High-accuracy positioning using EGNOS and Galileo products
Background & Objectives
At the core of the FIX_8 solution is a GNSS positioning technique known as Precise Point Positioning (PPP). PPP is a method of processing un-differenced pseudorange and carrier phase observations from a single GNSS receiver to compute positions with decimetric, or even centimetric accuracy, globally. In recent years, commercial services have applied the PPP technique to offer an accurate and flexible positioning solution which has proved particularly popular in the precision agriculture and offshore markets. The services rely on providing high-accuracy satellite data (orbit and clock predictions) to users for real-time operations. Using this satellite data and applying complex algorithms on the user-receiver, the services typically provide a positioning solution for a moving platform with a horizontal accuracy of 20 cm (95%) following a 30-minute convergence period. The solutions are completely independent of the distance to reference stations.
Using similar satellite data to the commercial service providers, FIX_8 can replicate these levels of positioning accuracy and has also shown the potential to reduce the convergence period. The FIX_8 project also explored the level of accuracy which could be achieved without subscribing to a service supplying this precise satellite data.
FIX_8 developed a prototype for a new GNSS service with a performance comparable to that of existing commercial high-accuracy differential services. It explored the performance which can be achieved with a stand-alone GNSS receiver using only free-to-air signals, assessing the accuracy which can be achieved today using EGNOS data within a PPP solution and in the future with using Galileo.
To implement a PPP solution, the GNSS errors which are usually removed or reduced by differencing in high accuracy applications must be compensated for using other error mitigation techniques. These techniques include combining observations to overcome ionospheric delay errors and applying models to estimate satellite antenna phase centre offsets, relativity effects, site displacement effects due to Earth tides, etc. These mitigation methods are included in the FIX_8 prototype. The FIX_8 processing algorithms also introduce a number of innovative techniques which are intended to improve upon classical dual frequency PPP solutions. It uses raw code and carrier phase observations, and includes the ionospheric delay on each satellite range as a state in the Kalman filter rather than forming an ionospheric free observable.
This is intended to reduce convergence time with respect to traditional PPP solutions. The performance of the prototype solution was initially investigated using a large number of static datasets. Dedicated field trials were then undertaken in conditions designed to be representative of those found in target applications. For each trial a centimetre level reference solution was used to determine the error in the FIX_8 solution. Orbit and clock data with varying accuracy was used to establish the relationship between these inputs and the accuracy of the resultant position solution. This then allowed an extrapolation to predict the level of accuracy which should be reached by applying FIX_8 processing algorithms on a Galileo receiver.
The first objective of FIX_8 was to develop a PPP solution with performance comparable to that of the existing commercial high-accuracy differential GNSS services. The prototype is a real-time software implementation of the positioning algorithms which would be applied on a GNSS receiver.
The software was designed to be flexible, allowing for modifications to classical PPP algorithms to be explored. FIX_8 has also aimed to demonstrate the accuracy which can be obtained from a stand-alone GNSS receiver using EGNOS-corrected orbit and clock data in place of commercial products. Finally, the prototype can be used to estimate the performance which could be achieved in the future using the Galileo Open Service and also to explore the potential of a high-accuracy Galileo Commercial Service.
Work performed & results
A baseline FIX_8 solution using precise orbit and clock products at a static site produced accuracies of 10 cm (95%) in the horizontal component and 20 cm (95%) vertically, similar to commercial PPP services. When precise satellite data was replaced with a broadcast GPS message with EGNOS corrections applied, the accuracy achieved was 30 cm (95%) for the horizontal component. Field trials demonstrated a typical accuracy of less than 20 cm (95%) for horizontal positioning onboard a boat using precise satellite data. The trials were not able to provide a comprehensive assessment of a FIX_8 solution based on EGNOS because in 2006 corrections were not available for a significant number of the GPS satellites in view. Results suggest that if corrections are available consistently for five or six satellites, a positioning accuracy of half a metre will be achieved. A 50 cm, or perhaps better, solution using only free-to-air signal represents a significant improvement on a standard EGNOS code position. Extrapolations to future Galileo operations suggest that a 50-cm solution should also be possible with a dual frequency receiver using only the Open Service. A 20-cm solution is possible if predicted clock and orbit data can be distributed without some of the constraints of the proposed dissemination approach for Galileo orbit and clock data. The provision of more precise satellite data, allowing a high accuracy positioning solution with a suitably equipped receiver, may become a Galileo Commercial Service.