Calibration is a critical step for all remote sensing data. Calibration ensures that the data received from any instrument are converted into meaningful and accurate measurements. This step also ensures that the available data is correct and reliable for those who want to use it.
The OCO-2 team will perform three types of calibration on the Observatory's science data stream:
- Radiometric calibrations that convert raw data numbers into spectral radiances (so we have a specific unit of measurement).
- Spectral calibrations that assign an appropriate wavelength to each of the samples that comprise the spectrum for each instrument channel (assigned wavelengths will help us start putting together the "picture").
- Geometric calibrations that provide the parameters required to accurately locate the footprint of each measurement on the Earth's surface (this tells us where we were looking).
The OCO-2 instrument focal planes will record the brightness of the incident spectral radiances as raw data numbers (DN). Data numbers are measures without units. For the OCO-2 mission, data numbers that represent spectral radiances range from 0 to 216. The OCO-2 Ground Data System will be 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-2 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 will be the critical component of the OCO-2 Level 1B Product.
A dedicated team will oversee the radiometric calibration process for the entire OCO-2 mission. This team will maintain the algorithms and update the parameters required to generate accurate radiances. Ongoing calibration exercises during the space operations of OCO-2 will 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. This aspect of the OCO-2 mission is vital as CO2 sources and sinks must be inferred from small (<2%) spatial variations in Xco2. OCO-2 will have a precision of <0.3% (1 ppm), thus allowing for the quantification 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 due exposure to the space environment. 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 OCO-2 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-2 science data, clear sky conditions will not be required to acquire this data. The mission will regularly perform four types of calibration using the OBC. Each calibration generates a unique data collection, which include:
- Cal_solar data: To collect these data, the instrument will deploy an attenuation screen in front of the telescope. The Observatory will point 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 will provide a means to calibrate the absolute instrument response as well as relative instrument response among the three OCO-2 spectrometers. The wavelengths where radiances appear in the Cal_solar data will 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 will acquire both data sets in sequence. Acquisition of the Cal_solar data will immediately precede the Cal_limb data. The instrument attenuation screen will remain deployed and the instrument telescope will continue to view the Sun. However, as the Observatory orbit progresses, the instrument's line of sight will pass through the Earth's atmosphere. Thus, Cal_limb spectra will contain both Solar absorption lines as well as absorption lines that are characteristic of the atmosphere's chemical content.
- Cal_dark data. The mission will employ two means to collect these data. Either the instrument will view the dark ocean at night, or the instrument will apply a cover to the viewing telescope. Cal_dark data will specify the focal plane response for a totally dark scene. Thus, these values will specify the "zero point" on the radiance scale. Once a calibration is applied, measurements that are equivalent to the "zero point" will indicate no incident light.
Cal_dark data will be collected at two points on the night side of the orbit. One set of Cal_dark data will 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 will be collected at different locations over the night side of the orbit. These data will 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 will turn on one of three small light bulbs. Light from the bulb will illuminate a reflector. The reflector will diffuse 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 will 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) will employ precise in situ measurements collected at the Earth's surface to estimate the solar radiation field at the top of the Earth's atmosphere. The calibration team will compare these data with measurements acquired by the Observatory. The comparisons will yield a correction table. Application of the correction table will force OCO-2 measurements to conform to the Vicarious Calibration experiment. This adjustment will provide both an absolute and channel-relative calibration for OCO-2 data products. See Kuze et al. (IEEE Trans. Geosci. Rem. Sens., 2010).
- "Flat fielding" will employ Earth scene statistics which will be collected from sets of Nadir Mode data. These statistics will 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, will indicate an error in the calibration process. When systematic effects are detected, the team will apply the Earth scene statistics to update the characterization of the On Board Calibrator.
- Cal_doppler observations will view the Sun over one entire day side of the orbit. These data will provide measurements of the solar spectrum over the full range of Doppler shifts that the Observatory encounters. These data will also provide a means to apply Doppler corrections to instrument wavelength measurements.
The figure below summarizes the calibration parameters that the OCO-2 mission will measure to track instrument degradation using on-orbit tests. The team will transfer the outcome of these studies to the Ancillary Radiometric Product (ARP). The ARP stores both instrument and On Board Calibrator (OBC) descriptive coefficients. A large number of the parameters are based on instrument characterization experiments that the mission will conduct before launch.
Routine updates are required to maintain the accuracy of radiances computed using these coefficients. Thus, the calibration team updates the ARP after each orbit repeat cycle. Each repeat cycle covers 16 days or 233 spacecraft orbits. After the calibration team creates a new file, and the mission approves the file for data production, the OCO-2 Ground Data System will implement the new ARP into the Operational Pipeline process.
The Ground Data System CalApp Product Generation Executive (PGE) will use the approved ARP to generate standard Level 1B Products.
The team will use iterative updates of the ARP to test and modify the algorithms used to produce the radiance parameters. As more data are collected, the calibration team will improve their assessment of:
- the impact of temperature on the variation of detector gain and offset,
- which temperatures correlate best with gain and offset changes,
- which mathematical functions best describe the observables, and
- the need, if any, to correct for stray light on instrument detectors.