Calibration ensures that the data received from the instrument are converted into meaningful and accurate measurements. The OCO-2 team will perform three types of calibration on the Observatory's science data stream:
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 area highlighted in the blue circle provides a physical overview of the On Board Calibrator that overlays the instrument telescope/collimator assembly.
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_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.
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:
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: