast enough to track these potential changes could be extremely useful. For this purpose we have designed the Naval Research Laboratory Floating Probe (FP). The FP consists of a metallic sphere containing a high-impedance amplifier and a capacitive divider network for scaling large voltages to the range that typical solid state circuits can handle. Operation is similar to other floating probes; when placed in the surrounding ionosphere via a long deployable boom, the sphere quickly attains a voltage that approximates its local plasma potential. By measuring the difference between the sphere potential and the spacecraft-body potential it is possible, under certain circumstances, to determine the payload potential relative to the surrounding region. The probe can be used for either positive or negative polarity measurements, however, the ‘‘worst case’’ time response is associated with negative charging since the probe must collect ions to reach the local plasma potential. We tested the FP in a large vacuum chamber and followed this with a successful flight on a sounding rocket (SPEAR III). We will discuss the FP design, construction, theory of operation, and some problems and limitations encountered during the testing of the instrument.