Development of the Orbiting Carbon Observatory Instrument Thermal Control System

The Orbiting Carbon Observatory (OCO) will carry a single science instrument scheduled for launch on an Orbital Sciences Corporation LeoStar-2 architecture spacecraft bus in December 2008. The science objective of the OCO instrument is to collect spaced-based measurements of atmospheric CO₂ with the precision, resolution, and coverage needed to identify CO₂ sources and sinks and quantify their seasonal variability. The instrument will permit the collection of spatially resolved, high resolution spectroscopic observations of CO₂ and O₂ absorption in reflected sunlight over both continents and oceans. These measurements will improve our ability to forecast CO₂ induced climate change. The instrument consists of three bore-sighted, high resolution grating spectrometers sharing a common telescope with similar optics and electronics. One spectrometer is used for O₂ observations with a 0.765 urn channel, while the weak and strong CO₂ bands are observed with 1.61 urn and 2.06 um channels, respectively. The high-resolution spectroreters will reasure reflected sunlight to retrieve the column-averaged CO₂ dry air mole fraction, X_CO2. An extensive validation and correlation measurement program was developed for this mission to ensure that X_CO2 can be retrieved with precisions of 0.3% (1 ppm) on regional scales (8^o × 10^o). The nominal mission lifetime for the instrument is 2 years. The OCO spacecraft will be placed in a sun-synchronous near-circular polar orbit with an inclination of 98.2 degrees, mean altitude of 705 km, 98.9 minute orbit period and 1:26 pm ascending node.

The thermal control system consists of passive and active elements to maintain the instrument within allowable flight temperature (AFT) limits. Passive thermal control includes multilayer insulation (MLI) blankets, thermal straps, and surface coatings to manage the transfer of waste energy from sources through structures and ultimately to radiators. Two detectors are cooled to 120 K and the third to 180 K by means of a mechanical pulse tube cryocooler. The optical bench is cooled to −5°C with an outboard radiator. The instrument was fully integrated in December 2007 and has undergone EMI/EMC, vibration and two thermal vacuum (TV) tests. A general overview of the thermal control system with an emphasis on the two-phase heat rejection system (HRS) as well as the cryogenic subsystem is presented. The paper describes the instrument thermal requirements, thermal control and analysis approach, key design drivers and analysis results.