Journal of Surveying Engineering

Real-time precise point positioning (PPP) is possible through the availability of the real-time satellite orbit and clock corrections to broadcast ephemeris, the real-time broadcast corrections (BCs). The real-time BCs are currently available in global as well as in regional reference frames. In this contribution, PPP usage and the performance of these global and regional BCs are analyzed for the North American Datum of 1983 (NAD83). The limitations of the current regional BC approach for NAD83 are identified and the coordinate differences it leads to compared with the traditional global BC approach are shown. Although the biases as a result of the different reference frame usage are shown to be subcentimeter, it is also shown how they can be reduced or eliminated by modifying either the PPP algorithm or the regional BC approach. Analyses were performed for three different PPP variants, a single-frequency ionosphere-free variant, a dual-frequency ionosphere-free variant, and a single-frequency ionosphere-corrected variant.

In this paper, the network real-time kinematic (NRTK) positioning method based on undifferenced (UD) observation corrections (abbreviated as URTK) are described, in which instantaneous ambiguity resolution can be carried out within rover’s precise point positioning (PPP) software, thus achieving position solutions equivalent to NRTK. The URTK method transforms the fixed double-differenced (DD) ambiguities of the reference network into UD ambiguities. With the transformed ambiguities, the residuals of the UD observation are used for interpolating the corrections at the user side. This approach breaks down the connections between the stations and satellites of the DD corrections in the current NRTK methods, such that corrections can be broadcast by the base station and automatically selected and optimized by a rover during the real-time kinematic (RTK) processing when the rover is transitioning into a more favorable subnetwork configuration, thus avoiding ambiguity reinitialization as a result of the change of reference. More importantly, this proposed approach provides a seamless integration of regional NRTK (PPP with regional augmentation) and global PPP-assisted kinematic surveying, which should be very useful for a wide range of applications. The key for realizing the improvements over standard NRTK is the development of a logical scheme that has the capability of rapid fixing of the single-differenced ionosphere-free ambiguity of the rover. Case studies at sea and on land demonstrate the advantages of this approach in terms of improvement in positioning accuracy, elimination of subsequent ambiguity reinitialization, and wider applications in the future.

As of this writing, there are two popular methods to perform precise surveying using the single dual-frequency Global Positioning System (GPS) receiver. One is the Real-Time Kinematic (RTK) technique. The other is the Precise Point Positioning (PPP) technique. The RTK technique requires GPS surveyors to have at least one GPS base station while the PPP technique needs a long observation period to resolve the carrier-phase ambiguities. This paper proposes a new, fast method that can conduct precise kinematic surveying using a single dual-frequency GPS receiver, overcoming the aforementioned problems. This new method, Absolute Plus Loop-based Accumulated-solution Time-relative (APLAT), combines the GPS absolute-positioning method and a loop-based accumulated-solution time-relative positioning method. In this APLAT method, the kinematic surveying trajectory must form a loop. The coordinates of the start-point of the loop can be precisely determined using the absolute positioning function of APLAT by making two short sessions of static observations. This function is useful if no control-point is available in the kinematic surveying. The relative positions of other kinematic surveying points in the loop are determined using the function of loop-based accumulated-solution time-relative GPS positioning. The integration of the absolute and the relative positioning functions allows kinematic surveying to precisely determine the absolute coordinates of each surveyed point in the loop. To determine the absolute coordinates of the start-point of a loop, only two short sessions (5 min each in this study) of static GPS observations (prior to starting and after completing the loop surveying) are required. The results of extensive absolute-positioning tests show that 3D-positioning standard deviation about 7 cm and root-mean-square (RMS) error about 13.4 cm can be achieved when the loop-duration is in the range of 40–80 min. Too short or long a loop-duration may degrade the absolute-positioning accuracy. Results from various relative positioning tests using the loop-based accumulated-solution time-relative method indicate that relative-positioning RMS errors of 1.3 cm, 2.6 cm, and 8.8 cm can be obtained for loop durations of 20 min, 40 min, and 60 min, respectively. This new APLAT method offers a new, fast, and precise (RMS error of 12 cm for absolute and RMS error of 8.8-cm relative positioning for 60-min loop duration) kinematic surveying method that is able to meet many GPS surveying and mapping requirements. This is particularly useful for postmission data processing in circumstances where single-base-station RTK or network RTK service is not available. In this study, the final precise-orbit and precise-satellite-clock data from the International GNSS Service are used to mitigate these errors. At present, this method is useful just for postmission applications, but it offers the flexibility of using absolute-positioning or relative-positioning function only, depending upon the specific surveying and mapping conditions and requirements. This method is a good complement to the PPP and RTK precise-positioning techniques currently in use. Due to the increasing spatial decorrelation of error sources, the positioning accuracy may degrade as the maximum spatial distance (approximately between the loop start-point and the loop middle-point) increases.

During attitude determination using the Global Positioning System (GPS), incomplete knowledge of the stochastic model of the observables often occurs in realistic applications, resulting in the failure of the traditional least-squares method for the Global Navigation Satellite System baseline model. This paper presents a single-frequency single-epoch GPS attitude determination method, which is aimed at attitude determination without the prior knowledge of measurement precision. The new approach is a combination of an analytical resolution and the constrained integer least-squares theory. By using the global minimizer of the fixed ambiguity objective function, our method identifies the most likely solution from all possible analytical candidates. Simulation experiments demonstrate the performance of the proposed algorithm for different noise levels and different baseline lengths. To determine the applicable scope of this scheme, the success rate gap of the least-squares method is also analyzed. Actual experiments also verify the effectiveness of the proposed method.

Precise point positioning (PPP) involves observations from a single global navigation satellite system (GNSS) receiver and benefits of satellite orbit and clock products obtained from the global infrastructure of permanent stations. PPP avoids the expense and logistic difficulties of deploying a network of GNSS receivers around survey areas in isolated places, such as the arctic or less populated areas. Potential accuracies are at the centimeter level for static applications and at the subdecimeter level for kinematic applications. Static and kinematic PPP based on the processing of global positioning system (GPS) observations is limited by the number of visible satellites, which is often insufficient for urban or mountain applications, or it can be partially obstructed or present multipath effects. Even if a number of GPS satellites are available, the accuracy and reliability can still be affected by poor satellite geometry. One possible way of increasing satellite signal availability and positioning reliability is to integrate GPS and GLONASS observations. This case study deals with the possibilities of combining GPS and GLONASS dual-frequency measurements on the static and kinematic PPP solution to reduce the convergence time and improve the accuracy of the solution. The results show that the addition of the GLONASS constellation does not always improve the convergence of static PPP; the kinematic results (car and walk trajectories) present better accuracy from the GPS + GLONASS solution rather than the GPS-only solution. The MagicGNSS software was used in processing of all observations.

The least-squares (LS) method is highly susceptible to outlying observations. For this reason, various types of robust estimators have been developed; for example, $M$ estimators. In this paper, it is proposed to use the sign-constrained robust LS (SRLS) method in surveying networks utilizing the shuffled frog-leaping algorithm (SFLA). The robustness of SRLS is directly implemented as constraints. Therefore, a penalty function approach is used to deal with the constraints. In addition, the performance of any stochastic optimization approach such as SFLA strongly depends on the search domain. Hence, a strategy to define the boundaries of the search domain has been developed for use in surveying networks. The results indicate that SRLS yields better results than the LS method even if there are more outliers among the observations.