Nadir vs. Off-nadir: Initial Look at LAPAN-A3 Off-nadir Acquisition Mode on its Spectral Quality

The maneuverability of a satellite to perform nadir and off-nadir offers an excellent opportunity for gathering valuable information in the time of disaster events. This paper aims to compare how LAPAN-A3 spectral quality changed with different acquisition modes. Three scenes of LAPAN-A3 with different acquisition modes ranging from nadir to extreme off-nadir mode were analyzed. Using Sentinel-2A Multispectral Instrument data as the reference, geometric co-registration was performed, and the Root Mean Square Error was kept at maximum (<2.5 pixels). Pseudo-invariant features were selected semi-automatically using threshold values based on the Normalized Difference Vegetation Index and Normalized Difference Water Index. The digital number produced then compared to Sentinel-2A data. Pearson correlation coefficient, as well as linear regression, was calculated for four corresponding bands. Generally, the results show a decreased correlation value between the corresponding datasets as the roll-angle increased. The correlation value went down from 0.623 (Off-Nadir) to 0.497 (Extreme off-nadir) on Near-infrared band.