Processing TES Level-1B Data

TES L1B Subsystem is a computer program that performs several functions for the Tropospheric Emission Spectrometer (TES). The term "L1B" (an abbreviation of "level 1B"), refers to data, specific to the TES, on radiometric calibrated spectral radiances and their corresponding noise equivalent spectral radiances (NESRs), plus ancillary geolocation, quality, and engineering data. The functions performed by TES L1B Subsystem include shear analysis, monitoring of signal levels, detection of ice build-up, and phase correction and radiometric and spectral calibration of TES target data. Also, the program computes NESRs for target spectra, writes scientific TES level-1B data to hierarchical- data-format (HDF) files for public distribution, computes brightness temperatures, and quantifies interpixel signal variability for the purpose of first-order cloud and heterogeneous land screening by the level-2 software summarized in the immediately following article. This program uses an in-house-developed algorithm, called "NUSRT," to correct instrument line-shape factors

In-Situ Focusing Inside a Thermal Vacuum Chamber

Traditionally, infrared (IR) space instruments have been focused by iterating with a number of different thickness shim rings in a thermal vacuum chamber until the focus meets requirements. This has required a number of thermal cycles that are very expensive as they tie up many integration and test (I&T)/ environmental technicians/engi neers work ing three shifts for weeks. Rather than creating a test shim for each iteration, this innovation replaces the test shim and can focus the instrument while in the thermal vacuum chamber. The focus tool consists of three small, piezo-actuated motors that drive two sets of mechanical interface flanges between the instrument optics and the focal- plane assembly, and three optical-displacement metrology sensors that can be read from outside the thermal vacuum chamber. The motors are used to drive the focal planes to different focal distances and acquire images, from which it is possible to determine the best focus. At the best focus position, the three optical displacement metrology sensors are used to determine the shim thickness needed. After the instrument leaves the thermal vacuum chamber, the focus tool is replaced with the precision-ground shim ring. The focus tool consists of two sets of collars, one that mounts to the backside of the interface flange of the instrument optics, and one that mounts to the backside of the interface flange of the focal plane modules. The collars on the instrument optics side have the three small piezo-actuated motors and the three optical displacement metrology systems. Before the instrument is focused, there is no shim ring in place and, therefore, no fasteners holding the focal plane modules to the cameras. Two focus tooling collars are held together by three strong springs. The Orbiting Carbon Observatory (OCO) mission spectrometer was focused this way (see figure). The motor described here had to be moved five times to reach an acceptable focus, all during the same thermal cycle, which was verified using pupil slicing techniques. A focus accuracy of .20.100 microns was achieved

Processing TES Level-2 Data

TES Level 2 Subsystem is a set of computer programs that performs functions complementary to those of the program summarized in the immediately preceding article. TES Level-2 data pertain to retrieved species (or temperature) profiles, and errors thereof. Geolocation, quality, and other data (e.g., surface characteristics for nadir observations) are also included. The subsystem processes gridded meteorological information and extracts parameters that can be interpolated to the appropriate latitude, longitude, and pressure level based on the date and time. Radiances are simulated using the aforementioned meteorological information for initial guesses, and spectroscopic-parameter tables are generated. At each step of the retrieval, a nonlinear-least-squares- solving routine is run over multiple iterations, retrieving a subset of atmospheric constituents, and error analysis is performed. Scientific TES Level-2 data products are written in a format known as Hierarchical Data Format Earth Observing System 5 (HDF-EOS 5) for public distribution

OCO-3 Version 11: Better Data and More Data

The Orbiting Carbon Observatory 3 payload was installed on the International Space Station (ISS) on May 10, 2019 and completed in-orbit checkout on August 6, 2019. To enable precise retrievals of the column-average carbon dioxide dry air mole fraction, high resolution spectra of reflected sunlight are collected in three infrared channels. Band 1 (“ABO2”) from 757.6 to 772.7 nm is used to measure molecular oxygen and solar-induced chlorophyll fluorescence, and Bands 2 and 3 measure weak and strong carbon dioxide absorption features from 1591.2 to 1622.7 nm (“WCO2”) and 2042.0 to 2082.8 nm (“SCO2”). The three individual grating spectrometers share a common entrance telescope with a 1.8˚ field of regard, divided into eight along-slit footprints. Spectra are acquired at 3 Hz from an altitude of 400-420 km, with each footprint covering roughly 5 km2. OCO-3 employs an agile two-axis Pointing Mirror Assembly to view Earth in nadir over land, near the glint spot over water, and observe in target and area map modes over locations of interest. The PMA also points into an onboard calibrator to view one of three tungsten halogen lamps or to collect dark measurements. On rare occasions OCO-3 has viewed the Moon but cannot safely view the Sun. The current Level 1B product (Version 10.3) contains a number of imperfections, especially from June 2020 to January 2021 when the instrument was most contaminated. For Version 11, due for public release in 2024, changes were made to ABO2 radiometric calibration, both to the spectrally flat absolute level and the in-band spectral shape. A drift of roughly 1 % per year in Lamp 1, which is assumed to have constant output in Version 10, was corrected. The ratios between preflight and in-orbit checkout were also revised and made footprint-dependent. For spectral shape, data shortly after decontamination is given the spectral shape of Lamp 2 instead of Lamp 1. For later orbits, the spectral shape is based on ratios of ocean spectra to earlier in the cycle. Additional refinements were made to the instrument line shape, spatial response function, and signal to noise coefficients. OCO-3 was scheduled for a three-year prime mission, which concluded successfully in September 2022. Originally, the payload was to be uninstalled to make space for another mission. More recently, an extension was approved for continued operations through the lifetime of ISS in 2029. When the next payload arrives, currently expected in December 2023, OCO-3 will be temporarily stowed for approximately 6 months instead of being disposed. After reinstallation and another 90-day in-orbit checkout, nominal science operations will resume with no further planned interruptions

Refined Spatial Response Functions for OCO-3

The Orbiting Carbon Observatory 3 (OCO-3) has been collecting high-resolution spectra of sunlight reflected by Earth in three near-infrared spectral channels since 2019. An external payload on the International Space Station, it uses the spare instrument from the OCO-2 mission, which continues to collect data as a dedicated satellite. The three individual long-slit imaging grating spectrometers have their own focal plane arrays and share a common entrance telescope. The OCO-3 telescope was modified to have a 1.8˚ field of view to yield a similar ground footprint size from the 400-420 km altitude ISS orbit as OCO-2 obtains with its 0.8˚ field of view from its 705km altitude orbit. During preflight testing, the OCO-3 instrument was illuminated by a collimator with slit and pinhole targets in front of a quartz tungsten halogen lamp. The slits and pinholes were scanned across the field of view using a two-axis stage to determine the field of regard, coalignment of the three spectrometers, and the 2D spatial response of each footprint. The initial Ancillary Geometric Product, containing the centers and widths of each footprint, was calculated using an average of columns in the spectral dimension. Subsequently, a defocus was identified that varies significantly in the spectral dimension. As a result, the widths of the along-slit footprints are much larger at one end of the spectral band than at the other. The effect is strongest for the “Strong CO2” channel with a center wavelength of 2.06 microns. An improved characterization of the column-dependent spatial response is key for absolute calibration using the Moon, and important for determining which science scenes are sufficiently uniform to allow good quality atmospheric retrievals. Here, we will summarize recent progress in characterization and its impacts on the OCO-3 data products

TES Level 1 Algorithms: Interferogram Processing, Geolocation, Radiometric, and Spectral Calibration

The Tropospheric Emission Spectrometer (TES) on the Earth Observing System (EOS) Aura satellite measures the infrared radiance emitted by the Earth's surface and atmosphere using Fourier transform spectrometry. The measured interferograms are converted into geolocated, calibrated radiance spectra by the L1 (Level 1) processing, and are the inputs to L2 (Level 2) retrievals of atmospheric parameters, such as vertical profiles of trace gas abundance. We describe the algorithmic components of TES Level 1 processing, giving examples of the intermediate results and diagnostics that are necessary for creating TES L1 products. An assessment of noise-equivalent spectral radiance levels and current systematic errors is provided. As an initial validation of our spectral radiances, TES data are compared to the Atmospheric Infrared Sounder (AIRS) (on EOS Aqua), after accounting for spectral resolution differences by applying the AIRS spectral response function to the TES spectra. For the TES L1 nadir data products currently available, the agreement with AIRS is 1 K or better