## 4 Discussion
### Is It Possible to Get an Early Information on Glyphosate Application by Satellite Data?  
Satellite sensors are characterized by their spatial, spectral and temporal resolutions. These properties, among other criteria, significantly determine whether they are suitable to support certain questions or not. The results of this preliminary study showed that, in terms of spectral resolution, hyperspectral satellite sensors may in principle monitor glyphosate application on agricultural crops. However, satellite-based hyperspectral remote sensing is still a matter of research. At present, three missions are in orbit. The Italian PRISMA mission was successfully launched in March 2019 [44] rotating around the earth in a sun-synchronous orbit at approximately 614 km altitude. Two other hyperspectral missions are currently operating onboard the International Space Station (ISS). That is the German Aerospace Center (DLR) Earth Sensing Imaging Spectrometer DESIS [45; 46], which was installed on ISS in August 2018 and the Japanese HISUI mission, which was deployed onboard ISS in December 2019 [47]. The PRISMA and HISUI hyperspectral instruments cover the full spectral range from the VIS to the SWIR (400 nm to 2500 nm) with 237 and 185 spectral bands respectively. DESIS is limited from 400 nm to 1000 nm equipped with 235 spectral bands. According to the results of our laboratory experiment, NDLI proved to be most suitable for early detection of glyphosate application. While this index will be supported by PRISMA and HISUI data, it will not be possible to derive the NDLI from DESIS data. CRI-1 and PRI, which proved best after NDLI, are ultra-narrow-band indices. They may be adequately resolved by DESIS with its very narrow spectral sampling capabilities (2.55 nm) but not by PRISMA and HISUI with a spectral resolution of about 12 nm. Despite this, the temporal resolution of the PRISMA satellite with a repeat cycle of 29 days is not appropriate for glyphosate monitoring. The sensors on-board acquire images on a more frequent basis but standardized radiometric and geometric correction of the data is still a difficult task to do for the future.  
Due to the high application potential of hyperspectral satellite imagery, several other missions are planned for the future. One of them is the German EnMAP mission [48] which is scheduled for launch in 2020. This mission meets the required spectral resolution and is of great interest as the pointing feature allows a target revisit within 4 days (depending on latitude). Because EnMAP is a research mission, just as the above mentioned, the satellite is not meant to deliver data on a regular basis nationwide. This is different from the future Copernicus hyperspectral mission CHIME [49]. It shall provide data of the same place on earth every 10 to 15 days. This is, however, not applicable for a satellite-based glyphosate monitoring. Moreover, one has to keep in mind that frequent cloud coverage will limit data usability in the mid-latitudes as with any other optical satellite mission and hamper a monitoring of agricultural fields.  
In terms of temporal resolution and coverage, the commercial PlanetScope multiple satellite constellation by Planet Labs, Inc., may be an interesting undertaking. The complete constellation has more than 120 active satellites in orbit and is able to image the entire land surface of the earth every day with a spatial resolution of 3 m. Although spectral resolution is limited, the suitability of the multispectral imagery for monitoring glyphosate application is still to be examined.  
Finally, it should be noted that spatial resolution of the aforementioned systems is approximately 30 m due to the trade-off between spatial and spectral resolution in remote sensing. Thus, hyperspectral satellites are generally not suitable for a monitoring in small-structured agricultural landscapes.