Journal of Surveying Engineering

The paper presents the application possibilities of the new method of determining the curvature of the axis of the railway track, called “the method of the moving chord.” Based on the estimation of the existing curvature, it becomes possible to identify the geometric layouts of the track in the horizontal plane. As shown in previous publications, for model layouts, the method of the moving chord shows full compliance of the obtained curvature diagrams constituting the basis for obtaining the corresponding geometric solution. Now, the question of its use for the estimation of the horizontal curvature of the axis of the operated railway track, based on Cartesian coordinates obtained by direct measurements, has been taken up. Measurement data from two geometric layouts of the track with clearly different values of the radius of the horizontal arc and the nature of the curvature present were used. In both cases, the obtained curvature diagrams are less regular and oscillatory in nature, which does not prevent the basic geometrical parameters of the measured layout from being estimated on their basis. The presented adaptation of the moving chord method to the adopted measurement procedure and the practical use of the obtained curvature diagram provide the method in question with appropriate implementation bases. The application of the presented procedure should significantly improve the process of identifying geometric track layouts.

Harris-Galveston Subsidence District (HGSD), in collaboration with several other agencies, has been operating a dense Global Navigation Satellite System (GNSS) network for subsidence and faulting monitoring within the Greater Houston region since the early 1990s. The GNSS network is designated HoustonNet, comprising approximately 250 permanent GNSS stations as of 2021. This paper documents the methods used to produce position time series, transform coordinates from the global to regional reference frames, identify outliers and steps, analyze seasonal movements, and estimate site velocities and uncertainties. The GNSS positioning methods presented in this paper achieve 2–4-mm RMS accuracy for daily positions in the north–south and east–west directions and 5–8-mm accuracy in the vertical direction within the Greater Houston region. Five-year or longer continuous observations are able to achieve submillimeter-per-year uncertainties (95% confidence interval) for both horizontal and vertical site velocities. Two decades of GNSS observations indicate that Katy in Fort Bend County, Jersey Village in northwestern Harris County, and The Woodlands in southern Montgomery County have been the areas most affected by subsidence ($1–2\mathrm{cm}/\mathrm{year}$) since the 2000s; the overall subsidence rate and the size of subsiding area ($>5\mathrm{mm}/\mathrm{year}$) have been decreasing as a result of the groundwater regulations enforced by HGSD and other local agencies. HoustonNet data and products are released to the public through HGSD. The primary products are the daily East-North-Up (ENU) position time series and site velocities with respect to the International GNSS Service (IGS) Reference Frame 2014 (IGS14), the stable Gulf of Mexico Reference Frame (GOM20), and the stable Houston Reference Frame (Houston20). The ENU position time series with respect to Houston20 are recommended for delineating subsidence and faulting within the Greater Houston region. The ENU time series with respect to GOM20 are recommended for studying subsidence and faulting within the Gulf coastal plain and sea-level changes along the Gulf Coast. The entire HoustonNet data set is reprocessed every a few years with updated positioning software, IGS and regional reference frames, and data analysis tools. We recommend that users use the most recent release of HoustonNet data products and avoid mixing old and new positions.

The precise point positioning technique (PPP), an absolute positioning method, is widely used in geodetic point positioning. This study investigates the accuracy of the technique through statistical comparisons of the root mean square errors (RMSE), which are calculated with coordinates obtained from online PPP services. For this purpose, we carried out a two-stage investigation. First, we chose two days, a high geomagnetic activity day and a quiet one. During the high geomagnetic activity day, the Kp index went above 7, and the Dst was below $-170\mathrm{nt}$. We uncovered the effect of geomagnetic activity by using the Automatic Precise Positioning Service (APPS), Canadian Spatial Reference System Precise Point Positioning (CSRS-PPP), and magic Global Navigation Satellite System (magicGNSS) PPP services. The results show that the three-dimensional (3D) RMSE are $\sim 1.5$ times higher during the high geomagnetic activity day during 1-, 2-, 4-, and 6-h session durations, than in the quiet day. However, when the session duration increases to 24 h, the effects of geomagnetic activities are significantly eliminated. In the second stage, we chose 31 consecutive days without significant geomagnetic activities and obtained PPP-derived coordinates for 1-, 2-, 4-, 6-, and 24-h from APPS, CSRS-PPP, magicGNSS, and Trimble real-time extended (RTX). We compared the PPP-derived coordinates with Australian Online GPS Processing Service (AUSPOS)- and Online Positioning User Service (OPUS)-derived reference coordinates. As a result, we determined that, as the session duration increases, the 3D RMSE decreases, and therefore the position accuracy increases. In terms of 3D RMSE, CSRS-PPP yielded the best results in all scenarios. Finally, we concluded that 3D RMSE decreased by approximately 55.8%, 18.1%, 7.7%, and 6.2%, when session durations increased from 1 h to 2 h, 2 to 4 h, 4 to 6 h, and 6 to 24 h, respectively.

To aid astronomical research in the identification and positioning of random stars in unfamiliar environments, this paper proposes a random star identification algorithm that does not require approximate station position. Rather, an observer calculates the angular distance between three selected stars at a unified time and forms an observation triangle using the angular distance difference method to match the observation triangle to a navigation triangle in a database of navigation stars. The detailed identification process is given in the article, and the distribution of the selected stars in the experiment is depicted intuitively. The measured data show that the success rate of random star recognition was as high as 100% when the observation interval between stars did not exceed 3 min. The article introduces the experiment and calculation of astronomical positioning by the identified stars, which realizes the identification and positioning of random stars without requiring approximate station position. We used 12, 8, or 4 stars with good spatial geometry as a positioning star group and analyzed the internal accord accuracy (Std) and external accord accuracy (Rms) of the positioning results of different stars. Multiple sets of experimental data show that 4 random stars with good spatial geometric distribution allowed for solving identification and positioning tasks with high efficiency and precision. Finally, four main sources of positioning error were analyzed: instrument angle measurement error, time error, star centroid extraction error, and positioning model error. The identification method proposed in this paper does not rely on station location information and has application value for rapid astronomical measurements in harsh environments, such as cloudy weather and cities with bright lights at night. In addition, the method can theoretically be used anywhere in the world.

Track irregularities generate train vibrations. The precise adjustment of a track after locking a long rail and dispersing its stress during railway operation and maintenance are crucial to ensure high-speed, stable, and safe train operation. Currently, the main method for rail adjustment utilizes the supporting software of a trolley to formulate an adjustment solution under the manual mode. Depending on the requirements of track irregularities, the deviations are modified in real time, and track irregularities are fed back by postadjustment deviations. Because the manual mode of adjustment is considerably influenced by human factors, the process is inefficient, time-consuming, and laborious. Accordingly, this paper proposed an iterative optimization adjustment method for ballastless track irregularity of railway (IOABLT). IOABLT utilizes feedback of the irregularity of adjusted rail alignments (obtained by the simplex method) of all segment units. This enables the method to modify the constraint boundary values and iteratively solve the corresponding adjustment model of the track. Then, the minimum and second minimum data sets of track irregularities are derived, and the total optimal solution of the adjustment model is determined based on the difference in the deviations of adjacent points and the change rate of the difference. The experimental results showed that this proposed method can automatically and efficiently achieve optimized rail adjustment under complex constraints. Further, it eliminates the influence of human subjective factors in formulating the precise track adjustment solution and attains optimal track regularity within the allowable adjustment range of the fastening system.

Satellite positioning methods [global navigation satellite system (GNSS) positioning] are used for various applications, which is especially true in the case of the real-time kinematic (RTK) method. This method provides real-time differential data for surveyors to achieve reliable positioning accuracy. In this paper, theoretical foundations for testing the GNSS receivers according to ISO 17123-8 standard are introduced. Using this standard, the positioning performance of a geodetic GNSS receiver Trimble R8s was evaluated using Bosnian Active GNSS Network Bosnia and Herzegovina positioning service (BIHPOS) and a conventional base-rover RTK solution. The main purpose of this study is to evaluate the relationship between the ranging pole support and the quality of the RTK position solution. Besides that, results from RTK positioning using a single base station versus a network of base stations are also compared. Several comprehensive tests were conducted, which involved the most common field procedures where the pole is hand-held by the surveyor and positioned vertically using a circular bubble, whereas in other experiments, a bipod was attached to the range pole, and the GNSS instrument was mounted directly to a tripod via optical plummet-equipped tribrach. The precision, absolute accuracy, and relative accuracy of positioning solutions obtained from these experiments were compared and discussed. The results indicate that regardless of how the RTK range pole was supported, the effect of rover pole holding is within the error of satellite surveys. Furthermore, it was confirmed that using a bipod or a steady hand can produce as accurate RTK results as a tripod.