# Landsat Surface Product Validation Instrumentation:
## 1 Big MAC Overview
### Purpose
#### 1.1.2 Measurement Technologies
It is important to note that a true Lambertian material cannot have a reflectance greater than 100%. However, because the SPARC target is specular, it can reflect more light in a given direction than an ideal Lambertian reflector. Thus, it is possible for a SPARC target to produce a LER much greater than 100% by selecting a sufficient mirror RoC. The reader will notice that this will be true for the SPARC reflectors used in this study to maximize the measured signal-to-noise radiance response but still avoid saturation nonlinearity. Two separate iterations of SPARC were used during BigMAC. A number of manually placed target arrays made of multiple mirrors were placed at the primary experiment site; Figure 6. These \"manual\" arrays were individually pointed by hand based on a set observation geometry determined by the solar azimuth angle at the measurement time and the predicted view angle of the sensor under test. Secondly, an automated version of SPARC was utilized. The FLARE represents a commercial, on-demand calibration network [27]. A network node was located in Arlington, SD, 30.8 km WNW from the BigMAC experiment site. This node is designed for GSD up to 60 m, and it was tasked with providing calibration against Planetscope and the assets available during the experiment. During field measurements, an ASD FieldSpec 4 spectrometer (Malvern Panalytical Ltd., Malvern, United Kingdom) with RCR foreoptic was deployed to measure downwelling irradiance over the course of the experiment. Measurements were made for the full sky global irradiance, with discrete shaded measurements separating the global, direct, and diffuse components. The direct solar irradiance was utilized to derive the atmospheric optical transmission and at-aperture radiance for the SPARC manual mirrors and automated FLARE node, while the diffuse-to-global irradiance ratio was utilized in calculating LER (\(\lambda\)) [24; 28]. A common approach in the vicarious calibration of Earth Observing systems is the ELM [22; 28; 29]. Two or more targets of known radiance (or reflectance) are presented to a sensor under test and the sensor's response is assessed. By performing regression against multiple radiance levels, a calibration gain and offset can be produced across the sensor's linear response range. An important consideration in ELM is the RAIFoV of the system under test [30]. The calibration targets must be large enough to contain pixels that are not contaminated by edge or adjacency effects. During BigMAC, multiple reflectance levels of grayscale tarps were used to perform ELM for the high-resolution UAS systems. However, these targets were not large enough to provide any radiometrically accurate pixels for the coarser resolution satellite systems. Leveraging the scalable SPARC methodology, targets of multiple at-aperture radiance or LER levels were used for a MELM regression applicable to UAS and Sentinel-2A MSI imagery [31]  
Arable (San Francisco, CA, USA) makes a low-cost field-deployable radiometer, known as the Arable Mark 2, primarily for agricultural applications. It has a multi-band downwelling and upwelling radiometer with set spectral filters covering the VNIR portion of  
Figure 6: Mirror array used to replicate a bright point target. the solar spectrum. This instrument is meant to be deployed throughout the agricultural growing season, is solar powered, and uploads data automatically via a cellular network interface. Details on the characterization and calibration of these units can be found in [32]. SDSU has used a number of these units over the past two years in an effort to evaluate their stability and accuracy for satellite calibration and validation [33]. Two units were deployed during BigMAC, as shown in Figure 2. The design of the large natural alfalfa target to border the Arabic field of view allowed the efficient collection of data for ASD-based systems and Arables.