Performance of Open Source Precise Point Positioning Software Using Single-Frequency GPS Data
This research aims to assess the performance of GPS Precise Point Positioning (PPP) with code and carrier phase observations from L1 signal collected from geodetic GPS receiver around the world. A simple PPP software developed for processing the single frequency GPS data is used as a main tool to assess a positioning accuracy. The precise orbit and precise satellite clock corrections were introduced into the software to reduce the orbit and satellite clock errors, while ionosphere-free code and phase observations were constructed to mitigate the ionospheric delay. The remaining errors (i.e. receiver clock error, ambiguity term) are estimated using Extended Kalman Filter technique. The data retrieved from 5 IGS stations located in different countries were used in this study. In addition, three different periods of data were downloaded for each station. The obtained data were then cut into 5-min, 10-min, 15-min and 30-min data segments, and each data segment was individually processed with the developed PPP software to produce final coordinates. Results indicate that the use of 5-min data span can provide a horizontal positioning accuracy at the same level as a pseudorange-based differential GPS technique. Furthermore, results confirm effects of station location and seasonal variation on obtainable accuracies.
Optimization of Satellite Combination in Kinematic Positioning Mode with the Aid of Genetic Algorithm
The basis of high precision relative positioning is the use of carrier phase measurements. Data differencing techniques are one of the keys to achieving high precision positioning results as they can significantly reduce a variety of errors or biases in the observations and models. Since GPS observations are usually contaminated by many errors such as the atmospheric biases, the receiver clock bias, the satellite clock bias, and so on, it is impossible to model all systematic errors in the functional model. Although the data differencing techniques are widely used for constructing the functional model, some un-modeled systematic biases still remain in the GPS observations following such differencing. Another key to achieving high precision positioning results is to fix the initial carrier phase ambiguities to their theoretical integer values. To obtain a high percentage of successful ambiguity-fixed rates, noisy GPS satellites have to be identified and removed from the data processing step. This paper introduces a new method using genetic algorithm (GA) to optimize the best combination of GPS satellites which yields the highest number of successful ambiguity-fixed solutions in kinematic positioning mode. The results indicate that the use of GA can produce higher number of ambiguity-fixed solutions than the standard data processing technique.
The VRS network-based technique has become the main precise GNSS surveying method especially for medium-range baselines (approximately 20-70 km). The key concept of this approach is to use the observables of multiple reference stations to generate the network correction in the form of a virtual reference station for mitigating distance-dependent errors including atmospheric effects and orbital uncertainty at the user’s location. Numerous GNSS data processing strategies have been adopted in the functional model in order to improve both the positioning accuracy and the success of ambiguity resolution. However, it is impossible to completely model the aforementioned errors. As a result, the unmodelled residuals still remain in the virtual reference station observables when the least squares estimation is employed. An alternative approach to deal with these residuals is to construct a more realistic stochastic model whereby the variance-covariance matrix is assumed to be homoscedastic. This research aims to investigate a suitable stochastic model used for the VRS technique. The rigorous statistical method, MINQUE has been applied to estimate the variance-covariance matrix of the double-difference observables for a virtual reference station to rover baseline determination. The findings of the comparison to the equal-weight model and the satellite elevation-based model indicated that the MINQUE procedure could enhance the positioning accuracy. In addition, the reliability of ambiguity resolution is also improved.
The key concept of the virtual reference station (VRS) network-based technique is to use the observables of multiple reference stations to generate the network corrections in the form of a virtual reference station at a nearby user’s location. Regarding the expected positioning accuracy, the novice GNSS data processing strategies have been adopted in the server-side functional model for mitigating distance-dependent errors including atmospheric effects and orbital uncertainty in order to generate high-quality virtual reference stations. In addition, the realistic stochastic model also plays an important role to take account of the unmodelled error in the rover-side processing. The results of our previous study revealed that the minimum norm quadratic unbiased estimation (MINQUE) stochastic model procedure can improve baseline component accuracy and integer ambiguity reliability, however, it requires adequate epoch length in a solution to calculate the elements of the variance-covariance matrix. As a result, it may not be suitable for urban environment where the satellite signal interruptions take place frequently, therefore, the ambiguity resolution needs to be resolved within the limited epochs. In order to address this limitation, this study proposed the stochastic model based on using the residual interpolation uncertainty (RIU) as the weighting schemes. This indicator reflects the quality of network corrections for any satellite pair at a specific rover position and can be calculated on the epoch-by-epoch basis. The comparison results with the standard stochastic model indicated that the RIU-weight model produced slightly better positioning accuracy but increased significant level of the ambiguity resolution successful rate.
Each GNSS constellation operates its own system times; namely, GPS system time (GPST), GLONASS system time (GLONASST), BeiDou system time (BDT) and Galileo system time (GST). They could be traced back to Coordinated Universal Time (UTC) scale and are aligned to GPST. This paper estimates the receiver clock offsets to three timescales: GPST, GLONASST and BDT. The two measurement scenarios use two identical multi-GNSS geodetic receivers connected to the same geodetic antenna through a splitter. One receiver is driven by its internal oscillators and another receiver is connected to the external frequency oscillators, caesium frequency standard, kept as the Thailand standard time scale at the National Institute of Metrology (Thailand) called UTC(NIMT). The three weeks data are observed at 30 seconds sample rate. The receiver clock offsets with respected to the three system time are estimated and analysed through the geodetic technique of static Precise Point Positioning (PPP) using a data processing software developed by Wuhan University - Positioning And Navigation Data Analyst (PANDA) software. The estimated receiver clock offsets are around 32, 33 and 18 nanoseconds from GPST, GLONASST and BDT respectively. This experiment is initially stated that each timescale is inter-operated with GPST and further measurements on receiver internal delay has to be determined for clock comparisons especially the high accuracy clock at timing laboratories.
Random and systematic errors affect navigation satellite observations on both pseudo-range and carrier phase. These errors are originated at satellites, propagation path and receivers. This study focuses on the GNSS receiver internal delay determination resulting from the receiver’s electronics circuit. The characterisation of the delay in the GNSS geodetic receivers is essential to enhance the accuracy for the time transfer and time comparisons as part of GNSS integrity chain determinations. The purpose of GNSS receiver internal delay at the National Institute of Metrology (Thailand), NIMT, is to estimate the characteristics and performances of the GNSS geodetic receiver used for international time comparisons. The experiments are simultaneously observed GNSS satellites by a GPS and a GNSS receivers and two separate antennas with short baseline (around 6 metres), where both receivers are connected to the identical external caesium frequency standard maintained as time and frequency standard of Thailand. The GPS receiver is well-defined for its receiver internal delay on the pseudo-range observation of C1, through comparisons using an internationally recognised method, while the geodetic GNSS receiver is to be defined on its receiver internal delay. These experiment observations started from 26 December 2017 to 17 January 2018 at NIMT, Pathumthani, Thailand. The determined unknowns are receiver position, receiver clock offset, tropospheric delay through the geodetic technique of static Precise Point Positioning observations with Bernese GNSS software version 5.2. Later the receiver internal delay of NIMT is computed and analysed. The receiver internal delay on GPS C1 code is successfully characterised, resulted as 346.0 nanoseconds as of this experiment.
Nowadays, the use of multi-Global Navigation Satellite System (GNSS) has improved positioning accuracy in autonomous driving, navigation and tracking systems utilized by general users. However, signal quality in urban areas is degraded by poor satellite geometry and severe multipath errors, which may disturb up to a hundred-meter-ranging error as a consequence. In this study, the performance of several satellite selection methods in multipath mitigation was evaluated, based on the concept that better quality signals and more accurate solutions will be obtained, the more multipath signals can be excluded. Three methods were performed and compared: 1) azimuth-dependent elevation mask based on fisheye image technique, 2) receiver autonomous integrity monitoring (RAIM), and 3) signal-to-noise ratio (SNR) mask in the SPP method. To examine the effect of the satellite selection methods on multipath error, the static test (single-point positioning (SPP) in real-time 1 Hz test) was performed in a multipath environment. The preliminary results showed a possible impact on improving the horizontal positioning accuracy of SPP. Among the three techniques assessed in this study, the results indicated that the SNR mask set at 36 dB-Hz in every elevation showed the most promising result. The SNR mask method could improve positioning accuracy by up to 46.80% compared to the SPP method.
Since its introduction in 1990s, the GPS Precise Point Positioning (PPP) technique has been widely used for many high precision positioning applications such as the study of tectonic plate motion, establishment of national and regional reference frames and so on. Among the GPS PPP software packages, the GIPSY-OASIS II software package is the one of the most popular software package used by many research institutes worldwide. The processing of GPS data with the GIPSY-OASIS II software requires three main steps. The first step is to compute a daily GPS solution for each station and the second step is to combine daily GPS solutions into a multi-day averaged solution. The final step is to transform these multi-day averaged solutions into the International Terrestrial Reference Frame (ITRF) coordinate solution and this step generally requires the use of available International GNSS service (IGS) stations to compute the required transformation parameters. In order to obtain high precision ITRF coordinate solutions, an investigation on a selection of IGS stations used for aligning the multi-day averaged solution into ITRF is therefore needed. This study aims to investigate the effect of number of IGS stations used for aligning the multi-day averaged solutions into the final ITRF coordinate solution in Thai region. Data from two different GPS campaigns (with epochs before and after the 2004 Sumatra- Andaman earthquake) measured by the Royal Thai Survey Department (RTSD) were used in this investigation. By varying the number of IGS station used in the alignment step, results indicate that the use of at least 16 IGS stations in the alignment process can produce reliable and accurate ITRF solutions especially those impacted by the large earthquake.