## 2 Materials and Methods
### Representing SIF Retrieval Methods  
The FIREFLY spectral range of 670-780 nm was designed to span the chlorophyll fluorescence emission spectrum (Figure 1), which encompasses both the oxygen A (\(\sim\)759-771 nm) and oxygen B (\(\sim\)688 nm) telluric atmospheric absorption features, and many solar Fraunhofer absorption features. This spectral range includes regions previously exploited for SIF retrievals, such as the \"split window\" either side of the oxygen A absorption feature (756-759 nm and 770.5-774.5 nm) [10]. As such, FIREFLY has access to many SIF retrieval approaches that differ in both their targeted spectral regions, and their fundamental mechanics. Since each SIF retrieval method considered will heavily contextualize the results, this study attempts to provide as wide a perspective as possible by utilizing two archetypes of SIF retrieval. Together, these two SIF retrieval archetypes approximate the mechanics of most major SIF retrieval methods, while remaining explicit in their propagation of uncertainty as deeply into their implementation as possible. The first of these archetypes is the \"direct\" retrieval, representing those methods that seek to remove non-SIF components of a signal, retrieving the residual of this process as the SIF signal. The second archetype is the \"indirect\" retrieval, representing methods that \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Attribute** & **Value** \\ \hline \multirow{3}{*}{Spectral} & Wavelength range (nm) & 670 to 780 \\  & Bands & 2160 \\  & Band interval & 0.051 \\ \hline \multirow{4}{*}{Spatial} & G-LiHT flight altitude (m) & 335 \\  & Spatial pixels (after \& binning) & 266 \\  & Field of view (degrees) & 23.5 \\  & Instantaneous field of view (degrees) & 0.142 \\  & Swath width (m) & 140 \\ \hline \multirow{2}{*}{Sensor} & Type & TE-cooled sCMOS \\  & Dynamic range (bits) & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: FIREFLY Attributes. use baseline non-SIF relationships between spectral bands or regions to infer the presence and quantity of an additive SIF signal in collected observations. Among contemporary methods that would be categorized here as \"direct\", some strip away non-SIF components of the signal explicitly, such as retrievals from inside the oxygen features that estimate and remove attributes of solar irradiance, target reflectivity, and atmospheric effects. To be used operationally, the approach of retrieving SIF directly from oxygen features relies on an estimate of the non-SIF component of the radiance at the sensor, which could in turn involve estimates of some combination of the following, depending on the coincident data available: top-of-atmosphere irradiance, atmosphere from top to target, downwelling irradiance, target reflectivity, and atmosphere from target to sensor. While the potential to retrieve these components is discussed in Section 4.1.2, quantitative analyses of the uncertainty in this SIF retrieval method are focused on the expected contributions of noise, and other instrument sources. Other direct methods remove non-SIF components implicitly, typically by identifying them as systematic variation via principal component analysis (PCA) or singular vector decomposition (SVD). Others still, including the prominent spectral fitting methods, use a combination of explicit knowledge of instrument and target function shapes, and the implicit structural properties of the signal, to derive a residual SIF function. Compared to the maturity and complexity of these approaches, the representations of direct retrievals in this study are simple, assuming that each band is used as an individual sample, with SIF and potentially confounding influences applied to it. While treating bands as independent samples may fall short of the potential performance of spectral fitting methods in fighting noise and removing systematic variation, this approach is employed here to provide transparent propagation of uncertainty, enabling independent evaluation of each potential source of uncertainty in SIF retrievals.