Development and Testing of a Multiple Filter Approach for Precise DGPS Positioning and Carrier-Phase Ambiguity Resolution

The most precise relative positioning obtained using differential GPS depends on accurately determining carrier-phase integer ambiguities. To achieve high precision, many current static and kinematic algorithms use a floating-point solution until enough information becomes available to fix the carrier-phase ambiguities accurately. A mew method is presented that uses a multiple model Kalman filter to resolve the carrier-phase integer ambiguities. This method starts with the floating-point results, yet smoothly and rapidly attains the precision of the correct fixed-integer solution, eliminating the need to decide when to switch from the floating to the fixed-integer solution. This method is based on a theoretically correct blending of solutions from multiple filters. This new technique is computationally efficient, providing a robust navigation solution useful in demanding applications such as precision landing and autonomous navigation. The new method was evaluated during static ground and flight tests. Initial results indicate that the new method is capable of quickly resolving the carrier-phase ambiguities and provides a highly accurate (centimeter-level) navigation solution. However, the performance of the new method depends on the correct carrier-phase ambiguity set being hypothesized by one of the multiple filters. Recommendations for further research are included

Multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution

Multi-frequency and multi-GNSS measurements from modernized satellites are properly integrated for PPP with ambiguity resolution to achieve the state-of-the-art fast and accurate positioning, which provides an important contribution to GNSS precise positioning and applications. The multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution, which is accomplished by a unified model based on the uncombined PPP, are thoroughly evaluated with special focus on Galileo and BDS

Enhancement of the accuracy of single epoch positioning for long baselines with application to structure deformation monitoring

Phd ThesisUsing single-epoch GPS positioning has many advantages, especially when monitoring dynamic targets (e.g. structural movements). In this technique, errors occurring in previous epochs cannot affect the current epoch’s accuracy. However, careful processing is required. This research uses the GPS Ambiguity Search Program (GASP) single-epoch software. Resolving the phase ambiguities is essential in this technique. Some statistical ambiguity resolution functions have been introduced to estimate the best values of these ambiguities. The function inputs are the base station position, the approximate roving receiver position, and the shared GPS phase measurements at both receivers. This work investigates different GPS pseudorange solutions to find the optimal ambiguity function inputs. The noise level in an undifferenced pseudorange coordinate solution is less than in the double-differenced case; thus, using it in the ambiguity function improves the results. Regional correlation between the pseudorange-computed positioning errors exists; therefore, applying a regional filter reduces their effects. Multipath errors approximately repeat themselves every sidereal day in the case of static or quasi-static receivers and applying a sidereal filter mitigates their effects. The IGS ionospheric model reduces the effect of the ionosphere on the GPS phase measurements. Also, a local code-based ionospheric correction model can be generated. Applying these models improves the quality of the phase measurements, which leads to improvement of the ambiguity function outputs. A Kalman filter applied to the code-based ionospheric model further improves the corrected phase measurements. There is a correlation between the ambiguity function outputs’ quality and the phase measurement residuals’ . Applying a threshold filter reduces the probability of obtaining inaccurate results. Data for various baseline lengths, with synthetic displacements added, indicate that the improved GASP results are reliable for monitoring movements exceeding 10 cm for baselines up to 60 km.Aleppo University, Syria, Postgraduate Research Studentship

Integrity monitoring scheme for undifferenced and uncombined multi-frequency multi-constellation PPP-RTK

The precise point positioning (PPP)-based real-time-kinematic (RTK) method attracts increasing attention from both academia and industry because of its potential for high accuracy positioning with a shorter convergence time compared to the traditional PPP. Besides high accuracy, integrity monitoring (IM) is indispensable for safety–critical real-time land vehicle and aviation applications. As the traditional advanced receiver autonomous integrity monitoring (ARAIM) method is designed for (smoothed) pseudorange-based positioning, the complexity of multi-frequency multi-constellation PPP-RTK using carrier phase measurements has not been given sufficient consideration. This study proposes an IM scheme for multi-frequency multi-constellation uncombined PPP-RTK applying the ARAIM theory, with a new comprehensive threat model to accommodate not only pseudorange measurements, but also carrier phase measurements, and other fault events arising from the network corrections that support PPP-RTK. Characteristics of different types of faults are analyzed with the aid of numerical experiments. In addition, the impact of ambiguity-fixed solutions on PPP-RTK integrity performance is investigated. The authors have also conducted case studies, including static and real-kinematic positioning experiments. Experiments have demonstrated that fast convergence in accuracy and the position error bounds, or protection levels, with a given integrity risk, in horizontal position components of PPP-RTK could be achieved. For the open sky environments on a highway, the protection levels estimated by PPP-RTK solutions have the potential to meet the alert limit requirement for road transportation using ambiguity-fixed PPP-RTK positioning under the assumption that the risks of wrong ambiguity fixing are very small and can be ignored

Multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution

Multi-frequency and multi-GNSS measurements from modernized satellites are properly integrated for PPP with ambiguity resolution to achieve the state-of-the-art fast and accurate positioning, which provides an important contribution to GNSS precise positioning and applications. The multi-frequency and multi-GNSS PPP phase bias estimation and ambiguity resolution, which is accomplished by a unified model based on the uncombined PPP, are thoroughly evaluated with special focus on Galileo and BDS

Trustworthy precise point positioning with global navigation satellite systems

With the modernization of the Global Navigation Satellite System (GNSS), GNSS precise point positioning (PPP) technology becomes popular benefiting from its wide coverage and high accuracy. However, PPP technology still has many challenges in terms of continuity, fast convergence, and integrity monitoring, and these unsolved issues result in limitations of engineering applications. In this thesis, a reliable PPP technology with GNSS is investigated. The main contributions of the thesis are as follows: (1) A new cycle slip repair method that uses multiple epochs of time-differencing and geometry-based observations are proposed which has a significant improvement in the success rate of cycle slip repairs compared to existing methods. The positioning results also reflect that this method can reduce position errors and improve the continuity of PPP technology. (2) A systematic comparison of current interpolation methods used for high-accuracy regional ionospheric corrections is presented. It is found that each method has essentially the same accuracy in a small regional network with only a few stations, while the Kriging interpolation method can significantly improve the accuracy when the size of the network increases. Besides, a new method for predicting the uncertainty after broadcasting by grid point is also proposed. It has been validated that it is significantly closer to reality than other existing methods. In addition, different ionospheric correction implementation methods at the user end are also compared. (3) A integrity monitoring scheme for use in PPP based on real-time kinematic (RTK) positioning networks (PPP-RTK) with regional atmospheric corrections has been developed, which is based on the impacts of faults on the estimators considering possible faults in undifferenced and uncombined measurements. (4) Procedures for integrity monitoring considering the risks caused by incorrect ambiguity fixing are investigated. Two different methods for considering the probability of wrong ambiguity fixing including categorizing it into unmonitored fault and categorizing it as an individual type of fault are proposed and analyzed. (5) An integrity monitoring (IM) scheme based on the single-epoch framework for PPP-RTK is also proposed in order to exclude the effects caused by using observations from multiple epochs. Different solutions and their related availability are evaluated based on the satellite geometry in the global area

Development and Simulation of a Pseudolite-Based Flight Reference System

Current flight reference systems are vulnerable to GPS jamming and also lack the accuracy required to test new systems. Pseudolites can augment flight reference systems by improving accuracy, especially in the presence of GPS jamming. This thesis evaluates a pseudolite-based flight reference system which applies and adapts carrier-phase differential GPS techniques. The algorithm developed in this thesis utilizes an extended Kalman filter along with carrier-phase ambiguity resolution techniques. A simulation of the pseudolite-based positioning system realistically models measurement noise, multipath, pseudolite position errors, and tropospheric delay. A comparative evaluation of the algorithms performance for single and widelane frequency measurements is conducted in addition to a sensitivity analysis for each measurement error source, in order to determine design tradeoffs. Other analyses included the use of optimal smoothing, non-linear filtering techniques, and code averaging. Specific emphasis is given to two alternate methods, both developed in this research, for handling the residual tropospheric error after applying a standard tropospheric model. Results indicate that the algorithm is capable of accurately resolving the pseudolite carrier-phase ambiguities, and providing a highly accurate (centimeter-level) navigation solution. The filter enhancements, particularly the optimal smoother and tropospheric error reduction methods, improved filter performance significantly

Multi-Antenna Vision-and-Inertial-Aided CDGNSS for Micro Aerial Vehicle Pose Estimation

A system is presented for multi-antenna carrier phase differential GNSS (CDGNSS)-based pose (position and orientation) estimation aided by monocular visual measurements and a smartphone-grade inertial sensor. The system is designed for micro aerial vehicles, but can be applied generally for low-cost, lightweight, high-accuracy, geo-referenced pose estimation. Visual and inertial measurements enable robust operation despite GNSS degradation by constraining uncertainty in the dynamics propagation, which improves fixed-integer CDGNSS availability and reliability in areas with limited sky visibility. No prior work has demonstrated an increased CDGNSS integer fixing rate when incorporating visual measurements with smartphone-grade inertial sensing. A central pose estimation filter receives measurements from separate CDGNSS position and attitude estimators, visual feature measurements based on the ROVIO measurement model, and inertial measurements. The filter's pose estimates are fed back as a prior for CDGNSS integer fixing. A performance analysis under both simulated and real-world GNSS degradation shows that visual measurements greatly increase the availability and accuracy of low-cost inertial-aided CDGNSS pose estimation.Aerospace Engineering and Engineering Mechanic

Modified RTK-GNSS for Challenging Environments

Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS) is currently the premier technique for achieving centimeter-level accuracy quickly and easily. However, the robustness of RTK-GNSS diminishes in challenging environments due to severe multipath effects and a limited number of available GNSS signals. This is a pressing issue, especially for GNSS users in the navigation industry. This paper proposes and evaluates several methodologies designed to overcome these issues by enhancing the availability and reliability of RTK-GNSS solutions in urban environments. Our novel approach involves the integration of conventional methods with a new technique that leverages surplus satellites—those not initially used for positioning—to more reliably detect incorrect fix solutions. We conducted three tests in densely built-up areas within the Tokyo region. The results demonstrate that our approach not only surpasses the fix rate of the latest commercial receivers and a popular open-source RTK-GNSS program but also improves positional reliability to levels comparable to or exceeding those of the aforementioned commercial technology