Robert E. Haring

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.

The Orbiting Carbon Observatory instrument: performance of the OCO instrument and plans for the OCO-2 instrument

NASAs Orbiting Carbon Observatory (OCO) was designed to make measurements of carbon dioxide concentrations from space with the precision and accuracy required to identify sources and sinks on regions scales (~1,000 km). Unfortunately, OCO was lost due to a failure of the launch vehicle. Since then, work has started on OCO-2, planned for launch in early 2013. This paper will document the OCO instrument performance and discuss the changes planned for the OCO-2 instrument.

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.

Spectral band calibration of the Total Ozone Mapping Spectrometer (TOMS) using a tunable laser technique

The Total Ozone Mapping Spectrometer (TOMS) provides daily global mapping of the total column ozone in the earth’s atmosphere. It does this by measuring the solar irradiance and the backscattered solar radiance in 6 spectral bands falling within the range from 308.6 nm to 360 nm. The accuracy of the ozone retrieval is highly dependent on the knowledge of the transfer characteristics and center wavelength for each spectral band. A 0.1 nm wavelength error translates to a 1.6% error in ozone. Several techniques have historically been used to perform the wavelength calibration of the TOMS instruments. These methods include the use of film and reference spectra from low-pressure spectral line lamps and the use of continuum sources with a narrow-band scanning monochromator. The spectral transfer characteristic of the Flight Model 5 instrument for the QuikTOMS mission was calibrated using a new technique employing a frequency doubled tunable dye laser. The tunable laser has several advantages that include a very narrow spectral bandwidth; accurate wavelength determination using a wavemeter; and the ability to calibrate the instrument system level of assembly (prior methods required that the calibration be performed at the monochromator sub assembly level). The technique uses the output from a diode-pumped solid state Nd:V04 laser that is frequency doubled to provide a continuous wave 532 nm pump laser beam to a Coherent Model 899-01 frequency doubled ring dye laser. The output is directed into the entrance port of a 6-inch diameter Spectralon integrating sphere. A GaP photodiode is used to monitor the sphere wall radiance while a Burleigh Wavemeter (WA-1500) is used to monitor the wavelength of the visible output of the dye laser. The TOMS field of view is oriented to view the exit port of the integrating sphere. During the measurement process the response of the instrument is monitored as the laser source is stepped in 0.02-nm increments over each of the six TOMS spectral bands. Results of the new technique allow establishing the wavelength center to a precision of better than 0.1 nm. In addition to the spectral band measurements, the laser provided a means to calibrate the radiometric linearity of the QuikTOMS instrument and yield new insights into the stray light performance of the complete optical system.

Oxygen A-band spectrometer breadboard for the Orbiting Carbon Observatory

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.

Fabrication and assembly integration of the orbiting carbon observatory instrument

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.

Wide-field-of-view imaging spectrometer (WFIS): from a laboratory demonstration to a fully functional engineering model

This paper presents the status of the ongoing development of the laboratory Wide Field-of-view Imaging Spectrometer (WFIS) and the new engineering model WFIS. The design is shown to provide a unique solution to wide field hyperspectral imaging with several advantages over traditional scanning systems. Tests of the engineering model, funded under NASAs Instrument Incubator program, take the WFIS to the next level of technology readiness. The WFIS is based on a patented optical design intended for optical remote sensing of the earth and the earths atmosphere in the hyperspectral-imaging mode. The design of the laboratory spectrometer and the initial test results obtained with it were presented at the 1999 SPIE Annual Meeting in Denver, Colorado (3759-32). Since that time, the laboratory unit has undergone several upgrades in the optical path and continues to be a pathfinder for the new engineering model instrument. The WFIS engineering model incorporates several improvements to provide increased wavelength coverage from the UV to the NIR and an increase in the field-of-view coverage to 120 degrees. It differs most significantly from the laboratory unit in that it is designed for flight. The status of the hardware, software, and the assembly of the engineering WFIS is discussed as well as an overview of the planned demonstration tests.

Wide-field-of-view imaging spectrometer (WFIS) engineering model laboratory tests and field demonstrations

The Wide Field-of View Imaging Spectrometer (WFIS) is a patented optical design allowing horizon to horizon imaging of the earth and earth’s atmosphere in the pushbroom-imaging mode from an aircraft or space platform. The design couples a fast, F/2.8, unobstructed all reflective telescope to an all-reflective three element imaging spectrometer using a unique field coupling mirror arrangement. Early laboratory demonstrations of the technology covered fields of view exceeding 70 degrees. The latest instrument, the incubator WFIS, demonstrate the field of view can be extended to 120 degrees. This paper summarizes the current ongoing work with the engineering model WFIS covering this field of view and a spectral range from 360 nm to 1000 nm. Also presented are the results of the latest laboratory and field demonstrations. The paper also identifies specific applications the technology is now addressing.

Field testing the wide-field-of-view imaging spectrometer(WFIS)

The Wide Field-of-view Imaging Spectrometer (WFIS), a high-performance pushbroom hyperspectral imager designed for atmospheric chemistry and aerosols measurement from an aircraft or satellite, underwent initial field testing in 2004. The results of initial field tests demonstrate the all-reflective instruments imaging performance and the capabilities of data processing algorithms to render hyperspectral image cubes from the field scans. Further processing results in spectral and photographic imagery suitable for identification, analysis, and discrimination of subjects in the images. The field tests also reveal that the WFIS instrument is suited for other applications, including in situ imaging and geological remote sensing.

Radiometric calibration of total ozone mapping spectrometer: flight model 5 (TOMS-5) aboard QuickTOMS

12 The Total Ozone Mapping Spectrometer - Flight Model 5 (TOMS- 5), aboard the QuikTOMS spacecraft, is designed to continue the measurement of the total column amount of ozone in the atmosphere in order to monitor the global trend. Since the predicted total ozone change due to man-made sources is very small, an accurate calibration of the measuring instrument is required. Since in the TOMS-5 experiment the total ozone amount is determined from the ratio of the measurement of the solar backscattered ultraviolet Earth radiance to the incident solar irradiance, the accuracy of the calibration of the instrument sensitivity to this ratio measurement is critical. The prelaunch calibration of TOMS-5 was designed to achieve a ratio calibration accuracy of 1% in addition to the uncertainties of the standards used. Multiple calibration techniques were employed to ensure the self- consistency of results of different techniques to eliminate any systematic errors. TOMS-5 prelaunch radiometric calibration was performed twice, one in 1996 and the second in 1999 due to the launch delay. The ratio calibration was reproduced within 0.5% from the tests of 1996 to those of 1999 while the calibration of the individual measurement modes agreed among the various techniques to within 1%.