Co-aligned Pointing for On-orbit Intercalibration Between Instruments on Separate Satellites

The CLARREO Pathfinder (CPF) payload, planned for flight as an external payload on the International Space Station (ISS), will perform co-aligned near-cotemporaneous observations with independent remote sensing instruments on other low-Earth-orbit (LEO) satellites. CPF's observations are required to occur within 10 minutes of the "target" instrument's observations, and to be co-aligned within 0.7 degrees of the "target" instrument's line of sight (LOS). These observations will allow for the highly radiometrically accurate hyperspectral measurements of CPF to be used to improve the on-orbit calibrations of these "target" instruments already on orbit. In order to take co-aligned measurements without any active communication or coordination between the instruments, CPF will intercalibrate with cross-track scanning instruments (nominally the Visible Infrared Imaging Radiometer Suite (VIIRS) and the Clouds and Earth's Radiant Energy System (CERES) instruments on a Joint Polar Satellite System (JPSS) satellite), whose behavior is sufficiently deterministic for short-term predictions of their pointing. This paper describes CPF's on-board real-time pointing algorithm that produces the desired co-aligned LOS during observation opportunities when ISS underflies the JPSS orbit (with appropriate timing); the algorithm assumes that the ephemeris and attitude of the target spacecraft are sufficiently predictable, but that the ephemeris and attitude of ISS (hosting CPF) cannot be predicted with sufficient accuracy in advance; and that therefore these must be near-real-time inputs to the pointing algorithm.