Comparative analysis of four different single-frequency PPP models on positioning performance and atmosphere delay retrieval

Single-frequency precise point positioning (SF-PPP) has attracted increasing attention due to its high precision and cost effectiveness. With various strategies to handle the dominant error, i.e., ionosphere delay, the ionosphere-float (IF), ionosphere-free-half (IFH), ionosphere-corrected (IC), and ionosphere-weighted (IW) SF-PPP models are certain to possess different characteristics and performance levels. This study is dedicated to assessing and comparing the four models from model characteristics, positioning performance, and atmosphere delay retrieval. The model comparison shows that IC and IW models are full-rank while IF and IFH models have a rank deficiency of size one that will result in biased estimations, which means the better solvability of IC and IW models. The experiments are carried out based on the 7-day Global Positioning System (GPS) observations collected at 57 global Multi-GNSS Experiment (MGEX) stations and Global Ionosphere Map (GIM) products. The results indicate that the IW model can accelerate SF-PPP convergence and achieve higher positioning accuracy compared to the other three SF-PPP models, especially in kinematic mode. With convergence criteria of 0.25 m in horizontal and 0.5 m in vertical, the east/north/up convergence times of IW model are 0.5/15.0/25.0 min and 0.5/16.0/36.5 min for static and kinematic modes, respectively. The IW model is able to achieve an instantaneous positioning accuracy of 0.28/0.35/0.75 m. In addition, a real kinematic test also demonstrates the best positioning solutions of IW model. Regarding troposphere delay retrieval, the IF, IFH, and IW models obtain a comparable daily accuracy of 3.0 cm on average, while the IC model achieves the worst accuracy of 8.0 cm. For precise ionosphere delay estimation, IW model only needs an average initialization time of 34.3 min, but a longer initialization time of 51.6 min is required for IF model. The daily precision of ionosphere delay estimation for IW model can reach up to 10.8 cm. At the present accuracy of GIM products, it is suggested that the IW model should be adopted for SF-PPP first due to its superior performance in positioning and atmosphere delay retrieval.     

Four-peak longitudinal distribution of the equatorial plasma bubbles observed in the topside ionosphere: Possible troposphere tide influence

In this paper we consider an idea of the troposphere tide influence on the character of the longitudinal variations in the distribution of the equatorial plasma bubbles (EPBs) observed in the topside ionosphere. For this purpose, the obtained EPB longitudinal patterns were compared with the thermosphere and ionosphere characteristics having the prominent “wave-like” longitudinal structures with wave number 4, which are uniquely associated with the influence of the troposphere DE3 tides. The characteristics of the equatorial mass density anomaly (EMA), equatorial ionization anomaly (EIA), zonal wind and pre-reversal E?×?B drift enhancement (PRE) were used for comparison. The equinox seasons during high solar activity were under consideration. It was obtained that the longitudinal patterns of the EMA and zonal wind show the surprising similarity with the EPB distributions (R???0.8, R???0.72). On the other hand, the resemblance with the ionosphere characteristics (EIA, PRE) is rather faint (R???0.37, R???0.12). It was shown that the thermosphere zonal winds are the most possible transfer mediator of the troposphere DE3 tide influence. The most successful moment for the transfer of the troposphere DE3 tide energy takes place in the beginning of the EPB production, namely, during the seed perturbation development.     

A priori zenith wet delays in the analysis of VLBI CONT sessions

Tropospheric delay is one of the major sources of error in VLBI (Very Long Baseline Interferometry) analysis. The principal component of this error can be accurately computed through reliable surface pressure data —hydrostatic delay— yet there is also a small but volatile component —wet delay— which is difficult to be modelled a priori. In VLBI analysis, troposphere delay is typically modelled in the theoretical delays using Zenith Hydrostatic Delays (ZHD) and a dry mapping function. Zenith Wet Delay (ZWD) is not modelled but estimated in the analysis process. This work studies inter alia the impact of including external GNSS estimates to model a priori ZWD in VLBI analysis, as well as other models of a priori ZWD.In a first stage, two different sources of GNSS troposphere products are compared to VLBI troposphere estimates in a period of 5 years. The solution with the best agreement to VLBI results is injected in the VLBI analysis as a priori ZWD value and is compared to other options to model a priori ZWD. The dataset used for this empirical analysis consists of the six CONT campaigns.It has been found that modelling a priori ZWD has no significant impact either on baseline length and coordinates repeatabilities. Nevertheless, modelling a priori ZWD can change the magnitude of the estimated coordinates a few millimeters in the up component with respect to the non-modelling approach. In addition, the influence of a priori ZWD on Earth Orientation Parameters (EOP) and troposphere estimates —Zenith Total Delays (ZTD) and gradients—has also been analysed, resulting in a small but significant impact on both geodetic products.     

中国中部低层大气行星波无线电探空仪的观测研究

通过分析武汉、宜昌和恩施气象局无线电探空仪2001-2003年的观测数据,研究了中国中部地区对流层和低平流层中行星波的特性.通过Lomb-Scargle(L-S)的周期图方法发现了周期为准16天和周期为准10天的谱分量占据着主导地位.观察发现,较大振幅的行星波振荡主要集中在5-15 km之间.准16天行星波沿纬圈向西传播,对应的纬圈波数大约为2,水平波长约为17 324.8 km,传播相速度约为-12.5 m·s-1(东向为正),通过计算准16天行星波在10 km以下相位随高度的改变可以得到其垂直波长大约为25-30 km,而在对波层顶附近其相位几乎没有发生改变,呈现出静态波特性.准10天行星波沿纬圈向东传播,对应的纬圈波数大约为4,水平波长约为8627.3 km,传播相速度约为10.0 m·s-1,垂直波长约为22-40 km.      

Long term changes of altimeter range and geophysical corrections at altimetry calibration sites

Accurate sea level trend determination is fundamentally related to calibration of both the instrument as well as to investigate if there are linear trends in the set of standard geophysical and range corrections applied to the sea level observations. Long term changes in range corrections can leak into the observed sea level record and be interpreted as part of the sea level trend. Particularly if these exhibit anomalous trend close to the satellite calibration sites.     

Progress in tropospheric ammonia retrieval from the MIPAS satellite instrument

The Michelson Interferometer for Passive Atmospheric Sounding is a polar orbiting high resolution mid-infrared emission limb sounder with a nominal vertical resolution of 3 km. Work to extend the list of routinely retrieved species led to the examination of ammonia, NH₃. In this paper, we explore the feasibility of upper tropospheric retrievals around 6-km altitude, discussing appropriate methods. We demonstrate a weak north to south gradient outside the error bars, using co-addition to improve signal-to-noise and compare these first results with a model.     

对流层散射传播机制与特性分析

文章从对流层的一般特性入手,介绍了目前对流层散射传播机制研究的三种主要理论模型,分析了对流层散射传播所具有的一系列突出优点,同时重点讨论了传输损耗特性和衰落特性,并提出了克服其不足的有效措施,最后对未来的研究重点进行了展望。     

Improvement of PPP-inferred tropospheric estimates by integer ambiguity resolution

Integer ambiguity resolution in Precise Point Positioning (PPP) can improve positioning accuracy and reduce convergence time. The decoupled clock model proposed by Collins (2008) has been used to facilitate integer ambiguity resolution in PPP, and research has been conducted to assess the model’s potential to improve positioning accuracy and reduce positioning convergence time. In particular, the biggest benefits have been identified for the positioning solutions within short observation periods such as one hour. However, there is little work reported about the model’s potential to improve the estimation of the tropospheric parameter within short observation periods. This paper investigates the effect of PPP ambiguity resolution on the accuracy of the tropospheric estimates within one hour.     

Combination methods of tropospheric time series

In this article we present two methods for combination of different Global Navigation Satellite Systems (GNSS) Zenith Total Delay (ZTD) time-series for the same GNSS site, but from different producers or different processing setups. One method has been setup at ASI/CGS, the other at KNMI. Using Near Real-Time (NRT) ZTD data covering 1 year from the E-GVAP project, the performance of the two methods is inter-compared and validation is made against a combined ZTD solution from EUREF, based on post-processed ZTDs. Further, validation of the ASI combined solutions is made against independent ZTDs derived from radiosonde, Numerical Weather Prediction (NWP) model and Very Long Baseline Interferometry (VLBI) ZTD.     

Precision analysis of troposphere sensing using GPS single-frequency signals

Various studies have been performed to investigate the accuracy of troposphere zenith wet delays (ZWDs) determined from GPS. Most of these studies use dual-frequency GPS data of large-scale networks with long baselines to determine the absolute ZWDs. For small-scale networks the estimability of the absolute ZWDs deteriorates due to high correlation between the solutions of the ZWDs and satellite-specific parameters as satellite clocks. However, as relative ZWDs (rZWDs) can always be estimated, irrespective of the size of the network, it is of interest to understand how the large-scale network rZWD-performance of dual-frequency GPS using an ionosphere-float model compares to the small-scale network rZWD-performance of single-frequency GPS using an ionosphere-weighted model. In this contribution such an analysis is performed using undifferenced and uncombined network parametrization modelling. In this context we demonstrate the ionosphere weighted constraints, which allows the determination of the rZWDs independent from signals on the second frequency. Based on an analysis of both simulated and real data, it is found that under quiet ionosphere conditions, the accuracy of the single-frequency determined rZWDs in the ionosphere-weighted network is comparable to that of the large-scale dual-frequency network without ionospheric constraints. Making use of the real data from two baselines of 15?days, it was found that the absolute differences of the rZWDs applying the two strategies are within 1?cm in over 90% and 95% of the time for ambiguity-float and -fixed cases, respectively.