Modeling and correction for the optical axis pointing error of an airborne electro-optical platform.

Optical axis pointing accuracy is an important index of airborne electro-optical (EO) platforms. In this work, we aim to correct the optical axis pointing angle of an airborne EO platform by digital compensation. First, a basic parameter model (BPM) of pointing error with clear physical significance is established by analyzing the physical structure and error source of the EO platform. Then, to suppress the nonlinear factors in the error, we propose an improved algorithm of a semi-parametric regression model based on the BPM. Numerical simulation analysis shows that the improved algorithm inherits the advantages of the BPM, such as fewer model parameters and clear physical significance, and can improve the correction effect. Finally, experimental results show that the mean square error of the azimuth angle is reduced from more than 110'' to less than 4'', and that of the elevation angle is reduced from more than 75'' to less than 3''. According to the results obtained, the proposed correction model can improve the optical axis pointing accuracy of an airborne EO platform quickly and effectively, which has significant application value.

Aseismic slip and recent ruptures of persistent asperities along the Alaska-Aleutian subduction zone.

The frictional properties and slip behaviors of subduction thrusts play a key role in seismic and tsunami hazard assessment, especially in weakly coupled "seismic gaps". Here, we rely on GPS observations in the Shumagin Gap of the Aleutian subduction zone to derive the slip distribution of the 2020 Mw 7.8 Simeonof Island, Alaska earthquake and of the subsequent afterslip during the first 87-day period. Our modeling results show that the mainshock ruptured at depths of ∼30-40 km beneath Simeonof Island. Kinematic and stress-driven models indicate that the afterslip occurred both updip and downdip of the mainshock rupture. Physically plausible locking models derived from interseismic GPS velocities suggest that the 2020 Simeonof and 2021 Mw 8.2 Chignik earthquakes ruptured persistent asperities on the subduction thrust. We infer that there are several additional persistent asperities at depths of 20-50 km west ∼157°W. However, it is still uncertain whether there are additional locked asperities at shallow depths because of the current lack of geodetic observations close to the trench.