The OCO-2 Ground Data System (GDS) will convert automated readings downloaded from the Observatory into data products that provide comprehensive mission results as well as material for further research and investigation. The GDS will provide the environment to generate two data products (i.e., L1 and L2) that will be available for distribution to both the scientific community and the general public. Both products will appear in the Hierarchical Data Format (HDF) v 5. The products will conform to the standards of the Earth Observing System Data and Information System (EOSDIS) Core System (ECS).
The OCO-2 Level 1B product will list calibrated and geolocated spectral radiances. Each Level 1B product will contain a unique record for every sounding that the OCO-2 instrument acquires while viewing the Earth during a single spacecraft orbit. Each sounding will consist of the three collocated spectra. Each of these spectra will correspond with one of the three spectrometer channels. If the instrument acquires data continuously over the entire daylight side of the Earth, the Level 1B Product will contain approximately 37,000 soundings. The Level 1B product also includes error measures and indicators that assess the quality of each acquired spectrum. These indicators denote which spectra are suitable for subsequent processing.The OCO-2 Level 2 product will provide several retrieved atmospheric and geophysical measures. The retrieved data in each product will be based on the spectra that were acquired during the same OCO-2 spacecraft orbit. The presence of clouds, thick aerosols and topographic inhomogeneities inhibits the retrieval process. A typical Level 2 Product will likely list the output from about 9,300 retrievals. The retrieved data will include vertical atmospheric profiles of temperature, CO2 and water vapor as well as aerosol content. Each vertical profile will appear as an array in the Level 2 Product. Each element in these arrays will represent a location in the vertical profile. Retrieved data will also include scalar measures of albedo, surface pressure and the column averaged dry air CO2 mole fraction (Xco2).
The Level 2 product will contain error measures and indicators that assess the quality of the data output. These indicators will specify whether the retrieved data sets are suitable for subsequent processing. Members of the OCO-2 science team will generate Level 3 and Level 4 products at their home institutions. The Level 3 product will map Xco2 to one part per million (ppm) accuracy over the Earth's surface in bins with dimensions of 1° in latitude and 1° in longitude. Each granule of the OCO-2 Level 3 product will represent data from one 16 day orbit repeat cycle. The Level 4 product will map the distribution of CO2 sources and sinks over the Earth's surface. Plots of the Level 4 product will display fluxes in CO2 abundance over the Earth's surface in regions with dimensions of 4° in latitude and 5° in longitude.
To enhance the quality and to verify the validity of mission data, OCO-2 will collect science observations in Nadir, Glint, and Target Modes. In Nadir Mode, the satellite points the instrument to the local nadir so that data can be collected along the ground track just below the spacecraft. Science observations will be collected at all latitudes where the solar zenith angle is less than 85°. Nadir Mode provides the highest spatial resolution on the surface and is expected to return more useable soundings in regions that are partially cloudy or have significant surface topography. Nadir observations may not provide adequate signal-to-noise over dark ocean surfaces.
In Glint Mode, the spacecraft points the instrument toward the bright "glint" spot, where solar radiation is specularly reflected from the surface. Glint Mode observations provide up to 100 times more signal than nadir observations. Thus, the use of glint measurements significantly improves the signal-to-noise ratio over the dark ocean. Glint soundings will be collected at all latitudes where the local solar zenith angle is less than 75°. The OCO-2 mission plans to alternate between Nadir and Glint Modes over sequential 16-day global ground track repeat cycles so that the entire Earth is mapped in each mode on monthly time scales.
In Target Mode, the Observatory will lock its view onto a specific surface location, and will retain that view while flying overhead. A target track pass can last for up to 9 minutes. Over that time period, the Observatory can acquire as many as 12,960 samples at local zenith angles that vary between 0° and 85°. The mission plans to conduct regular Target track passes over each of the OCO-2 calibration sites where the ground-based solar-looking Fourier Transform Spectrometers are located. Comparison of space- and ground-based measurements provides a means to identify and correct systematic and random errors in the OCO-2 Xco2 data products.