> Metrology > Instrumentation > arghXiv:2602.06104
Calibration of a Differential Refraction Comparator for Contested Horizon Observations
View PDF: Calibration of a Differential Refraction Comparator for Contested Horizon ObservationsAbstract: Observations taken along a sightline near the horizon confound two quantities: the geometric obscuration due to the figure of the Earth, and the refractive lift due to the vertical temperature gradient along the path. Where an observation is disputed, it is usually disputed on exactly this point, and neither party is in a position to separate the terms. We describe a differential comparator that measures the two simultaneously against a reference target of known height at known range, and report its calibration against 60 paired observations over water. Residual uncertainty in the separated refractive term is 0.6 arcseconds, which is sufficient to decide any observation of the class in question. The instrument was applied to one such observation in October 2025. The result is reported in Section 5. The observation remains contested.