Chair: H van der Marel (The Netherlands)

Water vapour is one of the most important constituents of the atmosphere. It plays a crucial role in many atmospheric processes covering a wide range of temporal and spatial scales. Furthermore, it is also the most important greenhouse gas and highly variable. Climate research and monitoring, as well as operational weather forecasting, need accurate and sufficiently dense and frequent sampling of the water vapour, to which existing GPS networks could contribute significantly. In order to be of any use for operational weather forecasting, firstly GPS networks must be able to provide integrated water vapour in near real-time (with a typical delay of one hour), and secondly GPS observations must be assimilated into numerical weather prediction (NWP) models.

Dual frequency GPS receivers enable the estimation of total electron content (TEC) along a given satellite-receiver signal path. By combining observations from regional and global networks of continuously operating dual frequency receivers, parameters describing the spatial and temporal distribution of total electron content can be derived. Such observations of TEC, available globally on a near real-time basis, allow an excellent opportunity for monitoring ionospheric signatures associated with space weather. For example, the development of ionospheric storms can be observed in global patterns of TEC, while small-scale irregularities in electron density (associated with scintillations) can be observed in short-term variations of TEC and/or spectral analysis of GPS phase observations.

The focus of the SSG is to explore the issues related to

Due to differences in the way the IWV and TEC are derived, it makes sense to organize the work in the special study group into an IWV and a TEC section.

The main objectives of the special study group are:

1. Identify key signatures observed in GPS-derived estimates of TEC, as associated with phenomena such as ionospheric and geomagnetic storms, scintillations, travelling ionospheric disturbances, magnetospheric substorms and auroral activity.
2. Assess methods to quantify the level and nature of ionospheric activity, based on TEC estimates
3. Explore key issues related to the feasibility of integrating TEC estimates, and TEC-based indices, into space weather forecasting and nowcasting – such issues include real-time requirements, and the temporal and spatial resolution necessary for reliable detection and prediction of ionospheric phenomena.
4. Identify key problems in GPS derived integrated water vapour, as associated with phenomena related to the near field of the antenna, such as multipath and phase variations, and local weather (gradients, mapping to the vertical), reprocessing and archiving of data, in relation to climate applications.
5. Explore key issues related to the assimilation of GPS derived integrated water vapour observations into numerical weather prediction (NWP) models – such issues include real-time requirements, temporal and spatial resolution, slant or vertical delays, temporal and spatial correlation and quality insurance issues.
6. Assess the potential impact of tropospheric tomography using GPS estimated slant delays.

The activities of the SSG include:

1. Compile a database including relevant literature and research groups
2. Facilitate discussions of key issues though web site and email
3. Periodic progress reports to members
4. Organization of working group meetings

  R.Weber (Austria)

Chair: R. Weber (Austria)

In recent years an increasing number of GPS reference stations have been established on both global and regional scales. Ideally, the latter should represent local densifications of the ITRF polyhedron.

While, at the outset, these stations were built up in most cases to monitor active tectonic regions, recently the augmentation of real time surveying and probing of the atmosphere have become more

important.

The work of this study group aims at the tie of regional GPS networks to the ITRF as well as to study ambiguity resolution within a network of multiple reference stations at baselines with a length of up to several tens of kilometres. Especially the appropriate modelling of ionosphere and troposphere path delays as the limiting factors for ambiguity resolution and the influence of antenna phase centre variations should be discussed. Concepts and realisations of virtual reference stations will be compared. RTK solutions within active reference station networks, the benefits of using combined GPS/GLONASS receivers as well as the use of predicted IGS orbits are also the subject of the investigations. Last, but not least, reliable error models of the baseline solutions have to be formulated.

To achieve these goals the SSG will:

Chair: M P Stewart (Australia)

The precision of raw carrier phase observations recorded by modern GNSS receivers is generally at the sub-millimetre level. However, in all but the most benign environments, the achievable resolution of GNSS positioning is one or more orders of magnitude worse. This discrepancy between the theoretical hardware-dependent precision of the raw observations and the practical accuracy of GNSS position solutions can, in part, be attributed to the effects of site-dependent electromagnetic scattering of incoming GNSS signals. If millimetre level (or better) GNSS accuracies are to be routinely achieved in the future, these electromagnetic scattering effects (commonly referred to as multipath and diffraction) must be eliminated. 

The goal of SSG 1.182 is to study GNSS multipath detection and mitigation techniques with the aim of improving existing high precision positioning accuracies. In the context of this SSG, multipath is loosely defined as the systematic errors in raw GNSS observations that are due to any signal scattering effect caused by the local environment surrounding an antenna. Furthermore, this SSG will focus on carrier phase and code-based multipath in terms of effects on receiver operation for high precision applications. Finally, within the scope of the group, the term GNSS is defined to encompass any type of global positioning system (for example, NAVSTAR-GPS, GLONASS-GPS and GALILEO), or systems simulating GNSS signals (such as pseudolite arrays).

1. Evaluate and compare existing and developing algorithms and techniques for multipath detection and mitigation.
2. Quantify and document the effectiveness of commercial receiver based multipath mitigation techniques for high precision positioning.
3. Investigate and document the properties of multipath in a variety of environments (particularly high risk environments).
4. Provide information and guidelines for multipath detection and elimination for high precision applications.

These objectives will be achieved by:

The outcome of these activities will be summarised in a final report containing classification of detection and mitigation techniques, a library of results from different environments and receivers, and a set of recommendations giving information on multipath detection and mitigation.