Different GNSS-based technologies (alone or augmented and/or hybridized) are currently being used for a wide range of positioning applications. GPS and GLONASS are operational systems, and Galileo and Beidou are now being deployed. Augmentation systems such as WAAS, EGNOS, MSAS and GAGAN are providing improved accuracy and integrity on top of the basic GNSS navigation solutions, and SDCM is progress. Real Time Kinematics (RTK), Precise Point Positioning (PPP) and the integration with sensors, mapping data, local ionospheric information and other non-GNSS signals are allowing the achievement of excellent accuracy and reliability levels, and spreading the application of GNSS based technologies to many different fields (precision agriculture, civil works, natural disaster warning and mitigation, marine precise positioning, mining, geodesy, mapping and surveying...). The GNSS receivers market is also continuously evolving, incorporating advanced features as the systems are being developed and improved, for being able to provide a wider and wider range of positioning navigation solutions to a growing community of users.

PPP is a positioning technique providing centimeter-level error. PPP algorithms process dual-frequency pseudorange and carrier-phase measurements from a single user receiver, using detailed physical models and corrections, and precise GNSS orbit and clock products calculated beforehand. PPP is different from other precise-positioning approaches like RTK in that no reference stations are needed in the vicinity of the user receiver. The only observation data that must be processed are measurements from the user receiver. Another advantage of PPP is that since the GNSS orbit and clock products are by nature global, the PPP solutions are also global, i.e., the PPP approach works for a receiver located anywhere on or above the Earth surface, and the resulting position is referred to a well-known terrestrial reference frame (normally ITRF). PPP can be applied in post-processing and also in real-time applications, provided that real-time input orbits and clocks are available. One disadvantage of standard PPP however is its relatively slow convergence time, which is on the order of half an hour for decimetric accuracy, as compared to nearly instantaneous convergence with centimetric accuracy in short-baseline RTK.

GMV's magicGNSS suite (magicgnss.gmv.com), allows a registered user to perform multi-GNSS Precise Orbit Determination (POD) processing based on observation RINEX files. GMV has also developed a dedicated infrastructure for the generation of precise real time orbit and clock products. This infrastructure acquires via NTRIP data streams from a worldwide station network, and produces orbit and clock updates from a combined multi-GNSS solution. A real-time multi-GNSS PPP client has been also developed and integrated in a portable hardware device supporting in-the-field real-time PPP. The mentioned device connects to a standard geodeticclass receiver through a serial interface to retrieve the observations, and features mobile communications with the PPP corrections server using a mobile network or Iridium. As a differentiating feature, we are also working on providing a reliability bound, together with accuracy, continuity and availability to our PPP solution, which we think will help widening its applicability range.

In this paper we are going to present:
- A description of the real-time server and PPP client developments undertaken, together with both the
server (i.e. orbit and clock) performances achieved and the resulting multi-GNSS positioning
performances.
- A critical analysis of the PPP problem, considering the GNSS constellations geometrical effects and how the quality of the orbit and clock products can affect the accuracy of the provided positioning solution.

Our objectives are:
- to identify potential accuracy improvement margins
- to analyze the feasibility of building a dedicated correction at server level to be delivered to the users for mitigating the effects of degraded geometry and/or degraded quality of the orbit and clock products on the position solution
- to define reliable error bounds for the provided positioning solutions
- to demonstrate that the inclusion of Galileo satellites in the PPP process can improve the provided positioing performances

We do already have a strong background in this area. We have already been working with a reliability concept for PPP, see reference 4., and have performed dedicated experimentation campaigns making use of multiconstellation real data for testing the algorithms feasibility in open sky, sub-urban and urban scenarios. We are working in laying down the foundations for an upper-level reliability concept for PPP, closely linked to the final user perspective, which might open a new field for GNSS applications (precision agriculture, civil works, natural disaster warning and mitigation, marine precise positioning, mining, geodesy, mapping and surveying...). The current demand for high precision GNSS based applications and serviced will keep on increasing in the next years, and we aim at achieving the challenging task of enabling all these present and future GNSS uses, those involving high positioning accuracy, as well as those in which a certain reliability bound is required.

Keywords: PPP, Galileo, reliability