# On the Potential of the RST-FLARE Algorithm for Gas Flaring Characterization from Space
## 2 Data and Methods
### Rst-Flare
#### 2.3.1 Flaring Sites Detection  
The difference between the BTs acquired in the MODIS MIR and TIR channels (i.e., bands 20 and 31) is investigated for describing the target behaviour. In detail, following RST prescriptions, the following ALICE\({}_{20-31}\) index (Equation (1)) is implemented at the pixel level [20]:  
\[\otimes_{20-31}(x,y,t)=\frac{BT_{20-31}(x,y,t)-\mu_{20-31}(x,y)}{\sigma_{20-31 }(x,y)}, \tag{1}\]  
where \(\mu_{20-31}(x,y)\) represents the value expected to measure for the signal under investigation (i.e., BT\({}_{20-31}\)) in unperturbed (or normal) conditions while \(\sigma_{20-31}(x,y)\) is its natural variability, for the pixel located at (\(x,y\)) and acquired at time \(t\). Both \(\mu_{20-31}(x,y)\) and \(\sigma_{20-31}(x,y)\) (namely the reference fields) are computed, for each location (\(x,y\)), processing several years of historical satellite records acquired in homogeneous spatiotemporal conditions (e.g., same geographic area, same month and hour of acquisitions) of the image at hand. As assessed by an independent work focused on RST [40], such satellite historical series should consist of about 80 imagery to guarantee representative background signal. Such a number, considering a monthly temporal window and a sensor offering a daily temporal resolution, roughly corresponds to 3-year long time series, representing indeed the minimum time span for a reliable RST implementation. Longer time series will obviously allow to better facing cloud and satellite geometry view issues The robustness of the ALICE index is intrinsic, because any signal fluctuation due to natural and observational (known and unknown) sources produces an increase in the standard deviation reference field (i.e., \(\sigma_{20-31}(x,y)\)), with a consequent reduction in the probability to detect an anomaly. Among these possible sources, the signal variability due to satellite viewing angles or atmospheric effects are also included, with the standard deviation accounting for these and protecting from possible false positives. According to RST configuration, gas flares are thermal features more difficult to discriminate, being generally persistent both in space and time. As investigated in [20], with reference to such targets, the ALICE index is expected to discriminate, among the measured radiant records, the most highly ones, associable to the major fluctuations in the combustion operations and/or to gas flaring in emergency conditions (extremely high and intense burst of flames are generally associated with these anomalous situations). Ref. [20] found that ALICE\({}_{20-31}\) values \(\geq\) 1.5 can be assumed as representative of thermal anomalies related to gas flaring, representing a significant signal deviation from what observed in normal operational conditions of the plant and likely related to unexpected events at the system, like accidents, malfunctions or extremely high oil production rates. When the position of flaring sites is not known, a time persistency criterion has to be applied to the ALICE\({}_{20-31}\)\(\geq\) 1.5 to flag a pixel as flaring site, for discriminating such hot objects from other high thermally emitting sources, such as wildfires, which may also affect the Nigerian soils [41]. In particular, considering, for each year, the localization of the identified thermal anomalies, their percentage of recurrence within the reference annual dataset (i.e., only cloud-free pixels acquired with SatZA \(<\) 40\({}^{\circ}\) and SolZA \(<\) 101\({}^{\circ}\)) was computed. The algorithm classifies a pixel as potentially flaring-affected by applying a threshold to the annual recurrence percentage of the previously detected anomalies. Such a value is derived by a series of tests based on the comparison between potential persistent source maps, obtained at different recurrence percentage values and a reference map (i.e., Google Earth). The flaring site maps obtained using a percentage ranging from 15% to 35%, with a step of 5%, were analysed in comparison with the above mentioned reference.