Communication links between GNSS satellites are required if the constellation shall operate for an extended time without ground contact. Under such circumstances, jamming resistance is considered a critical requirement and optical links can provide the ultimate protection from ground-to-space jamming. At the same time, low cost, high reliability and small size, mass and power are important to be implemented on a GNSS constellation. The activity shall review the requirements on intersatellite links based on the GNSS architecture.

Communication links between GNSS satellites are required if the constellation shall operate for an extended time without ground contact. Under such circumstances, jamming resistance is considered a critical requirement and optical links can provide the ultimate protection from ground-to-space jamming. At the same time, low cost, high reliability and small size, mass and power are important to be implemented on a GNSS constellation. The activity shall review the requirements on intersatellite links based on the GNSS architecture.

Carrier phase measurements are more precise than pseudorange measurements and are used for high precision services. The quality of the carrier phase measurements influences directly the Galileo/EGNOS Ground-Segment navigation and integrity algorithms output quality, as well as the GPS/Galileo professional and mass market user positioning accuracy. Unfortunately, phase tracking can be severely stressed under ionospheric scintillation (under severe scintillation, phase may change too fast for being tracked by the receiver).

Studies on high-sensitivity Rx have already focussed on weak-signal (e.g.

The accuracy of tropospheric excess path length estimation in a GNSS receiver can be increased by using an estimate of current or short-term evolution of atmospheric conditions. This information can be provided by ground or space based sensors and numerical weather prediction systems (like ECMWF for a global scale or HIRLAM/MM5 for regional scale). The models and algorithms to process this type of information and convert it into a message for local augmentation systems need to be develped.

The accuracy of tropospheric excess path length estimation in a GNSS receiver can be increased by using an estimate of current or short-term evolution of atmospheric conditions. This information can be provided by ground or space based sensors and numerical weather prediction systems (like ECMWF for a global scale or HIRLAM/MM5 for regional scale). The models and algorithms to process this type of information and convert it into a message for local augmentation systems need to be develped.

Background and justification: The

Communication techniques can be introduced in the Navigation domain to improve the performances. Easyness of the generation/reception of the signal, reduction of the intrasystem interference, higher orthogonality of the spreading sequences, efficiency of the message encoding mechanism to achieve higher data rate or lower BER, robustness of the signal tracking, higher efficiency in the exploitation of the available bandwidth, robustness to hostile RF environment, jamming and anti spoofing capability etc. are areas for possible improvement.

Extensive measurements over solar maximum will be carried out during the MONITOR activity under the GNSS Evolutions Programme and other experiments from external organisations. They will be able to provide a better insight on several processes affecting GNSS performance and they will have a proper basis for improved modelling. Such activities does not focus on a deep analysis of specific effects and the actual modelling of effects currently not considerd on average models.

While the MEOSAR is moving to FOC with Galileo satellites that are equipped with SAR transparent payloads, ESA is already investigating its future Galileo-2 (G2G) system for new features. In this framework, one of the aims of the ESA study on Future Concepts for Galileo Search And Rescue was to define a preliminary design of a new SAR payload capable of processing both First Generation Beacon (FGB) and future Second Generation Beacon (SGB) signals.