Development of an Automated Solar-Tracking and Control System for MICROTOPS II Observations and Field-Transfer Validation Against an AERONET CIMEL CE-318 for CAS500-4

Portable sun photometers are useful for atmospheric characterization at satellite calibration and validation (CAL/VAL) sites where permanent refFerence instruments are unavailable, but conventional MICROTOPS II operation requires manual solar pointing and is not well suited to unattended, temporally regular observations. This study developed an automated solar tracking and supervisory control system for MICROTOPS II and evaluated reference-based calibration transfer against a collocated AErosol RObotic NETwork (AERONET) CIMEL CE-318 for CAS500-4. Two Model 540 instruments were operated during three field days at the Jeonju_RDA AERONET site. A custom Python (version 3.10.20) controller automated serial communication, location and time synchronization, fixed interval scanning, data retrieval, calibration constant logging, and local network monitoring. Calibration parameters were estimated from two complete observation days and then frozen before evaluation on the remaining day in a threefold leave-one-day-out (LODO) design. The pooled out-of-fold (OOF) dataset contained 147 pooled OOF matched pairs for each aerosol channel and 147 matched pairs for precipitable water. CE-318-based calibration reduced the RMSE for NASCM540 to 0.0088–0.0188 across 340–870 nm and reduced PKNUM540 RMSE to 0.0117 at 440 nm and 0.0080 at 500 nm; the largest reduction was 89.8% for PKNUM540 500 nm aerosol optical depth (AOD). Residual performance was weakest at 870 nm for PKNUM540 (RMSE = 0.0282, R2 = 0.280). Cross-validated temperature compensation improved 1020 nm AOD estimates (RMSE 0.0234 to 0.0199) and precipitable water estimates (0.1177 to 0.1100 cm) but did not improve AOD estimates at 870 nm. The results demonstrate that automated acquisition and independent-day-wise calibration transfer can substantially reduce systematic error while revealing wavelength specific limitations that must be resolved before broader seasonal or spatial transfer for satellite CAL/VAL.