## 1 Introduction  
The aims for the future Terrestrial Reference System (TRS) are driven by the goals of the Global Geodetic Observing System (GGOS) [1]: 1 mm accuracy and long-term stability of 0.1 mm/year. The GGOS wants to achieve these goals by the collocated use of different observation techniques such as GNSS (Global Navigation Satellite System) measurements, VLBI (Very Long Baseline Interferometry), SLR (Satellite Laser Ranging), LLR (Lunar Laser Ranging), radar observations, ISL (Inter-Satellite Links) and gravity acceleration measurements. Furthermore, it is important for the GGOS to closely connect the different satellite layers from the Low Earth Orbit (LEO), via the Medium Earth Orbit (MEO), to the geosynchronous orbit (GSO). The concept of a Geodesy and Time Reference in Space (GETRIS) was first discussed by [2, 3]. The goal of GETRIS is to extend the TRS by a space-based relativistic reference system to contribute to the GGOS for the connection of the near Earth environment and deep space--the moon and beyond. With the complementary use of GNSS L-band measurements and dual one-way high-precision optical links between GSO and MEO satellites, a 1 mm orbit precision as well as a clock synchronization at the picosecond level shall be achieved. Dual one-way links are two one-way links that simultaneously measure between two entities, where both are transmitters and receivers. From a technical perspective, the optical links are comparable to the European Data Relay System (EDRS; [4, 5])'s links, which are used for data communication with geostationary (GEO) satellites. Due to its high modulation rate, the EDRS link would technically allow ranging and time transfer with high precision. To move toward a GETRIS, such optical links have to be adapted to perform calibrated ranging and time transfer. The GENESIS satellite mission [6] has a similar approach as the one we follow with the GETRIS concept. By a collocated use of geodetic techniques in space--GNSS, SLR, VLBI and DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite)--the time and space reference systems on Earth are aimed to be improved and homogenized. GENISIS also intends to equip the satellite with a high-stability clock. The goal is to realize a TRS with accuracy of 1 mm and long-term stability of 0.1 mm/year, which also matches the GGOS's [1] goal. A GENESIS-like concept on GNSS satellites would even enhance the contribution to the GGOS and GETRIS. The development of high-stability clocks has been rapidly growing in the recent past. The Cesium atomic clock (frequency stability in the low \(10^{-13}\) region at one day), the Rubidium Atomic Frequency Standard (RAFS) (frequency stability in the low \(10^{-14}\) region at one day) and the Passive Hydrogen Maser (PHM) (frequency stability in the high \(10^{-15}\) region at one day) are clock types that are already used in satellites of different GNSSs. Satellite missions to test clocks with a significantly increased precision are already in preparation. These high-stability clock concepts are, e.g., clocks based on iodine cells and using optical frequency standards with a targeted frequency stability in the low \(10^{-15}\) region for sample intervals between 100 s and 10,000 s [7; 8; 9]. The Deep Space Atomic Clock (DSAC; [10]), a trapped-ion atomic clock developed by the National Aeronautics and Space Administration (NASA), already demonstrated stability of \(2\times 10^{-15}\) at 1 day and \(3\times 10^{-15}\) at 23 days [10; 11]. Along with the ESA mission Atomic Clock Ensemble in Space (ACES; [12; 13]), a new clock concept is tested, which combines an Active Hydrogen Maser (AHM) and a laser-cooled Cesium clock, called the Project d'Horloge Atomique a Refroidissement d'Atomes en Orbite (PHARAO) [12; 14; 13]. The expected frequency stability and precision is at the level of 3 \(\times\) 10\({}^{-15}\) at 300 s, 3 \(\times\) 10\({}^{-16}\) at 1 day and 1 \(\times\) 10\({}^{-16}\) at 10 days.