A METHOD FOR MEASURING LARGE CHANGES IN THE PAYLOAD VOLTAGE OF ROCKETS AND SATELLITES

We present a method for measuring large changes in the electrical pote ntial 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 streamin g electrons or during the operation of active experiments, such as ion /electron beams or electromagnetic tethers. Charging and discharging e vents 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 ra nge that typical solid state circuits can handle. Operation is similar to other floating probes; when placed in the surrounding ionosphere v ia a long deployable boom, the sphere quickly attains a voltage that a pproximates its local plasma potential. By measuring the difference be tween the sphere potential and the spacecraft-body potential it is pos sible, 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 m ust 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, const ruction, theory of operation, and some problems and Limitations encoun tered during the testing of the instrument.