Typical POD (Precision Orbit Determination) requirements for geodetic quality LEO satellites are currently at the few centimeter level (1-ċ) in the radial component and the few decimeter level in the horizontal components, which is usually met using post-processing methodologies.

These requirements have been demonstrated to have been met with, for example, ICESat (600 km) and GRACE (400 km). Other LEO satellites have positioning requirements that are meter level to tens of meters. Even on ICESat, for example, the real time requirements are several meters, but this also includes the ability to predict the ICESat position to tens of meters accuracy over 48 hours. Different methodologies are used on ICESat to achieve the respective real time and post-processing requirements. In the case of ICESat with a laser altimeter to measure changes in the polar ice sheets, factors such as errors in the z-component of the LEO orbit are important considerations, as well as motion of the respective ITRF origin with respect to the actual Earth center of mass. This paper will address these issues and the role of the current IGS products in meeting the requirements.

In the near term (e.g., ~5 years), satellites already in orbit will not be influenced by expansion of the navigation satellites with, for example, GALILEO, because of the receiver design of these in-orbit satellites. However, as new receivers with multi-constellation capability become available, the fundamental question that LEO planners will need to address is the gain in LEO orbit accuracy with such multi-constellations and the concomitant impact on processing strategies and the potential introduction of new error sources. This paper will delineate considerations for the future in high accuracy POD applications for geodetic quality applications, such as advanced ICESat or GRACE missions. If, for example, GALILEO were available today with the high accuracy geodetic LEOs, what would be gained? And what is the accuracy requirement expected for LEOs on the next generation of advanced geodetic quality satellites?

Finally, looking to the time horizon, navigation satellite constellations have been proposed for future applications at the Moon and Mars. While the user base of such constellations would be far smaller than Earth applications, the extension of the methodologies used at Earth warrant consideration for the lunar and planetary applications should similar navigation constellations be implemented. What new challenges are posed by such applications and can IGS contribute?

The two GRACE satellites, flying in a LEO formation with an inter-satellite distance of about 200 km, can be used to form a continuously observed GPS baseline in space. When the ambiguities are resolved for such a space-borne GPS baseline, very accurate inter-satellite orbit information between the two GRACE satellites can be obtained with mm-accuracy. The strength of such a baseline can be further improved by adding KBR measurements, mainly improving the along-track orbit information. Such a GPS baseline can be seen as an orbiting 3D-vector in space, of which the magnitude and orientation is governed by the gravity field and the non-conservative forces acting on the satellites. If we increase the number of LEO satellites to more than two, as e.g. in the case of the six COSMIC satellites, we finally end up with a global coverage and a fully connected, continuously tracked LEO constellation network with baseline lengths up to 13’000 km.

Using simulated GPS data for the COSMIC constellation, we already showed that ambiguity resolution considerably increases the strength of the LEO network and strongly ties the LEO constellation to the GPS constellation of ~30 satellites, thus forming a LEO/GPS dual constellation. Based on such a hybrid constellation, orbits of the GPS satellites can be estimated simultaneously with the orbits of the LEO satellites, even without using measurements from the ground GPS network. However, in such a case the orbit of one reference satellite has to be kept fixed and assumed to be known or a corresponding datum constraint has to be applied to the dual satellite constellation. The dual LEO/GPS constellation forms a unique network with satellites orbiting the Earth at very different altitudes. Any change in the Earth’s gravity field or Earth’s rotation and geocenter motion differently affects the LEO satellites close to the Earth and the GPS satellites high above the Earth. Finally, adding the network of ground stations as a third level, the dual satellite constellation can be firmly tied to the Earth’s surface.

In this contribution we study, how different scenarios of LEO formations and constellations combined with the GPS system affects the IGS products. Thereby, real measurements from the GRACE mission as well as simulated GPS measurements for the 6 COSMIC satellites and other constellations are used.

The European Global Navigation Satellite System (GNSS) Galileo is going to be built up within the next years. A first test satellite has been started at the end of last year, the start of a second one is scheduled for September 2006. The In-Orbit-Validation (IOV) phase with a certain number of operative satellites is planned to begin at the end of 2008.

The Galileo Geodetic Service Provider (GGSP) is a project funded within the sixth framework programme of the European Commission. A consortium of seven institutions has been established to set up a GGSP prototype for the development of the Galileo Terrestrial Reference Frame (GTRF) and the establishment of a service with products and information for the potential users.

The presentation gives an overview about the project. Main topics, first steps, expected results and products as well as future expectations will be outlined. The strategy in principle will be presented, the network of the various work packages will be shown.

The CODE analysis center has been computing orbit products for both the GPS and the GLONASS satellite constellation for around 3 years. GPS and GLONASS orbits are generated simultaneously in a rigorous GNSS analysis. This is unique within the IGS, all the more because the complete orbit product palette is covered: rapid solution (since the beginning of May 2003), final solution (since 8 June 2003), and ultra-rapid solution (since 30 July 2003). We give an overview of our GNSS analysis activities and discuss issues we consider relevant for a combined analysis of two or more navigation satellite systems. Also, specialties such as POD for GPS satellites being repositioned or inclusion of "fast moving" ground stations in a global analysis are highlighted.