INSTRUMENT FOR MEASURING SPACECRAFT POTENTIAL

A high-resolution, low-energy electron spectrometer can be deployed on rockets or satellites to measure absolute spacecraft floating potenti al relative to true (Earth) ground with an accuracy of +/-0.2 V. Featu res due to the photoionization of nitrogen and atomic oxygen by an ext remely sharp solar extreme ultraviolet line (304 Angstrom, He II) appe ar in the electron spectra collected by the instrument. At altitudes w here the photoelectron production is local (below 250 km), the peaks t hat appear in tile spectrum are created by electrons of a known kineti c energy: the energy of the ionizing photon minus the energy needed to ionize either atomic oxygen or molecular nitrogen, The spacecraft pot ential is determined from the apparent shift in the energy of the phot oelectron spectral peaks. The instrument is best suited for use in the daytime atmosphere at altitudes between 150 and 250 km. The spectrome ter is based on an earlier design, that of the photoelectron spectrome ter of the Atmosphere Explorer satellites. Enhancements of the earlier design, including a threefold greater throughput and a specialized sc an mode, greatly improve the determination of spacecraft potential. Ti le placement of the instrument on the spacecraft is critical to the su ccess of the measurements.