The Orbiting Carbon Observatory Mission
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Acta Astronautica 56 (2005) 193–197 www.elsevier.com/locate/actaastro The orbitingcarbon observatory mission David Crispa, Christyl Johnsonb,∗ aJet Propulsion Laboratory, California Institute of Technology, MS 241-105, 4800 Oak Grove Drive, Pasadena, CA 91109, USA bNasa HQ, Office of Earth Science, Code YF, 300 E Street SW, Washington DC, USA Abstract The OrbitingCarbon Observatory (OCO) mission was selected by NASA’s Office of Earth Science as the fifth mission in its Earth System Science Pathfinder (ESSP) Program. OCO will make the first global, space-based measurements of atmospheric CO2 with the precision, resolution, and coverage needed to characterize sources and sinks of this important green-house gas. These measurements will improve our ability to forecast CO2-induced climate change. OCO will fly in a 1:15 PM sun-synchronous orbit, sharingits groundtrack with the Earth ObservingSystem (EOS) Aqua platform. It will carry high-resolution spectrometers to measure reflected sunlight in the molecular oxygen (O2) A-band at 0.76 m and the CO2 . X bands at 1.61 and 2 06 m to retrieve the column-averaged CO2 dry air mole fraction, CO2 . A comprehensive validation X and correlative measurement program has been incorporated into this mission to ensure that CO2 can be retrieved with precisions of 0.3% (1 ppm) on regional scales. © 2004 Elsevier Ltd. All rights reserved. 1. Introduction measurements provide strongevidence for a northern hemisphere sink, they cannot discriminate the relative Over the past 40 years, measurements from a global roles of the North American and Asian continents and network of ground-based stations indicate that only the ocean basins. They also cannot fully explain why about half of the CO2 that has been released into the the annual buildup of atmospheric carbon varies from atmosphere by fossil fuel combustion, biomass burn- 1 to 7 Gtons per year in response to steadily rising ing, and other human activities has remained there fossil fuel emission rates. (Fig. 1). The rest has apparently been absorbed by the These uncertainties complicate efforts to predict fu- oceans and by land-based ecosystems. Unfortunately, ture atmospheric CO2 concentrations or their effects the existingCO 2 monitoringnetwork does not provide on the climate, because they limit our ability to predict the coverage or spatial resolution needed to identify how the sinks might change as the climate evolves. and monitor these CO2 sinks. In particular, while these They also complicate efforts to monitor compliance to proposed green-house gas emission treaties that give ∗ Correspondingauthor. Tel.: +1 202 358 1683; credit for CO2 sinks. fax: +1 202 358 2769. To address these issues, NASA selected the Orbit- E-mail address: [email protected] (C. Johnson). ingCarbon Observatory (OCO) as the fifth mission 0094-5765/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2004.09.032 194 D. Crisp, C. Johnson / Acta Astronautica 56 (2005) 193–197 X and temporal gradients of CO2 alongthe ground track. X The space-based CO2 measurements will be com- plemented by in situ and remote sensingdata from a ground-based validation and correlative measurement X network to ensure that the space-based CO2 mea- surements have precisions of 0.3% (1-ppm CO2)on regional scales at monthly intervals. Once validated, these measurements will be incorporated into sophis- ticated source–sink inversion models to characterize the geographic distribution of CO2 sources and sinks over two annual cycles. The requirements for the space-based measurements were derived from end-to-end observation system sim- ulation experiments [4]. The weak CO2 band near 1.61 m was selected for CO2 column measurements because this spectral region is relatively free of ab- Fig. 1. Top: Atmospheric CO2 concentrations measured at Mauna sorption by other gases (Fig. 2b). Measurements in Loa Observatory since 1958 [2]. Bottom: Atmospheric carbon this band are also ideal for studyingnear-surface CO 2 increases from fossil fuel combustion (red) are compared to the sources and sinks because high-resolution spectra of measured atmospheric carbon buildup (blue) [3]. this band are most sensitive to the CO2 concentration near the surface. in Earth System Science Pathfinder (ESSP) Program. The absorption depth of these lines therefore in- OCO is designed to make global, space-based mea- creases almost linearly with the CO2 number den- surements of atmospheric CO2 with the spatial res- sity and path length, such that high-resolution spectro- olution and accuracy needed to characterize surface scopic measurements yield their greatest information sources and sinks. This paper summarizes the factors content near the surface. that influenced the design of the OCO mission, and Bore-sighted measurements in the 0.76 mO2 A- provides a brief description of the implementation ap- band provide direct constraints on the atmospheric proach. pressure of the reflectingsurface ( Fig. 2a). This infor- mation must be combined with the CO2 column esti- mates to derive the column-averaged CO2 dry air mole X 2. Measurement approach fraction, CO2 . Aircraft studies show that A-band ob- servations can provide surface pressure estimates with Modelingstudies with source–sink inversion mod- accuracies of ∼1 mb (O’Brien and Mitchell, 1992). els [1] indicate that our understandingof CO 2 sources A-Band spectra also provide a sensitive indicator of and sinks could be improved substantially if data clouds and optically thick aerosols, which preclude from the existingground-basedCO 2 monitoring full column measurements of CO2. network were augmented by global, space-based Finally, spectra of the strong2 .06 m band will measurements of the column-integrated CO2 dry provide independent constraints on the aerosol opti- (X ) ∼ air mole fraction CO2 with accuracies of 0.3% cal properties at near-infrared wavelengths, dramat- X (1 ppm out of 370 ppm). The OCO mission incorpo- ically improvingthe accuracy of CO2 retrievals in rates space- and ground-based elements to address aerosol-laden conditions [4]. Bore-sighted measure- this need. The space-based observatory will collect ments in this band also provide information about the high-resolution spectra of reflected sunlight in the atmospheric temperature and humidity alongthe opti- 0.76 mO2 A-band, and the CO2 bands at 1.58 and cal path, minimizingsystematic errors associated with 2.06 m(Fig. 2. These data will be analyzed with a uncertainties in these parameters. A single sounding simultaneous retrieval algorithm to estimate spatial consists of near-simultaneous, bore-sighted spectra in D. Crisp, C. Johnson / Acta Astronautica 56 (2005) 193–197 195 Fig. 2. The three spectral channels sampled by the OCO instrument. The O2 A-Band at 0.76 yields constraints on clouds, aerosols, and surface pressures (a). The CO2 column abundance is retrieved from the CO2 bands near 1.61 m (b) and 2.06 m (c). the 0.76 mO2 A-band and the CO2 bands at 1.61 and each OCO spectrometer will have a 10 km-wide cross- 2.06 m. track field of view (FOV) at nadir. This FOV is di- The spectral range for each channel includes the vided into 10 (or more) cross-track elements. Sound- complete molecular absorption band as well as some ings are collected at a rate of 45 soundings per sec- nearby continuum. This minimizes biases due to un- ond as the spacecraft moves alongits groundtrack at certainties in atmospheric temperature and provides 6.78 km/s. This yields ∼740 soundings per degree of constraints on the optical properties of the surface latitude alongthe orbit track. At this samplingrate, albedo and aerosols. The spectral resolvingpower for thousands of samples are collected on regional scales each channel was selected to maximize the sensitivity duringeach 16-day groundrepeat cycle. to variations in the column abundances of CO2 and The OCO instrument incorporates independent O2, and to minimize the impact of systematic mea- bore-sighted, long-slit, imaging, grating spectrom- surement errors. A spectral resolvingpower, / ∼ eters for the 1.61 m and 2.06 mCO2 bands and 21,000 separates individual CO2 lines in the 1.61 and the 0.76 mO2 A-band. These three spectrometers 2.06 m regions from weak H2O and CH4 lines and are integrated into a common structure to improve from the underlyingcontinuum. For the O 2 A-band, a rigidity and thermal stability. They use similar optical resolvingpower of 17,500 is needed to distinguishthe designs, consisting of an optimized 100 mm diameter, O2 doublets. With these resolvingpowers, the OCO f/2 telescope that focuses light on a long, narrow slit retrieval algorithm can characterize the surface re- that is aligned perpendicular to the orbit track. Behind flectance throughout the band and solve for the wave- the slit, the light is collimated, dispersed by a grating, length dependence of the aerosol scattering, minimiz- and focused by a camera lens, formingan imageof a X ing CO2 retrieval errors contributed by uncertainties spectrum on a focal plane array (FPA). The spectrum in the continuum level. is dispersed across the FPA in the direction orthogo- While many soundings must be collected on re- nal to the slit, and (cross-track) spatial information is gional scales to adequately characterize regional vari- recorded alongthe slit. X ations in CO2 on monthly time scales, contiguous The OCO instrument will be manufactured by spatial samplingis not required because CO 2 diffuses Hamilton Sundstand Sensor Systems, in Pomona Cal- over a large area as it is mixed through the column. ifornia. This is the same organization that supplied However, the full atmospheric column must be sam- the last 4 total ozone mappingspectrometer (TOMS) pled to provide constraints on surface CO2 sources instruments to NASA. and sinks. Clouds and optically thick aerosols pre- clude measurements of the complete column.