## 2 Materials and Methods
### Sensor Data Collection and Pre-Processing  
L8, S2A and ASTER data from the years 2015 through 2018 were collected and processed for the current study (Figure 2). These years represent the time period (ongoing) where data are available for all three sensors. Landsat 8 (L8) data were collected from the NASA Earth Observing System (EOS) [61], for the years 2015 through 2018. L8 was initially known as the Landsat Data Continuity Mission and was launched on 11 February 2013. Similar to the previous generations of Landsat, L8 has a 16 day repeat cycle. The L8 OLI sensor captures nine spectral bands, including a panchromatic (PAN) band. The visible, near infrared (VNIR) and shortwave infrared (SWIR) bands are produced with a 30 m spatial resolution while the PAN band is produced at 15 m spatial resolution. The L8 swath width is 185 km. We downloaded L8 images covering the Paths 19-20, Row 39. While Path 20, Row-39 covers 100% of our study area, Path 19, Row-39 covers almost 50% of the eastern side of the study area. The reason for keeping two different combinations of Paths and Rows is to accumulate more dates in the temporal domain for training the RF. A sharp edge is observed in the L8 image for the dates where only one pass (Path 19, Row-39) covering approximately half of the study area was available (Figure 3). Figure 1: Location map of research setting in Apalachicola Bay, FL. Figure 2: Image pre-processing workflow. \(\text{T}_{\text{n}}\) represents a given satellite image acquisition date. We pre-processed the L8 data for years 2015 through 2018 using the analytics tool provided by the EOS cloud-based platform [61]. This service removes the need for the user to download and store the data prior to pre-processing locally, making this part of the workflow more reproducible, efficient, and less error-prone. The L8 data were subjected to the EOS imagery pre-processing pipeline which includes radiometric calibration of digital numbers into at-sensor radiance, raster filters to remove noise, reprojection to a common projection system (UTM zone 16N, WGS 1984), cloud detection and masking. NDVI was computed using EOS with band 4 (Red) and band 5 (NIR) reflectance as input and the resulting NDVI image output was in geotiff format [61]. EOS employs the canonical NDVI formula [62], expressed mathematically as:  
\[\mathrm{NDVI}=\frac{\mathrm{NIR-RED}}{\mathrm{NIR+RED}} \tag{1}\]  
Sentinel-2A (S2A) data were also collected from EOS. S2A was launched as part of the European Space Agency Copernicus program on 23 June 2015 [61]. S2A has 13 spectral channels including three VNIR bands with 10 m spatial resolution, two NIR bands with 10 and 20 m spatial resolutions. S2A has the widest swath width of the three sensors in the proposed virtual constellation at 290 km. Similar to L8, S2A data were collected and pre-processed using EOS for the years 2015 through 2018. Band 8A (NIR) and band 4 (Red) reflectance values were used for S2A NDVI calculation. All available S2A images from tiles T16 RFT, T16 RGU, T16 RGT, T16 RFU, and T16 RGT were used to produce NDVI images. These 5 tiles combined to cover the full study area, but not all tiles had images available on all desired dates. Similar to L8, a sharp edge was observed in the S2A image for those dates where less than five of the mentioned tiles are available. Additionally, S2A data tiles contain some overlap for images acquired on the same date. Image reprojection and cubic convolution resampling was done using ArcGIS to calculate the pixel value in the overlapped portion of the S2A input image [63] (Figure 3). ASTER data were acquired through the United States Geological Survey (USGS) Earth Explorer via the LP-DAAC site (NASA Land Processes Distributed Active Archive Center n.d.). ASTER is a joint program between NASA and Japan's Ministry of Economy, Trade and Industry (METI). ASTER global observation data have been publicly available since the year 2000.