# The Ground-Based Absolute Radiometric Calibration of the Landsat 9 Operational Land Imager
## 2 Methodology
### Automated Measurements: The Radiometric Calibration Test Site (RadCaTS)
#### 2.2.3 Development of Custom Ground-Viewing Radiometers for RadCaTS  
In 2002, the RadCaTS concept started with a one-channel proof-of-concept radiometer that used a light-emitting diode (LED) as a detector. The decision to use an LED as a detector was influenced by the successful Global Learning and Observations to Benefit the Environment (GLOBE) program, which promotes the collection and analysis of environmental data as well as the collaboration of students around the world [71; 72; 73; 74]. The preliminary RadCaTS radiometer was based on the GLOBE sun photometer, and prototypes were constructed, characterized, and field-tested using various VNIR LEDs. Modest improvements to the radiometer design over the next few years included the addition of two more channels in the VNIR region, as well as powered optics to better control the field of view [75]. The proof-of-concept instruments allowed the data processing architecture to be developed and tested, and it was determined that to be a truly operational and well-calibrated radiometric resource for the Earth observation community, new radiometers had to be developed.  
In 2012, the prototype three-channel VNIR GVRs were replaced with a new version developed by UArizona to transition to a truly operational and autonomous test site. Improvements include a temperature-controlled focal plane, interference filters for spectral selection, and a solar power supply for remote operation. The eight spectral channels of the GVRs used at the RadCaTS are nominally centered at 400 nm, 450 nm, 500 nm, 550 nm, 650 nm, 850 nm, 1000 nm, and 1550 nm, and they have 20 nm bandwidths (full width at half maximum, FWHM). The focal plane of each GVR is mounted 1.5 m above the ground, which corresponds to a spot-size diameter on the ground of 27 cm, as defined by the 10\({}^{\circ}\) full field of view for each channel. The GVRs were designed, developed, tested, and characterized at UArizona specifically for the spectral and spatial characteristics of the surface at Railroad Valley.  
In the original operational phase in 2012, there were three GVRs deployed to the RadCaTS, and in 2014, a fourth one was deployed. The development of more GVRs continued throughout the years, along with the addition of a satellite communication network that allows data to be uploaded daily to UArizona for processing. There are currently seven GVRs at the RadCaTS in a nadir-viewing configuration. In the past, two GVRs were mounted in the viewing configuration of the GOES-East and GOES-West geostationary satellites for work with the Advanced Baseline Imager (ABI), but they were reverted to nadir-viewing in 2020 [45; 46]. The positions and number of the GVRs required to obtain the same uncertainty in the surface bidirectional reflectance factor (BRF) as the reflectance-based approach were assessed using high-resolution satellite imagery [33; 75; 76; 77]. Examples of the current GVRs at the RadCaTS are shown in Figure 2, which also shows a recent prototype design that uses a new mount for the radiometer head. The new mount allows for 90 cm of linear motion, which increases the spatial sampling. The current positions of the seven GVRs are shown in Figure 3.  
Figure 2: (a) An example of two GVRs at the RadCaTS. Currently, six of the seven have a static mount. (b) A GVR with a linear translation mount that provides 90 cm of motion.