Space Geodesy and Time Transfer: From Satellite to Science

Dear Colleagues,

Since ESA selected GENESIS as a mission for the FutureNAV programme in November 2022, space geodesy has been involved in the development of strategies to combine four space-based techniques: SLR, GNSS, DORIS, and VLBI. GENESIS will establish the implementation of these four techniques in space for the first time, while on-ground GNSS is usually used alongside one of the other techniques. The number of geodetic observatories using all four techniques is increasing and with it the ability to study collocations in space and on the ground. This includes the possibility of finding systematic errors in the techniques used and exploiting the opportunities arising from the use of common parameters in joint estimation processes. Clock and troposphere ties play a role in the analysis, alongside the use of additional supporting measurements, such as wet delays determined by water vapour radiometers. In this regard, developments in precise clocks, such as optical clocks, but also clock synchronization through fibre links on the ground, should be mentioned. Reducing the clock parameters via least-squares adjustment can lead to more consistent combination strategies.

This Special Issue seeks to include studies covering different aspects related to the combination of space geodetic techniques and time metrology, including, but not limited to, the following: the analysis of combinations at different levels; studies of systematic errors in various space geodetic techniques; the use of inter-satellite links and extended satellite constellations; future GNSS with additional instruments; the use of ties, particularly the common clock; the verification of relativity with satellites; the use of the theory of relativity in studies related to the future determination of gravitational potentials with optical clocks; time synchronization; the development of new types of clocks for use on the ground and in space; and satellite missions related to time, among others. We also welcome the submission of studies focusing on the following topics:

• Reference frames;
• Systematic errors in space geodetic techniques;
• Importance of collocation (space and ground) in space geodesy (clocks, ties, troposphere), specifically common analysis and instrumentation;
• Comparison of time transfer methods;
• Relativistic geodesy;
• Definition of time scales in space;
• Current, future, and proposed satellite geodesy missions;
• Synergy between optical time synchronization, ranging, and data transfer.

Dr. Anja Schlicht
Dr. Pierre Exertier
Guest Editors

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