: Measurement Approach

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Measurement Approach

The retrieval of a global geographic distribution of CO₂ sources and sinks is the principal science objective of the OCO-2 mission. The OCO-2 mission will not, however, directly measure CO₂ sources and sinks. Instead, sophisticated computer-based data assimilation models that employ column averaged dry air CO₂ mole fraction (Xco₂) data will infer the location of these sources and sinks.

To ascertain representative values of Xco₂, the OCO-2 instrument will measure the intensity of reflected sunlight off of the Earth's surface at specific wavelengths. Gas molecules in the atmosphere absorb radiation at characteristic wavelengths. Thus, as light passes through the Earth's atmosphere, the presence of these gases leaves a distinguishing fingerprint on the residual radiation. The OCO-2 spectrometers will detect these molecular fingerprints, and the level of absorption displayed in these spectra will be indicative of the abundance of molecules in the region where the measurement was acquired.

The OCO-2 measurement approach will concentrate on gathering data that reflect chemical abundance near the Earth's surface, where almost all of the CO₂ sources and sinks are located.

Thus, in order to infer the presence of sources and sinks, the light detected by the instrument must penetrate through the full height of the atmosphere. The presence of clouds and optically thick aerosols can block part of the distance, and thus preclude measurement through the complete atmospheric column. Inhomogeneous conditions such as large topographic variations within individual soundings can introduce an additional uncertainty in the length of the light column, which also has a detrimental effect on resulting Xco₂ measurements. To allay these concerns, the OCO-2 instrument will acquire a large number of densely-spaced samples. Each sample will cover an area of about 3 km² when the instrument is viewing locations at nadir, along the spacecraft's ground track. The OCO-2 instrument can gather as many as 37,000 of these soundings on the sunlit side of any orbit. With measurement footprints of this size and density, the OCO-2 instrument will acquire an adequate number of high quality soundings even in those regions where clouds, aerosols and topographic variations are present.

OCO-2 mission designers selected three specific Near Infrared (NIR) wavelength bands. The OCO-2 instrument will measure intensity over all three of these bands at the same location on the Earth's surface at simultaneously. Each of the three selected wavelength bands provides a specific contribution to measurement accuracy. The weak CO₂ band with wavelengths in the vicinity of 1.61 µm is most sensitive to the CO₂ concentration near the surface. Since other atmospheric gases do not absorb significant energy within this spectral range, the 1.61 µm band measurements are relatively clear and unambiguous.

Accurate derivation of Xco₂ using space-based readings of the CO₂ absorption requires comparative absorption measurements of a second atmospheric gas. The concentration of molecular oxygen (O₂) is constant, well known, and uniformly distributed throughout the atmosphere. Thus, O₂ is the best candidate for reference measurements. The O₂ A-band wavelengths in the vicinity of 0.76 µm will provide the required absorption spectra. Light at this wavelength is just beyond the red end of the spectrum that is visible to the human eye. The O₂ A-band spectra indicate the presence of clouds and optically thick aerosols that preclude full column measurements of CO₂. Observations from this band will be used to infer the total atmospheric pressure, as well as to measure the length of the path of solar light as it passes through the atmosphere.

The strong CO₂ band with wavelengths in the vicinity of 2.06 µm will provide a second and totally independent measure of the CO₂ abundance. The 2.06 µm band spectra are very sensitive to the presence of aerosols. The ability to detect and mitigate the presence of aerosols enhances the accuracy of Xco₂. The 2.06 µm band measurements are also sensitive to variations in atmospheric pressure and humidity along the optical path. These variations in pressure and humidity have a known impact on Xco₂ measure.

The coverage capability of an orbiting observatory ensures a continuous, uniform measure over all regions of the Earth. The Observatory will fly in a polar, sun-synchronous orbit, providing global coverage with a 16-day repeat cycle. On each orbit, the Observatory path will cross the equator at approximately 1:18 PM local time. Acquisition at this time of day is ideal for spectroscopic observations of CO₂ that use reflected sunlight as the high sun maximizes the measurement signal-to-noise ratio. Furthermore, since Xco₂ measurements tend to be near their daily average value at this time of day, the Observatory data will be highly representative of the region where they were acquired. Coordination of the orbit with the A-train facilitates carbon cycle science by enabling the integration of OCO-2 observations with those of other instruments that fly aboard the Aqua and Aura spacecraft. Among these measurements are the temperature, humidity, and CO₂ retrievals from Atmospheric Infrared Sounder (AIRS), the cloud, aerosol and ocean color observations as well as carbon source and sink measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS), and the CH₄ and CO retrievals from Tropospheric Emission Spectrometer (TES).

This diagram displays the spectral reflectance of common Earth surfaces at the wavelength of the three OCO-2 channels.

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.

MPEG animation of typical spacecraft maneuvers in nadir mode -- 2.80 MBs.

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 roughly monthly time scales.

MPEG animation of typical spacecraft maneuvers in glint mode -- 1.45 MBs.

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 based and ground based measures provides a means to identify and correct systematic and random errors in the OCO-2 Xco₂ data products.

MPEG animation of spacecraft maneuvers from nadir mode to target mode and back to nadir mode -- 2.14 MBs.

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