Relative Azimuthal-Angle Matching (RAM): A Screening Method for GEO-LEO Reflectance Comparison in Middle Latitude Forests

Abstract

This study introduced a data screening method for comparing the reflectances in middle latitude forest regions collected by a Geostationary Earth Observing (GEO) satellite and a Low Earth Orbit (LEO) satellite. This method attempts to reduce the differences between the relative azimuth angles of the GEO and LEO observations. The method, called relative azimuthal-angle matching (RAM), takes advantage of the high temporal resolution of the GEO satellites, which enables collection of a wide range of relative azimuth angles within a day. The performance of the RAM method was evaluated using data in the visible and near-infrared bands collected by the Himawari-8/Advanced Himawari Imager (AHI) and the Terra/Moderate Resolution Imaging Spectroradiometer (MODIS). The consistency of the reflectance pairs of MODIS and AHI selected by the RAM method was compared with the consistency of the reflectance pairs acquired simultaneously by the two sensors. The data were matched pixel-by-pixel after applying atmospheric corrections and cloud screening. The results show that RAM improved the reflectance ratio by approximately 10% for the red and NIR bands on average relative to the simultaneous observations. Significant improvements in the two bands were observed (20%), especially among data collected in the fall and winter. Performance of RAM depends largely on season. Especially in summer, the reflectance pair chosen by RAM showed less consistency than solar zenith-angle matching (SZM). The results also indicated the relatively large influence of the spectral response functions on the green and red bands of the two sensors.

Seasonal Comparisons of Himawari-8 AHI and MODIS Vegetation Indices over Latitudinal Australian Grassland Sites

“…As the AHI phase angles were much lower than MODIS, particularly during the most active grass growing and peak greenness seasons, AHI observations were acquired under stronger backscatter orientations relative to MODIS during the same periods. The backscatter orientation of AHI had a negative effect on NDVI values and a positive effect on EVI and EVI2 values, in agreement with a previous study [12]. Similar results were found in all four grassland areas (Table 3), in which MODIS NDVI values were consistently higher than AHI NDVI, and MODIS EVI and EVI2 values were consistently lower than AHI EVI and EVI2, respectively.…”

“…Whereas the temporal resolution of MODIS is 1-2 days, the Advanced Himawari Imager (AHI) on board the Himawari-8 can provide data at 10-minute intervals [1,5,6]. GEO satellite sensors with subhourly image capture intervals enable greater opportunities to acquire cloud-free observations [7][8][9] and expand upon LEO-based land applications [10][11][12][13][14][15][16].…”

Improved Characterisation of Vegetation and Land Surface Seasonal Dynamics in Central Japan with Himawari-8 Hypertemporal Data

“…AHI data from the early morning or late afternoon are acquired at larger SZA than those from polar-orbiting satellites at mid-latitudes. Likewise, the SZA diurnal variation of AHI data could be larger than their seasonal counterpart with their relative azimuth angles (RAA) very different from those encountered by polar-orbiting satellites 43 . Examining the NDVI variations with respect to these SZA and RAA variations is important to improve our understanding of the AHI NDVI behaviors.…”

Sun-Angle Effects on Remote-Sensing Phenology Observed and Modelled Using Himawari-8

“…Another potential pathway, although it has yet to be tested, is the combined use of measurements taken by two or more GEO platforms located at different longitudes which gives at least two view angles for any given location, or even combining the measurements from GEO and LEO platforms. Of course, the second pathway demands many efforts in inter-calibrating measurements taken by multiple sensors [90], which is currently a rapidly developing field [91][92][93].…”

“…Conclusions drawn from these studies can be used to make decisions whether constant SZA or constant RAA is to be targeted (as the two cannot be fixed in the same time). A conceptually similar approach was proposed recently to match GEO (H-8 AHI) and LEO (Terra MODIS) based on the criteria of equal SZA or equal RAA [93]. Second, from a modelling point of view, the inversion of more complicated 3-D radiative-transfer models may become feasible with the combined use of high-resolution spaceborne light-detection and ranging (LiDAR, e.g., those from NASA's Global Ecosystem Dynamics Investigation, or GEDI mission), multi-spectral / multi-angle measurements, from which a more reliable surface reflectance correction for BRDF effect can be achieved.…”