## 4 Discussion
However, the stability of the band-to-band registration from the whiskbroom design is more challenging to achieve with the modern sensor approach making use of the multiple FPA pushbroom design [70]. Because the detectors for the different bands are separated in the along-track direction, there is a time delay between the different bands as they image the same location on Earth. For OLI 8, this is estimated to be 1.1 s [70], which results in terrain parallax creating challenges in the removal of high altitude cirrus (i.e., band-to-band registration between the cirrus and optical channels) and changes in satellite position and attitude that must be accounted for to establish the line of sight [44; 70; 71; 72; 73; 74; 75]. Despite efforts to standardize the continuity of land surface products between subsequent generations of satellite sensors [76], our results suggest caution in combining data from sensors with greatly different spectral and radiometric properties for peatland biogeochemicalmodeling. There is a further consideration of the sensor design related to the offset between the two rows of FPAs in both OLI 8 and Sentinel-2; one set has a slightly forward view of the Earth while the other set has a slightly rearward view [71; 72; 75]. This is an important consideration for peatlands in the summer, where the vascular plant component has stronger BRDF properties. The difference in the radiance collected by the two sets of FPAs for targets with substantial BRDF properties may be uncorrectable [73], but this effect is mitigated by updated band-specific gain values. The challenge of removing high-altitude cirrus is seen in our results with the Sentinel-2 imagery from 24 May to 19 August. Full removal of high-altitude cirrus from satellite imagery is not a trivial problem [77; 78; 79; 80; 81]. On Landsat 8 OLI, the cirrus band is affected by spectral cross-talk with contamination from the neighboring SWIR band [71] and both Sentinel-2 and Landsat 8 OLI have various magnitudes of band spatial misregistration. This leads to a potential problem for the cirrus removal if the channels have different parallax angles [80; 81]. A further consideration requiring more in-depth analyses is that data from all sensors underwent atmospheric correction through different models. Landsat TM5 Collection 1, Tier 1 imagery was atmospherically corrected using the Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) [82]. The Landsat 8 OLI Collection 1, Tier 1 imagery was atmospherically corrected using the Landsat Surface Reflectance Code (LaSRC) optimized for Landsat 8 [83]. Sentinel-2A was atmospherically corrected with Sen2 cor [45] and the SASI and CASI HSI were atmospherically corrected with ENVI FLAASH 5.4.1 or ATCOR 4.7.0. While there is no comprehensive comparison between these five approaches to atmospheric correction, differences are expected [84]; ATCOR and FLAASH are based on MODTRAN, while LEDAPS and LaRSC implement the MODIS/6S and Vectorial 6S (6 SV) models, respectively. Sen2 Cor implements a custom algorithm (Sentinel-2 Atmospheric Correction--S2 AC) based on the libRadtran4 code [85]. Differences have been shown between the approaches for the two Landsat sensors, where LaSRC processing provides improvements in the surface reflectance over LEDAPS [83]. Similarly, differences have been observed between ATCOR and FLAASH for airborne HSI [86]. However, more extensive comparisons are needed between the HSI and satellite approaches to determine the uncertainty introduced by the different implementations of atmospheric correction and their effect on peatland biogeochemical models. Finally, our results contribute to the growing body of literature illustrating hyperspectral imagery improves the retrieval of fundamental peatland characteristics over multispectral data at all spatial scales. The HSI used here both have a 14-bit dynamic range. From approximately 1000 m AGL flight altitude, neither the CASI nor SASI were close to saturation; over the peatland only approximately one-third of the dynamic range was utilized.