Single-frequency Precise Point Positioning Research Articles

The GFZ real-time GNSS precise positioning service system and its adaption for COMPASS

Motivated by the IGS real-time Pilot Project, GFZ has been developing its own real-time precise positioning service for various applications. An operational system at GFZ is now broadcasting real-time orbits, clocks, global ionospheric model, uncalibrated phase delays and regional atmospheric corrections for standard PPP, PPP with ambiguity fixing, single-frequency PPP and regional augmented PPP. To avoid developing various algorithms for different applications, we proposed a uniform algorithm and implemented it into our real-time software. In the new processing scheme, we employed un-differenced raw observations with atmospheric delays as parameters, which are properly constrained by real-time derived global ionospheric model or regional atmospheric corrections and by the empirical characteristics of the atmospheric delay variation in time and space. The positioning performance in terms of convergence time and ambiguity fixing depends mainly on the quality of the received atmospheric information and the spatial and temporal constraints. The un-differenced raw observation model can not only integrate PPP and NRTK into a seamless positioning service, but also syncretize these two techniques into a unique model and algorithm. Furthermore, it is suitable for both dual-frequency and sing-frequency receivers. Based on the real-time data streams from IGS, EUREF and SAPOS reference networks, we can provide services of global precise point positioning (PPP) with 5–10cm accuracy, PPP with ambiguity-fixing of 2–5cm accuracy, PPP using single-frequency receiver with accuracy of better than 50cm and PPP with regional augmentation for instantaneous ambiguity resolution of 1–3cm accuracy. We adapted the system for current COMPASS to provide PPP service. COMPASS observations from a regional network of nine stations are used for precise orbit determination and clock estimation in simulated real-time mode, the orbit and clock products are applied for real-time precise point positioning. The simulated real-time PPP service confirms that real-time positioning services of accuracy at dm-level and even cm-level is achievable with COMPASS only.

An improved approach to model ionospheric delays for single-frequency Precise Point Positioning

PPP with low-cost, single-frequency receivers has been receiving increasing interest in recent years because of its large amount of possible users. One crucial issue in single-frequency PPP is the mitigation of ionospheric delays which cannot be removed by combining observations on different frequencies. For this purpose, several approaches have been developed, such as, the approach using ionospheric model corrections with proper weight, the GRAPHIC (Group and Phase Ionosphere Calibration) approach, and the method to model ionospheric delays over a station with a low polynomial or stochastic process. From our investigation on the stochastic characteristics of the ionospheric delay over a station, it cannot be precisely represented by either a deterministic model in the form of a low-order polynomial or a stochastic process for each satellite, because of its strong irregular spatial and temporal variations. Therefore, a novel approach is developed accordingly in which the deterministic representation is further refined by a stochastic process for each satellite with an empirical model for its power density. Furthermore, ionospheric delay corrections from a constructed model using GNSS data are also included as pseudo-observations for a better solution. A large data set collected from about 200 IGS stations over one month in 2010 is processed with the new approach and several commonly adopted approaches for validation. The results show its significant improvements in terms of positioning accuracy and convergence time with a negligible extra processing time, which is also demonstrated by data collected with a low-cost, handheld, single-frequency receiver.

Real-time single-frequency precise point positioning: accuracy assessment

The performance of real-time single-frequency precise point positioning is demonstrated in terms of position accuracy. This precise point positioning technique relies on predicted satellite orbits, predicted global ionospheric maps, and in particular on real-time satellite clock estimates. Results are presented using solely measurements from a user receiver on the L1-frequency (C1 and L1), for almost 3 months of data. The empirical standard deviations of the position errors in North and East directions are about 0.15 m, and in Up direction about 0.30 m. The 95% errors are about 0.30 m in the horizontal directions, and 0.65 m in the vertical. In addition, single-frequency results of six receivers located around the world are presented. This research reveals the current ultimate real-time single-frequency positioning performance. To put these results into perspective, a case study is performed, using a moderately priced receiver with a simple patch antenna.

Single Frequency Ionosphere-free Precise Point Positioning: A Cross-correlation Problem?

Single Frequency Ionosphere-free Precise Point Positioning: A Cross-correlation Problem?This research investigates the feasibility of applying the code and the ionosphere-free code and phase delay observables for single frequency Precise Point Positioning (PPP) processing. Two observation models were studied: the single frequency ionosphere-free code and phase delay, termed the quasi-phase observable, and the code and quasi-phase combination. When implementing the code and quasi-phase combination, the cross-correlation between the observables must be considered. However, the development of an appropriate weight matrix, which can adequately describe the noise characteristics of the single frequency code and quasi-phase observations, is not a trivial task. The noise in the code measurements is highly dependent on the effects of the ionosphere; while the quasi-phase measurements are basically free from the effects of the ionospheric error. Therefore, it is of interest to investigate whether the correlation between the two measurements can be neglected when the code measurements were re-introduced to constrain the initial parameters estimation and thereby improving the phase ambiguities initialization process. It is revealed that the assumed uncorrelated code and quasi-phase combination provided comparable if not better positioning precision than the quasi-phase measurement alone. The level of improvement in the estimated positions is between 1 - 18 cm RMS.

High accuracy precise point positioning using a single frequency GPS receiver

One of the dominant error sources for single frequency GPS users is the ionosphere. The magnitude of this error can vary from a few meters to hundreds of meters, especially during severe ionospheric disturbance periods. For undifferenced processing using a single frequency single receiver unit, the ionospheric error cannot be cancelled out using a differential process. Therefore, one needs to rely on using appropriate models or algorithms to correct this delay to achieve high accuracy point positioning. This paper reports on a study undertaken to evaluate two different ionospheric error mitigation methods for single frequency Precise Point Positioning (PPP) processing. The two methods studied were ionospheric modeling and ionosphere-free combination. The feasibility of using either of these methods was assessed using four processing options: code-only processing; code and carrier phase combination; single frequency ionosphere-free linear combination employing an additive combination of code and carrier phase measurements, which was abbreviated as the quasi-phase measurements; and the code and quasi-phase combination

Ionospheric Effect Mitigation for Real-Time Single-Frequency Precise Point Positioning

ABSTRACT: Precise Point Positioning (PPP) has received increased interest within the GPS community for a number of reasons: simplified field operation, cost-effectiveness, and no base station requirement. PPP with dual-frequency receivers and precise GPS products has demonstrated centimeter to decimeter positioning accuracy. Using real-time precise GPS products, this accuracy is also obtainable in a real-time mode. This paper investigates PPP using single-frequency GPS data which will be of interest to a broad range of applications because the majority of GPS users are using single-frequency GPS receivers. Since the ionospheric effect will be the biggest error source for single-frequency receiver based precise point positioning, different methods for ionospheric effect mitigation will be assessed and compared so that sub-meter positioning accuracy becomes obtainable.

An Evaluation of Various Ionospheric Error Mitigation Methods used in Single Frequency PPP

Precise Point Positioning (PPP) using dual frequency GPS receivers is capable of providing centimetre level point positioning accuracy anywhere around the world, without the need for a base station. However, when using single frequency GPS receivers, the accuracy of the positioning decreases, particularly in the height component. One main factor for this degradation in accuracy is the unmodeled ionospheric error. This paper investigates the performance of three different ionospheric error mitigation methods used in single frequency PPP in the Australian Region. They are the GRAPHIC (GRoup And PHase Ionospheric Correction) algorithm, the Global Ionospheric Maps (GIMs) and the Klobuchar model. Numerical results show that the GRAPHIC and GIMs methods are able to provide point positioning accuracy better than 1m for session duration less than an hour using geodetic quality single frequency receivers. For 12 to 24 hours data sets, the positioning accuracy can be as good as <0.1m.

Hospital volume specialization, guidelines and outcomes in cancer treatment: importance in quality of cancer care

Motivated by the IGS real-time Pilot Project, GFZ has been developing its own real-time precise positioning service for various applications. An operational system at GFZ is now broadcasting real-time orbits, clocks, global ionospheric model, uncalibrated phase delays and regional atmospheric corrections for standard PPP, PPP with ambiguity fixing, single-frequency PPP and regional augmented PPP. To avoid developing various algorithms for different applications, we proposed a uniform algorithm and implemented it into our real-time software. In the new processing scheme, we employed un-differenced raw observations with atmospheric delays as parameters, which are properly constrained by real-time derived global ionospheric model or regional atmospheric corrections and by the empirical characteristics of the atmospheric delay variation in time and space. The positioning performance in terms of convergence time and ambiguity fixing depends mainly on the quality of the received atmospheric information and the spatial and temporal constraints. The un-differenced raw observation model can not only integrate PPP and NRTK into a seamless positioning service, but also syncretize these two techniques into a unique model and algorithm. Furthermore, it is suitable for both dual-frequency and sing-frequency receivers. Based on the real-time data streams from IGS, EUREF and SAPOS reference networks, we can provide services of global precise point positioning (PPP) with 5–10 cm accuracy, PPP with ambiguity-fixing of 2–5 cm accuracy, PPP using single-frequency receiver with accuracy of better than 50 cm and PPP with regional augmentation for instantaneous ambiguity resolution of 1–3 cm accuracy. We adapted the system for current COMPASS to provide PPP service. COMPASS observations from a regional network of nine stations are used for precise orbit determination and clock estimation in simulated real-time mode, the orbit and clock products are applied for real-time precise point positioning. The simulated real-time PPP service confirms that real-time positioning services of accuracy at dm-level and even cm-level is achievable with COMPASS only.