On-orbit calibration of the Tiny Ionospheric Photometer on the COSMIC/FORMOSAT-3 satellites

Abstract

The Tiny Ionospheric Photometer (TIP) instrument is a compact, high sensitivity UV photometer that observes the nighttime ionosphere of the Earth at 135.6 nanometers. The airglow emission from the nighttime ionosphere is produced primarily by radiative recombination of oxygen ions and electrons. The TIP instruments were launched aboard the Constellation Observing system for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3) satellites in April 2006 and have been operating nearly continuously since then. There are six TIP instruments, one on each of the six COSMIC/FORMOSAT-3 satellites. Each instrument consists of a photomultiplier tube operated at the prime focus of an off-axis parabolic mirror and a bandpass filter. The instrument's bandpass is limited by the long-wavelength quantum efficiency fall-off of the cesium iodide photocathode in the photomultiplier tube and, to eliminate radiation shortward of 132.5 nm, by the use of a heated strontium fluoride filter. The sensitivities of the instruments were estimated to be ~400 ct/s/Rayleigh. In addition to the TIP instruments, the COSMIC/FORMOSAT-3 satellites also carry the GPS Occultation Experiment (GOX) instruments, which are high accuracy Global Positioning Satellite receivers used for measuring tropospheric temperature/humidity profiles and for observing the ionospheric electron density. We present our technique for using tomographic inversion of coincident GOX and TIP ionospheric observations to determine and monitor the onorbit sensitivity of the TIP instruments.