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Atmospheric delays, e.g., ionospheric delay and tropospheric delay, are the dominant error sources for the Global Navigation Satellite System (GNSS), especially for Precise Point Positioning (PPP). The common method for eliminating ionospheric delay is to form an ionosphere-free (IF) observable, which is a linear combination of observables on two frequencies such as GPS L1 and L2. As for the tropospheric delay, the dry component can be precisely corrected by empirical models, while the wet component is usually estimated as unknowns. However, the higher-order ionospheric (HOI) terms are not totally cancelled out in the (first-order) IF observable and as such, when not accounted for, they degrade the accuracy of other parameters. The impact of HOI corrections is well documented in the literature. This paper investigates the temporal effects of HOI terms on estimated tropospheric parameters, i.e., zenith tropospheric wet delay (ZWD) and north and east gradients. For this purpose, observations from over 100 stations with good global coverage were used considering various geographic and geophysical conditions. The results of numerical experiments show that HOI effects have a significant impact on the estimated tropospheric parameters, and the influence is dependent on location and time. The maximum differences of ZWD estimates reach over 20 mm during periods of activity such as solar storms and geomagnetic storms. Additionally, the north gradients are more likely to be affected by HOI effects compared with east gradients. In particular, the tropospheric gradient component is most affected for low latitude station during daytime. Additionally, the effects of bending errors and HOI terms on slant tropospheric delay at low elevation angles are much larger than those at high elevation angles.

The Effects of Higher-Order Ionospheric Terms on GPS Tropospheric Delay and Gradient Estimates