Calibration Overview

Calibration ensures that the data received from the instrument are converted into meaningful and accurate measurements. The OCO team will perform three types of calibration on the Observatory's science data stream:

• Radiometric calibrations that convert raw data numbers into spectral radiances.
• Spectral calibrations that assign an appropriate wavelength to each of the samples that comprise the spectrum for each instrument channel.
• Geometric calibrations that provide the parameters required to accurately locate the footprint of each measurement on the Earth's surface.

Calibration Approach

The OCO instrument focal planes record the brightness of the incident spectral radiances as raw data numbers (DN). Data numbers are measures without units. For the OCO mission, data numbers that represent spectral radiances range from 0 to 65535. The OCO Ground Data System is responsible for the conversion of these data numbers into wavelength dependent measurements that are expressed in meaningful physical units. The physical units that the OCO mission will use for spectral radiance are photons per square meter per steradian per second. Radiometric calibration is the process that collects and applies the parameters needed to convert the instrument output into physical measures. The radiance measure generated by the calibration process is the critical component of the OCO Level 1B Product.

A dedicated team will oversee the radiometric calibration process for the entire OCO mission. This team will maintain the algorithms and update the parameters required to generate accurate radiances. Ongoing calibration exercises ensure that the mission obtains bias-free radiance measurements. Accurate radiance measures are crucial to retrieve Xco₂ with the precision needed to determine the geographic distribution of CO₂ sources and sinks.

The calibration team will characterize the instrument on the ground before Observatory launch. The characterization exercise will yield the initial set of parameters required to convert instrument data numbers into incident radiances. Once the instrument begins to operate in flight, its behavior will change. For the remainder of the mission, the calibration team will use on-board calibration capabilities to track instrument behavior, and modify the calibration parameters to ensure accurate assessment of instrument measure.

The area highlighted in the blue circle provides a physical overview of the On Board Calibrator that overlays the instrument telescope/collimator assembly.

The OCO mission will employ an On Board Calibrator (OBC) to detect changes in the instrument gain and wavelength response. While the spacecraft flies over the dark side of the Earth, the instrument will automatically collect calibration data using the OBC. Unlike the OCO science data, clear sky conditions will not be required to acquire these data. The mission will regularly perform four types of calibration using the OBC. Each calibration generates a unique data collection. These data collections include:

• Cal_solar data: To collect these data, the instrument deploys an attenuation screen in front of the telescope. The Observatory points the instrument telescope at the Sun while the instrument line of sight is above the Earth's atmosphere. The radiances and the wavelengths of the spectral lines in the solar spectrum are well established. Thus, radiances recorded in the Cal_solar data provide a means to calibrate the absolute instrument response as well as relative instrument response among the three OCO spectrometers. The wavelengths where radiances appear in the Cal_solar data also provide a means to calibrate the spectral wavelength associated with each spectral sample.

• Cal_limb data: These data are an extension of the Cal_solar data. The Observatory acquires both data sets in sequence. Acquisition of the Cal_solar data immediately precedes the Cal_limb data. The instrument attenuation screen remains deployed and the instrument telescope continues to view the Sun. However, as the Observatory orbit progresses, the instrument's line of sight passes through the Earth's atmosphere. Thus, Cal_limb spectra contain both Solar absorption lines as well as absorption lines that are characteristic of the atmosphere's chemical content.

• Cal_dark data. The mission employs two means to collect these data. Either the instrument views the dark ocean at night, or the instrument applies a cover to the viewing telescope. Cal_dark data specify the focal plane response for a totally dark scene. Thus, these values specify the "zero point" on the radiance scale. Once a calibration is applied, measurements that are equivalent to the "zero point" indicate no incident light.
  
  Cal_dark data are collected at two points on the night side of the orbit. One set of Cal_dark data are always collected at the same relative location of the Observatory orbit relative to the day-night terminator. These data monitor long-term drift of the zero point. A second set of Cal_dark data are collected at different locations over the night side of the orbit. These data monitor shifts in the zero-point offset that are associated with changes in instrument or spacecraft temperature.

• Cal_lamp data. To collect these data, the instrument turns on one of three small light bulbs. Light from the bulb illuminates a reflector. The reflector diffuses the light to produce a uniform field that is directed into the instrument telescope. Since the spatial and spectral distribution from these bulbs is uniform and well known, Cal_lamp data provide the "flat fields" that are used to define the relative radiometric response for each detector on the focal plane.

Additional calibration data come from observations that the mission will schedule when needed. Calibration planners will issue special command procedures to implement these activities. Examples of these are:

• Vicarious Calibration (VC) employs precise in situ measures collected at the Earth's surface to estimate the solar radiation field at the top of the Earth's atmosphere. The calibration team compares these data with measurements acquired by the Observatory. The comparisons yield a correction table. Application of the correction table forces OCO measurements to conform to the Vicarious Calibration experiment. This adjustment provides both an absolute and channel-relative calibration for OCO data products.

• "Flat fielding" employs Earth scene statistics collected from sets of Nadir Mode data. These statistics provide a means to verify the sample-relative gain coefficients generated by the On Board Calibrator. A consistent decrease in the radiance for one sample, as compared to a neighbor sample at the same wavelength in the same spectrum, indicates an error in the calibration process. When systematic effects are detected, the team applies the Earth scene statistics to update the characterization of the On Board Calibrator.

• Cal_doppler observations view the Sun over one entire day side of the orbit. These data provide measurements of the solar spectrum over the full range of Doppler shifts that the Observatory encounters. These data also provide a means to apply Doppler corrections to instrument wavelength measurements.

For more details about the calibration process, proceed to the next page.