Computing the Apparent Elevation of a Near-Earth Spacecraft at Low Elevation Angles for an Arbitrary Refraction Model

This article gives an exact expression for parallax adjustments to apparent elevation of objects at finite distances beyond the Earth atmosphere suitable for use at the very low elevation angles required by the Deep Space Network (DSN) Services Preparation Subsystem (SPS) Metric Prediction Generator (MPG). The parallax depends entirely on the astronomical refraction function and does not require a separate model, as was used in the Network Support Subsystem (NSS) Metric Prediction (MP) application. Rather, the effects of refraction and finite target distance are combined into a single computation. Results are compared with the parallax model used by NSS MP software, where a sizable difference at low elevation angles is noted. The NSS model is known to break down at low elevations, so this behavior was expected. The study also displays the performance of the method using a hybrid Lanyi/Berman–Rockwell refraction model being prepared for MPG use.