Abstract
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.
Highlights
Global Navigation Satellite System (GNSS) signals are delayed and refracted by the neutral atmosphere and ionosphere individually when propagating through the atmosphere, phenomena that are usually referred to as the “tropospheric delay” and “ionospheric delay.” The former is one of the major error sources in GPS positioning, which contributes a bias of several decimeters in humid regions in the vertical direction even when recorded meteorological data are simultaneously exploited in tropospheric models [1,2]
Considering that an higher-order ionospheric (HOI) term is a function of total electron content (TEC) and geomagnetic field, three experiments were designed to investigate the relationship between HOI terms and tropospheric parameter estimation in different situations
The first two experiments showed the impact of HOI effects on zenith tropospheric wet delay (ZWD) estimation in different TEC and geomagnetic conditions, using both daytime and night-time data, as well as data collected from active and quiet ionospheric periods
Summary
Global Navigation Satellite System (GNSS) signals are delayed and refracted by the neutral atmosphere and ionosphere individually when propagating through the atmosphere, phenomena that are usually referred to as the “tropospheric delay” and “ionospheric delay.” The former is one of the major error sources in GPS positioning, which contributes a bias of several decimeters in humid regions in the vertical direction even when recorded meteorological data are simultaneously exploited in tropospheric models [1,2]. Global Navigation Satellite System (GNSS) signals are delayed and refracted by the neutral atmosphere and ionosphere individually when propagating through the atmosphere, phenomena that are usually referred to as the “tropospheric delay” and “ionospheric delay.”. The former is one of the major error sources in GPS positioning, which contributes a bias of several decimeters in humid regions in the vertical direction even when recorded meteorological data are simultaneously exploited in tropospheric models [1,2]. For GNSS applications which demand high accuracy, such as precise point positioning (PPP) and real time kinematic positioning (RTK), especially during the peaks of the solar cycle, higher-order ionospheric (HOI) terms, i.e., the second- and third-order delays, need to be considered, as they can cause range errors of a few centimeters to tens of centimeters [8]
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