## 3 Remote Sensing Image Sources  
Regarding remote sensing, most published papers use satellites as image sources. In recent years, the use of automatic unmanned vehicles (UAVs) has increased. Although UAVs offer higher spatial resolution and much higher flexibility in terms of temporal resolution [11], their low spectral resolution, different regulations restricting the ground sample distance (GSD), and the cost of UAVs are slowing down their application in remote sensing. In [11], the authors also pointed out the limitations of UAVs in terms of meteorological factors (wind and rain), since similar meteorological conditions might limit the use of remote sensing (clouds); however, we consider this to not make a big difference. In fact, in regions characterized by high cloud coverage with low rain, the UAVs might be helpful for gathering information.  
Concerning the remote sensing images obtained from satellite sources, there is huge variability in their characteristics according to the selected satellite. A recent survey analyzed the satellite sources for remote sensing applications in coastal and marine areas [12]. According to their results, the most used ones are the Landsat, followed by MODIS, Sentinel 1, and SPOT 5. Nonetheless, the studied field affects the highly used satellite classification. Landsat, MODIS, and Sentinel 2 are the most used in grassland monitoring, according to [13]. Furthermore, the studied area also affects the selected satellites. According to [14], a review of remote sensing use carried out in China that includes several monitored elements indicated that the most used satellite is Landsat, followed by MODIS, but other satellites not even included in previous surveys appear in a relatively high position, such as ASTER and Gaofen. Some satellites offer an extraordinary historical record since they provide years of uninterrupted global data, such as MODIS and Landsat [15].  
To provide more accurate data about the most used satellites, a search on [1] included the words remote sensing + environmental monitoring and the three most used satellites according to [13]. We selected both Landsat 8 and Sentinel since they provide similar data in terms of spatial resolution (with the resolution of Sentinel 2 being 10 m, which is better than Landsat 8, 30 m) and quite identical spectral resolution (11 bands for Landsat 8 and 12 bands for Sentinel 2). The temporal resolution is better for Sentinel 2 (the revisit time is equal to 8 days for Landsat 16 and only 2.3 days for Sentinel 2). Landsat 8 and Sentinel 2 are recent satellites launched in 2013 and 2015, respectively. In the period from 2002 to July 2022, we found 1320 results (published papers on [1]) which included Landsat. For the same period, the number of papers using Sentinel was 1335. The numbers in terms of usage are very similar. Nonetheless, the differences are more evident if we focus the search on the last three years, from January 2020 to July 2022. We found a total of 406 papers for Landsat 8 and 490 for Sentinel 2. If we pay attention to the topics linked to the papers using Landsat 8, Sentinel 2 appears in the first position. Nevertheless, for the results related to Sentinel 2 images, Landsat 8 appears as the fifth topic. This might suggest that several researchers using Landsat images might be using Sentinel 2 to combine or compare information, but it is not evident when Sentinel 2 images are being used. Another search was performed for the papers focused on MODIS. The total number of publications from 2002 to July 2022 was 1680, which is considerably more than for Sentinel 2 or Landsat 8. This can be explained due to MODIS being launched earlier. If we focus on the period January 2020 to July 2022, the number of papers using MODIS was 399, which is notably lower than for Landsat 8 or Sentinel 2. Thus, we can conclude that the ranking of remote sensing satellites providing multispectral images in terms of usage since 2020 has Sentinel 2 as the most used, followed by Landsat 8 and MODIS.