# The Ground-Based Absolute Radiometric Calibration of the Landsat 9 Operational Land Imager
## 2 Methodology
### On-Site Personnel: The Reflectance-Based Approach
#### 2.3.3 Atmospheric Measurements  
The atmospheric characterization required during a vicarious calibration field campaign has traditionally been performed using a suite of custom-built and commercial instruments that temporarily operate onsite throughout the time of interest. The goal of such measurements is to determine the atmospheric transmittance, which is used in a radiative transfer code to determine the TOA spectral radiance for a specific time, date, as well as illumination and view geometry.  
The research groups in this work make base measurements of solar irradiance extinction due to absorption and scattering with a ten-channel multispectral automated solar radiometer (ASR) that uses narrow bandwidth interference filters for spectral selection. The ten spectral channels are centered at 380 nm, 400 nm, 441 nm, 520 nm, 611 nm, 670 nm, 780 nm, 870 nm, 940 nm, and 1030 nm [26]. The ASR tracks the sun and measures the incoming solar irradiance extinction caused by atmospheric absorption and scattering. The data are then used to derive the total atmospheric spectral optical depths, which can be further broken down into subcomponents such as molecular, aerosol, ozone, and water vapor optical depths [99; 100]. In desert sites used by UArizona, the aerosols are modeled by a power-law distribution, and the Angstrom exponent is used to define the aerosol size distribution. Columnar water vapor is determined through a modified Langley approach [27; 28], while the columnar ozone amount is obtained from the OMI or OMPS. Ancillary data such as temperature and pressure are measured using additional equipment brought to the site.  
Figure 10: (a) Landsat 9 OLI image of Ivanpha Playa on 13 November 2021, 18:19 UTC (WRS-2 path 40, row 35); (b) Landsat 9 OLI image of Brookings, South Dakota on 27 June 2022, 17:11 UTC (WRS-2 path 29, row 29). Each true color image was created using OLI bands 4, 3, and 2. The yellow arrows in each image denote the approximate location of the ground sites.  
Following a manual setup procedure that includes alignment with the sun, the ASR tracks the sun throughout the day through a quadrant cell that is coregistered to the optical axis of the radiometer. Measurements are typically made every minute during the surface BRF data collection process, which is described in the next section. The spectral optical depths are determined using a retrieval based on the Langley method [101]. The optical depth results are used in an inversion method to determine the ozone optical depth and an aerosol size distribution parameter [100]. As mentioned, the work presented here assumes that the aerosol size distribution can be defined by a power law. The main advantage of this assumption is that it only requires one value, known as the Angstrom turbidity parameter, to define the aerosol size distribution.  
In the final step of ASR data processing, the optical depths are determined from 350 nm to 2500 nm for use as inputs to the radiative transfer code. The columnar water vapor amount is derived from the ASR data using a modified Langley approach [102]. The Angstrom exponent determined using ASR measurements is also used to compute the Mie scattering phase functions, which are also used in the radiative transfer code.  
SDSU uses ASR measurements in conjunction with Langley analyses to derive instantaneous optical depth values. The initial model is propagated from the sun through the atmosphere to ground level, predicting spectral radiance at the reflectance standard, spectral radiance at the grass target, and the diffuse-to-global irradiance ratio of the sky. The predicted results are compared to the radiance measurements made with the portable spectroradiometer, while the diffuse-to-global ratio is compared to a Yankee Environmental Systems shadow-band radiometer to identify any anomalies in the model predictions. The model is then updated and rerun until a fit to reality is obtained.