More accurate positioning through real-time Tropospheric Delay Service
Positioning with the use of satellites has many variables. One of the more unresolved is the troposphere itself (the ‘air’). This tech-owner developed a service to guarantee much more accurate positioning by eliminating the tropospheric delay to the satellite signal. This results in a disruptive improvement of GNSS positioning, with accuracy towards to sub-cm domain. The primary users for the service are those requiring extremely accurate positioning and comlplies with SSR and PPP.
Summary: Near future GNSS promises to bring more accurate positioning based on an increasing number of visible GNSS satellites, but also due to its open dual frequency capability enables estimation of the actual signal delay introduced by the earth’s highly variable ionosphere. This leaves the troposphere as the main unresolved error source for atmospheric GNSS signal propagation. The Tropospheric Delay Service aims to remove this error source. Recent Numerical Weather Models (NWM) have undergone significant improvements: with very high spatial resolutions (1-3 km). These models are therefore a great source to estimate and correct GNSS tropospheric errors. This service takes advantage of NWM data to determine the GNSS tropospheric delay and thus improve real-time GNSS positioning accuracy. An end-user provides the service with his position, upon which the service calculates the local actual tropospheric delay for each of the satellites visible to the user. The service will use state-of-the-art tropospheric delay models and slant ray tracing algorithms. The service is expected to drastically improve GNSS positioning accuracy towards sub-cm domain. In detail: with the advent of the Galileo satellite navigation system more accurate position, timing and navigation services will become available. This gain in accuracy stems not only from the increased amount of GNSS satellites but also from additional open signals transmitted at currently unused frequencies. As the ionosphere is a dispersive medium to certain bands of radio frequencies the propagation delay varies for the different signal frequencies. As a result the position error introduced by the ionosphere can be accurately corrected for based on dual frequency GNSS. The Open (E1, E5) signals will provide all users with unrestricted dual frequency positioning. In addition to the ionosphere the lower, neutral part of the earth’s atmosphere also introduces position errors that must be corrected. Unfortunately this so-called tropospheric refraction is non-dispersive and dual frequency measurements cannot be utilized to cancel its effect. To correct GNSS positioning for all atmospheric propagation errors, this tropospheric error must be determined and compensated in a different manner, independent of the GNSS signal itself. Recent developments in numerical weather models have led to very accurate weather predictions with high spatial resolution (grid cells of 3 km or smaller). These state-of-the-art weather models can also be used to compute the tropospheric error correction for GNSS. The objective of this service will be to provide a highly accurate tropospheric delay service to GNSS end users. Combined with dual frequency ionospheric error correction, the service will enable correction of all atmospheric signal propagation errors to achieve a positioning accuracy in the sub-cm domain. The service will be based on up-to-date local weather conditions from a numerical weather model, combined with a state-of-the-art tropospheric delay model. The weather model data will be provided by meteorological service providers. Timely and locally valid provision of the service information will be provided through Internet information exchange. The end user provides its actual coordinates on earth to the service upon which the tropospheric delay service provides the local, actual tropospheric delay values for each of the GNSS satellites in view based on ray tracing through the current state of the Numerical Weather Model. The various requirements of the different kinds of applications and end-users are considered central in the design and development of the tropo correction service
Innovations & Advantages
- extremely increased accuracy for positioning applications (error margins reduced to sub-cm domain)
- integration possible with PPP services (‘precise point positioning’)
- enhancing possibilities of using Galileo and GNSS
- global coverage
Various critical applications of satellite navigation (e.g. automatic landings in aviation, autonomous road vehicles) require accuracy and reliability beyond what is currently available. By developing a high quality correction service, GNSS accuracy can be improved to a level where its use in such critical applications becomes feasible. The service is especially applicable to all areas of mobility, including aviation, shipping, road and rail transport. Other applications requiring high accuracy positioning will also benefit, such as high accuracy geodesy, offshore manufacturing and exploration, smart farming. The service could contribute to the optimization of current mobility problems such as clogged airways, harbor approaches. It can assist in the development of autonomous road vehicle systems. It will increase the uptake and use of Galileo in both present and new applications.
The tropo-correction service proposed for development is currently not yet available for end-users and as such completely new. The market segments mentioned above are closely matched with the primary target users for the service: mobility applications and other users requiring high accuracy positioning. By considerably enhancing general GNSS accuracy and reliability, the service has the potential to affect a very wide range of markets and applications. The commercial potential of the service is significant: the markets addressed are large, and the service has a clear added value. The use of GNSS in critical services (which was previously not possible) can provide significant cost savings.
Currently for highly accurate positioning the ‘Precise Point Positioning’ (PPP) technology is provided worldwide by service providers such as OmniSTAR, VERIPOS, GEO++, etc. for customer applications such as offshore mining, precision farming, scientific applications, cadastre, and more. This PPP service provides augmented GNSS positioning and timing, based on highly accurate ephemerids (GNSS satellites orbit and clock information) and atmospheric signal propagation information. PPP is able to provide position accuracies up to few cm. Its tropospheric delay estimate is based on a network solution, where local site-augmented tropospheric surface parameters of the involved reference stations are used. Currently the tropospheric residual error is considered as the most prominent error that limits the PPP accuracy performance. This envisioned Tropospheric Delay Service could be seamlessly integrated into commercial PPP services, for instance taking advantage of the SSR information exchange standardization that is developed within PPP.
Current and Potential Domains of Application
GNSS positioning requiring real-time high accuracy for which currently PPP is used, or beyond current PPP accuracy. The following applications are in view: - precision pharming, - offshore and onshore mining, - cadastre, - automotive, - maritime, - aeronautical navigation solutions, e.g. GBAS up to Cat III performance, - high accuracy geology