Abstract
Atmospheric delays are known to cause biases in Global Navigation Satellite System Reflectometry (GNSS-R) altimetry applications, such as for sea-level monitoring. The main quantity of interest is the reflection-minus-direct or interferometric atmospheric delay. Recently, we have presented a rigorous raytracing procedure to account for linear and angular refraction in conjunction with reflection as observed from near-surface platforms. Here, we demonstrate the feasibility of simplifying the ray trajectory by imposing a rectilinear wave propagation model. Two variants were assessed, based on the apparent or refracted satellite direction on the one hand and the geometric or vacuum conditions on the other hand. The former was shown to agree with rigorous results in terms of interferometric radio length while the latter agreed in terms of the interferometric vacuum distance. Upon a judicious combination of the best aspects of the two rectilinear cases, we have defined a mixed variant with excellent agreement with rigorous raytracing in terms of interferometric atmospheric delay. We further showed that mapping functions developed for GNSS positioning cannot be reused for GNSS-R purposes without adaptations. Otherwise, the total atmospheric delay may be underestimated by up to 50% at low elevation angles. The present work facilitates the adaptation of existing atmospheric raytracing software for GNSS-R purposes.
Highlights
Global Navigation Satellite System Reflectometry (GNSS-R) (Cardellach et al 2011; Jin et al 2014; Zavorotny et al 2014) has been widely demonstrated for long-term ground-based coastal sea level altimetry (Larson et al 2013; 2017)
Atmospheric refraction is known to cause a propagation delay which produces a bias in GNSS-R altimetry, depending on the satellite elevation angle and the reflector height
We show that the large-scale atmospheric geometric delay can be well captured by a judicious choice of rectilinear raypaths
Summary
Global Navigation Satellite System Reflectometry (GNSS-R) (Cardellach et al 2011; Jin et al 2014; Zavorotny et al 2014) has been widely demonstrated for long-term ground-based coastal sea level altimetry (Larson et al 2013; 2017). Atmospheric refraction is known to cause a propagation delay which produces a bias in GNSS-R altimetry, depending on the satellite elevation angle and the reflector height. Nikolaidou et al Earth, Planets and Space (2020) 72:91 other effects, the interferometric propagation delay can be expressed as τi = 2H sin e where e is the satellite elevation angle and H is the reflector height, i.e., the vertical distance between the receiver and the reflecting surface.
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