Ahmed El-Mowafy

Analysis of web-based GNSS post-processing services for static and kinematic positioning using short data spans

Abstract In this paper the use of some of the currently available web-based online engines for the processing of static and kinematic data from a single dual-frequency receiver is evaluated. The study focuses on using these processing services for the determination of the coordinates of temporary control stations from a few hours of observations in the static mode, and in positioning in the kinematic mode, such as in hydrographic mapping, checking as-built utilities and Geographic Information Systems (GIS) data collection. The different services are briefly presented and compared. The static testing was performed using the AUSPOS and CSRS-PPP services. The results show that precision at mm to cm level can be achieved from the former and within a decimetre from the latter. Kinematic testing using the CSRS-PPP service was performed. Its positioning results were compared with independent differential processing of the same data. The impact of varying the data length on the achieved precision is investigated and the resulting positioning error levels are quantified. In general, the precision varies from a few cm to a few decimetres, and it generally improves with longer observation periods. The CSRS-PPP smoothing performance is also evaluated when breaks of satellite visibility take place and compared with the case of good visibility. Some recommendations are given to surveyors interested in using this type of processing service.

On biases in precise point positioning with multi-constellation and multi-frequency GNSS data

Various types of biases in Global Navigation Satellite System (GNSS) data preclude integer ambiguity fixing and degrade solution accuracy when not being corrected during precise point positioning (PPP). In this contribution, these biases are first reviewed, including satellite and receiver hardware biases, differential code biases, differential phase biases, initial fractional phase biases, inter-system receiver time biases, and system time scale offset. PPP models that take account of these biases are presented for two cases using ionosphere-free observations. The first case is when using primary signals that are used to generate precise orbits and clock corrections. The second case applies when using additional signals to the primary ones. In both cases, measurements from single and multiple constellations are addressed. It is suggested that the satellite-related code biases be handled as calibrated quantities that are obtained from multi-GNSS experiment products and the fractional phase cycle biases obtained from a network to allow for integer ambiguity fixing. Some receiver-related biases are removed using between-satellite single differencing, whereas other receiver biases such as inter-system biases are lumped with differential code and phase biases and need to be estimated. The testing results show that the treatment of biases significantly improves solution convergence in the float ambiguity PPP mode, and leads to ambiguity-fixed PPP within a few minutes with a small improvement in solution precision.

Estimation of multi-constellation GNSS observation stochastic properties using single receiver single satellite data validation method

Abstract The single receiver single satellite validation method is a technique that screens data from each satellite independently to detect and identify faulty observations. A new method for estimation of the stochastic properties of multi-constellation GNSS observation is presented utilising parameters of this validation method. Agreement of the characteristics of the validation statistics with theory is used as the criterion to select the best precision of the observations, spectral density and correlation time of the unknowns. A curve fitting approach in an iterative scheme is employed. The method is applicable to any GNSS with any arbitrary number of frequencies. Demonstration of the method results and performance is given using multiple-frequency data from GPS, GLONASS and Galileo in static and kinematic modes.

Using Peer Assessment of Fieldwork to Enhance Students' Practical Training.

Fieldwork training is a key component of several practical disciplines. In this study, students’ peer assessment of fieldwork is explored as a method to improve their practical training. Peer assessment theories are first discussed. A framework for peer assessment of fieldwork is proposed, and the steps taken for preparation of students for this task are discussed. A developed marking, feedback and moderation tool of assessment are presented. Application of peer assessment in the field was investigated over a period of two years in one undergraduate unit in the geospatial discipline as an example. Reliability of peer assessment was estimated by measuring the difference between assessments carried out by groups of peer assessors, and its validity was measured by comparing students’ marks with those given by tutors. Results show that students have gained from the peer assessment process, mainly as a formative form of assessment, by better understanding and endeavouring to achieve the objectives of field tasks. Tutors use differences among assessments made by groups of students compared to tutors’ assessments to identify field components that need better explanation of their content and assessment criteria.

Machine automation using RTK GPS positioning

Machine automation is a challenging application that demands real-time positioning at the centimeter-level accuracy for precise jobs in the open fields. This positioning accuracy can be obtained by using a Global Positioning System (GPS) and employing a single reference station or through utilizing the service of a network of multiple reference stations. The paper first addresses the principles of the RTK positioning from a single reference station and next from a network. The potential of integrating the RTK positioning system and GIS to increase efficiency for machine automation in are explored. Such integration is proposed for implementation in a Supervisory Control and Data Acquisition (SCADA) system. The functionality and process automation of the proposed integrated system are discussed. The logical sequence of the soft PLC process program is illustrated. Two scenarios are presented for field machine automation to precisely operate according to previously preset plans. In addition, an efficient architecture of the integrated system is proposed. To evaluate the obtainable positioning precision when the system is mounted on machines, the RTK system was tested first when using a single reference station and next when using a RTK network. Results show that the system was successful in almost all time to achieve the target positioning precision at the cm level.

Triple-frequency GNSS models for PPP with float ambiguity estimation: performance comparison using GPS

This contribution proposes two new precise point positioning (PPP) models that use triple-frequency data, designed to accelerate convergence of carrier-phase float ambiguities. The first model uses a triple-frequency ionosphere-free linear combination that has minimum noise propagation and geometry-preserving properties. The second model uses a mixed code and carrier-phase linear combination with the same properties. A third model was also implemented, which uses individual uncombined triple-frequency measurements. The three models were validated using triple-frequency GPS data and their performance was compared to the traditional dual-frequency model in terms of the convergence time taken to achieve and maintain a uniform three-dimensional accuracy of 5 cm. Testing includes PPP processing of 1-h measurement blocks using 1–8 days of data from three locations in Australia. It was shown that all the three triple-frequency models had improved solution convergence time compared to the traditional PPP dual-frequency model although they gave almost similar accuracy and precision. The convergence time, when using the triple-frequency ionosphere-free model improved, by 10%, the improvement was 9% when using the mixed code-phase model, whereas the individual uncombined model resulted in 8% improvement.

Improved Coordinate Transformation in Dubai Using a New Interpolation Approach of Coordinate Differences

Abstract In this study, an improved coordinate transformation method between geodetic reference frames in Dubai, UAE, is investigated. In the first phase of this method, a similarity transformation is performed using a seven-parameter model between points known in the two frames involved in the transformation process. Next, the coordinate differences between the transformed coordinates and their known values in the frame into which they are transformed are computed. A database is then produced containing these coordinate differences. To transform coordinates of other points, their coordinates are initially transformed using the similarity transformation parameters estimated in the first step. Next, the coordinate corrections (as differences) at these points are interpolated using the generated database and added to the initial transformed coordinates. Instead of using the common method of double interpolation from a grid for this purpose, a new method is presented. The method directly interpolates coordinate differences at a required point from coordinate differences of the nearest three known points. These differences in the spatial domain form a triangle. Each triangle is assumed to be a surface. The coordinate difference at the required point is estimated on that surface by geometry. Results from testing show that coordinate transformation at accuracy of 2.1 cm on average was achieved in Dubai, which is better than using only the current traditional similarity transformation. Results also show that the proposed interpolation method has a very good performance.

PILOT EVALUATION OF INTEGRATING GLONASS, GALILEO AND BEIDOU WITH GPS IN ARAIM

Abstract In this pilot study, availability of the Advanced Receiver Autonomous Integrity Monitoring (ARAIM) when integrating various combinations of satellite constellations including; Galileo, GLONASS and BeiDou with GPS is investigated. The Multiple Hypothesis Solution Separation method was applied using one month of real data. The data was collected at stations of known positions, located in regions that have different coverage levels by the tested constellations. While most previous studies used simulated data, the importance of using real data is twofold. It allows for the use of actual User Range Accuracy (URA) received within the satellite navigation message, which is a fundamental component for computation of the integrity protection level; and the computation of vertical position errors to validate the integrity approach. Results show that the vertical position error was always bounded by the protection level during the test period and the ARAIM availability can reach 100% of the time when using all constellations even though some constellations are yet incomplete.

Validation of BeiDou Observations

Abstract This study presents validation of BeiDou measurements in un-differenced standalone mode and experimental results of its application for real data. A reparameterized form of the unknowns in a geometry-free observation model was used. Observations from each satellite are independently screened using a local modeling approach. Main advantages include that there is no need for computation of inter-system biases and no satellite navigation information are needed. Validation of the triple-frequency BeiDou data was performed in static and kinematic modes, the former at two continuously operating reference stations in Australia using data that span two consecutive days and the later in a walking mode for three hours. The use of the validation method parameters for numerical and graphical diagnostics of the multi-frequency BeiDou observations are discussed. The precision of the system’s observations was estimated using an empirical method that utilizes the characteristics of the validation statistics. The capability of the proposed method is demonstrated in detection and identification of artificial errors inserted in the static BeiDou data and when implemented in a single point positioning processing of the kinematic test.

Diagnostic Tools Using a Multi-Constellation Single-Receiver Single-Satellite Data Validation Method

The use of single-receiver single-satellite data validation parameters for numerical and graphical diagnostics of the multi-frequency observations is presented. This method validates Global Navigation Satellite System (GNSS) measurements of a single receiver where data from each satellite are independently processed using a geometry-free observation model with a reparameterised form of the unknowns. The method is applicable to any GNSS with any number of frequencies. The diagnostic tools are based on checking agreement of characteristics of the validation test statistics against theory. The use of these diagnostics in static and kinematic modes is demonstrated using multiple-frequency data from three GNSS constellations; Global Positioning System (GPS), Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) and Galileo.