Randy Pollock

Preflight Spectral Calibration of the Orbiting Carbon Observatory

We report on the preflight spectral calibration of the first Orbiting Carbon Observatory (OCO) instrument. In particular, the instrument line shape (ILS) function as well as spectral position was determined experimentally for all OCO channels. Initial determination of these characteristics was conducted through laser-based spectroscopic measurements. The resulting spectral calibration was validated by comparing solar spectra recorded simultaneously by the OCO flight instrument and a collocated high-resolution Fourier transform spectrometer (FTS). The spectral calibration was refined by optimizing parameters of the ILS as well as the dispersion relationship, which determines spectral position, to yield the best agreement between these two measurements. The resulting ILS profiles showed agreement between the spectra recorded by the spectrometers and FTS to approximately 0.2% rms, satisfying the preflight spectral calibration accuracy requirement of better than 0.25% rms.

Preflight Radiometric Calibration of the Orbiting Carbon Observatory

This paper describes the radiometric calibration of the original Orbiting Carbon Observatory. The calibration process required characterizing both the dark current level and gain coefficients of each instrumental channel. The dark response was characterized with extensive testing and revealed some unexpected instrument behavior. The gain coefficients were characterized via illumination of the instrument spectrometers with a laboratory calibrated integrating sphere source. Comparison between the spectrometer output and a calibrated photodiode led to a set of calibration coefficients for each spectrometer channel. The calibration coefficients were validated by a novel approach involving observation of the solar spectrum through a transmission filter. Validation occurred by examination of both the ratio of the filtered to unfiltered spectra and retrievals of geophysical quantities such as surface pressure. The linearity of the calibration was established to approximately 0.2%. Finally, using the calibration data from the integrating sphere, a simple noise model was developed for each channel of the instrument. A summary of the signal to-noise performance is included.

The Orbiting Carbon Observatory instrument optical design

The Orbiting Carbon Observatory (OCO) will measure the distribution of total column carbon dioxide in the Earths atmosphere from an Earth-orbiting satellite. Three high-resolution grating spectrometers measure two CO2 bands centered at 1.61 and 2.06 μm and the oxygen A-band centered at 0.76 μm in the near infrared region of the spectrum. This paper presents the optical design and highlights the critical optical requirements flowed down from the scientific requirements. These requirements necessitate a focal ratio of f/1.9, a spectral resolution of 20,000, and precedence-setting requirements for polarization stability and the instrument line shape function. The solution encompasses three grating spectrometers that are patterned after a simple refractive spectrometer approach consisting of an entrance slit, a two-element collimator, a planar reflection grating, and a two-element camera lens. Each spectrometer shares a common field of view through a single all-reflective telescope. The light is then re-collimated and passed through a relay system, separating the three bands before re-imaging the scene onto each of the spectrometer entrance slits using an all-reflective inverse Newtonian re-imager.

Characterization of the OCO-2 instrument line shape functions using on-orbit solar measurements

Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0.25%) compared to previous spaceborne grating spectrometers. Onorbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O2 A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5% of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O2 A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas.

Current development status of the Orbiting Carbon Observatory instrument optical design

The Orbiting Carbon Observatory, OCO, is a NASA Earth System Science Pathfinder (ESSP) mission to measure the distribution of total column carbon dioxide in the earths atmosphere from an earth orbiting satellite. NASA Headquarters confirmed this mission on May 12, 2005. The California Institute of Technologys Jet Propulsion Laboratory is leading the mission. Hamilton Sundstrand is responsible for providing the OCO instrument. Orbital Sciences Corporation is supplying the spacecraft and the launch vehicle. The optical design of the OCO is now in the detail design phase and efforts are focused on the Critical Design Review (CDR) of the instrument to be held in the 4th quarter of this year. OCO will be launched in September of 2008. It will orbit at the head of what is known as the Afternoon Constellation or A-Train (OCO, EOS-Aqua, CloudSat, CALIPSO, PARASOL and EOS-Aura). From a near polar sun synchronous (~1:18 PM equator crossing) orbit, OCO will provide the first space-based measurements of carbon dioxide on a scale and with the accuracy and precision to quantify terrestrial sources and sinks of CO2. The status of the OCO instrument optical design is presented in this paper. The optical bench assembly comprises three cooled grating spectrometers coupled to an all-reflective telescope/relay system. Dichroic beam splitters are used to separate the light from a common telescope into three spectral bands. The three bore-sighted spectrometers allow the total column CO2 absorption path to be corrected for optical path and surface pressure uncertainties, aerosols, and water vapor. The design of the instrument is based on classic flight proven technologies.

Preflight Spectral Calibration of the Orbiting Carbon Observatory 2

This paper describes the preflight spectral calibration methods and results for the Orbiting Carbon Observatory 2 (OCO-2), following the approach developed for the first OCO. The instrument line shape (ILS) function and dispersion parameters were determined through laser-based spectroscopic measurements, and then further optimized by comparing solar spectra recorded simultaneously on the ground by the OCO-2 flight instrument and a collocated high-resolution Fourier transform spectrometer (FTS). The resulting ILS profiles and dispersion parameters, when applied to the FTS solar data, showed agreement between the spectra recorded by the spectrometers and FTS to approximately 0.2% RMS, satisfying the preflight spectral calibration accuracy requirement of <0.25% RMS. Specific changes to the OCO-2 instrument and calibration process, compared to the original OCO, include stray-light protection; improved laser setup; increased spectral sampling; enhanced data screening, and incremental improvements in the ILS, dispersion, and FTS optimization analyses.

System for establishing best focus for the Orbiting Carbon Observatory instrument

A technology for establishing best focus of an optical system is described. This technology was recently used to establish best focus of the Orbiting Carbon Observatory spectrometers while the instrument was undergoing thermal-vacuum testing. Three piezo-actuated motors were used to adjust the tip, tilt, and piston of a focal plane assembly relative to the spectrometers optical system. A set of optical displacement sensors measured tip-tilt-piston throughout the focusing process. With best focus established and confirmed using a pupil-slicing technique, the corresponding sensor measurements were used to specify the geometry and dimensions of a precision-ground shim ring.

Identification and Correction of Residual Image in the

The detector used for the O2 A-band (0.76 μm) of the National Aeronautics and Space Administrations Orbiting Carbon Observatory (OCO) employed a HyViSI Hawaii-1RG sensor, operating at 180 K in a rolling read-out mode. During the thermal vacuum testing of the flight instrument, it was discovered that the detector exhibited residual images that lasted for many seconds and were of sufficient magnitude to compromise the mission objectives. Independent testing of flight-spare detectors revealed that the problem was common to all and was not simply a fault of the flight detector. The residual image was found to depend upon even-order derivatives of the spectrum, and its decay was a function of the number of frames rather than time. An empirical model was developed, which represented the measured spectrum in terms of the true spectrum and a history of all previous changes in the spectra. On the basis of the model, an algorithm was devised to correct spectra for the effects of residual image, using a time-marching analysis of a history of previous spectra. The algorithm was tested with spectra acquired during the second thermal vacuum test of OCO and was found to reduce the effect of residual image to almost the noise level of the detector. Numerical simulations indicate that residual image has a negligible impact on retrieved concentrations of O2 and CO2 once the spectra have been corrected.

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The Oxygen A-band spectrometer breadboard was developed to demonstrate alignment and focus methodologies planned for the spectrometers to be used for the Orbiting Carbon Observatory (OCO). The OCO is a proposed Earth System Science Pathfinder (ESSP) mission to provide the first global CO2 measurements from space with a relative accuracy of 1-ppm on scales of 2.5 × 105 km2. The flight system uses three refractive spectrometers to measure column CO2 at 1.58 and 2.06-micrometers and column O2 in the oxygen A-band at 0.76 micrometers. This paper describes a relatively fast, f/2, high resolution grating spectrometer breadboard designed, manufactured, and tested in less than 6 months. The breadboard successfully validates the optical design and alignment approach to be used for the three spectrometers that comprise the OCO instrument.

A-Band of the Orbiting Carbon Observatory

Final assembly and integration of the Orbiting Carbon Observatory instrument at the Jet Propulsion Laboratory in Pasadena, California is now complete. The instrument was shipped to Orbital Sciences Corporation in March of this year for integration with the spacecraft. This observatory will measure carbon dioxide and molecular oxygen absorption to retrieve the total column carbon dioxide from a low Earth orbit. An overview of the design-driving science requirements is presented. This paper then reviews some of the key challenges encountered in the development of the sensor. Diffraction grating technology, lens assembly performance assessment, optical bench design for manufacture, optical alignment and other issues specific to scene-coupled high-resolution grating spectrometers for this difficult science retrieval are discussed.