(1) RIGTC/GOP Geodetic observatory Pecny, Zdiby, Czech Republic, (2) GFZ German Research Centre for Geosciences, Potsdam, Germany, (3) Technical University of Ostrava, Czech Republic, (4) Royal Observatory of Belgium, Brussels, Belgium, (5) Belgian Institute for Space Aeronomy, Brussels, Belgium, (6) Vienna University of Technology, Austria, (7) E-GEOS, s.p.a. & ASI, Matera, Italy, (8) Institute of Computer Science of the Czech Academy of Sciences, Prague, Czech Republic, (9) Bundesamt für Kartographie und Geodäsie, Frankfurt am Mein, Germany, (10) University of Luxembourg, Luxembourg, (11) GeF Laboratory, ESGT – CNAM, Le Mans, France, (12) Institute of Geodesy and Geoinformatics, Wroclaw University of Environmental and Life Sciences, Wroclaw, Poland

In this paper, a concept is presented which makes use of inter-satellite ranging (ISR) techniques for monitoring the atmospheric state. Thereby, for this study it is assumed that future GNSS satellites are equipped with an ISR payload and that each satellite is supposed to connect to other satellites in different orbital planes sequentially. For further analysis a Galileo constellation of 24 (27 and 30) MEO, four IGSO and one MEO spare satellite was simulated over a period of one year to reconstruct the paths of the ranging signals and to identify events when the signal is travelling through the atmosphere. Based on the detailed analysis of the transit events we identify inter-satellite ranging not only as complementing technique to improve the satellite orbit and clock accuracy of the GNSS constellation but also its large potential for monitoring the state of the neutral atmosphere and ionosphere. Thereby, due to the unique observation geometry between satellites in MEO, IGSO or in transfer orbit, range measurements can be obtained for specific observation arcs. Some of them show similarities to radio occultation profiles on long baselines, others allow e.g. to permanently scan down to specific layers in the atmosphere. Latter is possible if e.g. a spare satellite is placed in orbit so that a permanent link is installed to another MEO satellite in the same orbital plane. In addition to the geometric analysis, also a propagation analysis was carried out to identify atmospheric losses, which may harm signal reception. So far studies dealing with inter satellite ranging do not consider signal paths through the atmosphere. Thus, ISR link budgets were recalculated for various frequencies in the microwave band in order to get a first guess about the atmospheric losses and whether signal reception is possible after the inter-satellite link has passed the atmosphere. Considering frequencies at three bands (K, S and C), while for K-band signal reception is guaranteed only under specific atmospheric conditions, S- or C-band seems to be most promising when the entire atmosphere (ionosphere and neutral atmosphere) is monitored.

In the last years real-time Precise Point Positioning (PPP) became a well-known GNSS positioning technique which is nowadays already used for various applications. Combining precise satellite positions and clock corrections with zero-difference observations from a dual-frequency GNSS receiver PPP is able to provide position solutions at decimeter to centimeter level. However, these corrections are insufficient to fix the ambiguities, which is why PPP still suffers from long initialization periods until the solution converges to the desired accuracy. This long convergence time is one of the most limiting factors of real-time PPP with regard to numerous applications. This contribution shall give an overview on the work performed in the research project PPPServe (funded by the Austrian Research Promotion Agency – FFG), which aimed at the development of appropriate algorithms for real-time PPP with special emphasis on the ambiguity resolution of zero-difference observations. It shall especially deal with the process and obstacles of calculating the so-called wide-lane and narrow-lane phase-delays which allow PPP-base ambiguity fixing in real-time. Furthermore, the achieved quality and the temporal stability of the estimated phase delays as well as the coordinate convergence period and coordinate quality achieved at the rover site will be discussed on basis of the most recent results.

PPP denotes a positioning technique, where code and phase measurements from a single GNSS (Global Navigation Satellite System) receiver were used to produce precise positions with the aid of globally valid precise orbits and clock corrections in general provided by organization like the International GNSS Service (IGS). Typically an ionosphere free linear combination is used to eliminate the ionospheric delay. Other error terms like tropospheric delay or receiver clock biases can be estimated. Even though real-time PPP is in the starting phase there are a number of applications already in place which make use of the technique. On the other hand there are still many unsolved problems like the prevention of float-ambiguities during PPP processing and the availability of real-time correction data, which again directly influences the position accuracy. As PPP is close to become a major technique in the field of sciences research activities there is an in- creased interest to provide the estimated parameters like coordinates or tropospheric delays also in real-time. Therefore the IGS real-time working group started to provide the user-community with real-time GNSS data and derived products such as precise clock corrections and orbits. Organizations or reference stations providing real-time GNSS data can participate in the working group or provide their data-streams via a central service. In this context also the Institute of Geodesy and Geophysics of Technical University Vienna (TUW) contributes by means of an individual clock and orbit data stream. This presentation deals with the accuracy of real-time PPP coordinate solutions obtained by means of the TUW data stream in comparison to RTK techniques and discusses also the related period of convergence. Furthermore post-processed site-coordinates and tropospheric delays calculated by the PPP technique based on the one hand on IGU products and on RT-data corrections on the other are analyzed.

After the annexation of Austria into the German Reich the “Berufsordnung der Offentlich bestellen Vermessungsingenieure“ (ObVI) was introduced in 1940. By analysis of documents in the German Federal Archives (Berlin) statements about the political orientation of the profession and the licensing procedure are possible. Within the group of ObVI the former “Ingenieurkonsulenten fur Vermessungswesen“ were the third largest group. The surveyors had to take the licensing procedure or they had to close their offices. The number of finally approved ObVI's is significantly lower than the number of independent surveyors in Austria in 1938. The result of the approval process was like in German Empire since 1938 a market adjustment. As part of the process, the political reliability and the ancestry of the candidates has been verified. In some cases, the authorization was denied for political reasons. In most cases, the rejection was based on age or lack of skills.

Today PPP is a well-known technique of GNSS based positioning used for a wide range of post-processing applications. Using observations of a single GNSS receiver and applying precise orbit and clock information derived from global GNSS networks highly precise positions can be obtained. The atmospheric delays are usually mitigated by linear combination (ionosphere) and parameter estimation (troposphere). Within the last years also the demand for real-time PPP increased. In 2012, the IGS real-time working group started a pilot project to broadcast real-time precise orbits and clock correction streams. Nevertheless, real-time PPP is in its starting phase and currently only few applications make use of the technique although SSR-Messages are already implemented in RTCM3.1. The problems of still limited accuracy compared to Network-RTK as well as long convergence times might be solved by almost instantaneous integer ambiguity resolution at zero-difference level which is a major topic of current scientific investigations.

Precise Point Positioning (PPP) denotes a GNSS (Global Navigation Satellite System) based positioning technique, where dual-frequency code and phase measurements from a single receiver are used to calculate precise site coordinates at the sub-decimeter level. The data processing relies on precise satellite orbits and clock correction information determined from observation data of a global reference station network provided by organizations such as the International GNSS Service (IGS). Typically, the ionospheric delay is almost completely eliminated by means of the ionosphere-free linear combination, while the tropospheric delay and the receiver clock bias are estimated parameters along with the site coordinates. Introduced for the first time about 14 years ago the PPP technique was mainly used in post-processing applications. Barriers for a more intense use of PPP were a lack of accurate real-time orbit and clock products, the still poor knowledge of receiver and satellite calibrations biases and last, but not least, long coordinate filter convergence times due to complex or incomplete integer ambiguity fixing. However, to meet the increasing demand of upcoming real-time (RT) applications IGS has initiated a real-time working group to investigate the feasibility of real-time GNSS data distribution and the generation of derived products such as precise clock corrections and orbits. Scientific organizations and companies operating reference stations can participate in the working group either by delivering their data-streams via a central service or by providing real-time GNSS products. This article deals with the contributions of the Institute of Geodesy and Geophysics, Technical University of Vienna (TUW) to the IGS Real-Time Working Group and with the quality of PPP positioning obtained using the RT-data stream established at our institute. Aside from the positioning aspect the potential of PPP to derive related products such as tropospheric delays to contribute to weather forecast models is discussed. Finally prospects as well as current barriers of PPP in view of the upcoming new GNSS systems and signals are highlighted.

Precise Point Positioning (PPP) is a modern Global Navigational Satellite Systems (GNSS) processing technique that enables the estimation of precise three-dimensional coordinates by means of code and phase measurements from a single GNSS receiver. To enhance the position accuracy so-called precise ephemerides are used, that are freely offered by analysis centers. Further, during PPP processing a lot of additional model corrections have to be considered (phase wind-up, tidal effects, instrumental biases, etc.). These would have cancelled in a relative positioning method (e.g. RTK) as a result of building differences between observations of multiple receivers. Usually, the ephemerides from external sources are globally valid and their transmission in real-time needs lower rates compared to RTK-services. Therefore, PPP offers the possibility of highly precise positioning in areas, where neither a dense reference station network, nor a good mobile data link is available. The resulting positions as well as troposphere or clock parameters are solely referred to the geodetic datum of the ephemerides. Nevertheless, real-time PPP is only in its starting phase, since the necessary correction data are available only for a short time with an adequate accuracy. It was not until 2011, when the transmission of PPP specific corrections for satellite orbits and clocks was standardized in the RTCM document (version 3.1) for the first time. Applying these so-called SSR (State Space Representation) corrections it is finally possible to process coordinates with an accuracy of only few centimeters in real-time. Nevertheless, for sub-decimeter accuracies an initialization phase of up to 30 minutes is necessary. This convergence time arises from the fact that ambiguities within PPP usually cannot Precise Point Positioning und Möglichkeiten für präzise Echtzeit-Anwendungen Katrin Huber, Roman Lesjak, Graz und Fabian Hinterberger, Wien Vermessung & Geoinformation 1/2015 38 be fixed to integer values, as their estimates contain not only the real ambiguities, but also instrumental biases that would have cancelled in RTK as a result of differencing observations. The real-valued parts of these phase biases can only be eliminated by phase corrections calculated in a network solution. This contribution presents the current developments concerning PPP for real-time applications, as well as research projects at the Working Group Navigation of the Institute of Geodesy at the TU Graz, that among others investigate also the fixing of integer ambiguities within PPP processing. The integer ambiguity fixing would shorten the initialization phase of PPP solutions significantly, and make the technique more attractive for many real-time applications.