# On the Integration of VLBI Observations to GENESIS into Global VGOS Operations
## 1 Introduction  
Under the Global Geodetic Observing System (GGOS) initiative, the International Association of Geodesy (IAG) set the accuracy and stability requirements for a Terrestrial Reference Frame (TRF) at 1 mm and 0.1 mm/year to meet the scientific needs of various Earth science disciplines [1]. The ITRF2020 is the latest realization of the International Terrestrial Reference Frame (ITRF) [2]. It is established by observations from four space geodetic techniques: Very Long Baseline Interferometry (VLBI), Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). These techniques are combined with terrestrial local tie measurements at co-location sites. Despite representing the most accurate realization of a TRF, the long-term position and time evolution of the ITRF2020 is estimated to be on the order of 5 mm and 0.5 mm/year, respectively, [2]. In the ITRF2020, a large fraction of discrepancies between terrestrial local ties and space geodesy estimates exceed 5 mm. A useful tool to improve the combination of the individual techniques is the concept of space ties [3], the co-locations of several spacegeodetic techniques on satellites, with the aim of strengthening inter-technique ties. Mission proposals, such as GRASP [4] and E-GRASP [5], which aimed to combine instruments of GNSS, SLR, VLBI, and DORIS on a single Earth-orbiting satellite, have not been approved in the past. In November 2022, the European Space Agency's (ESA) Ministerial Council approved the GENESIS satellite mission as a component of the FutureNAV program [6]. Scheduled for launch in 2028, GENESIS will be a satellite platform flying in a polar orbit at an altitude of about 6000 km. It will carry instruments of all four space geodetic techniques. These instruments will be co-located with carefully calibrated reference points on board the satellite. The ground stations of all space geodetic techniques will be indirectly interconnected through observations of the instruments on the GENESIS satellite (see Figure 1). The primary objective of the mission is the implementation of in-orbit space ties, which have the potential to improve the accuracy and stability of future realizations of the ITRF. With the GENESIS mission approved, it is now up to the geodetic community to develop optimal observing and analysis strategies. At the time of writing, the design of the VLBI transmitter onboard the GENESIS satellite is still under development. As part of the GENESIS Science Exploitation Team (GSET), Working Group 3, which includes experts in the field of VLBI, is providing advice and assistance to ESA in this design process. The key factors driving the antenna design include regulations set forth by the International Telecommunication Union (ITU), radio frequency interference (RFI), and the compatibility with legacy S/X and VLBI Global Observing System (VGOS) stations. VLBI observations of Earth-orbiting satellites are not standard and are not performed routinely yet. However, in several test experiments (e.g., [7; 8; 9]) it was demonstrated that the technical challenges inherent to VLBI observations of near-field targets can be overcome by adjustments to the standard VLBI processing pipeline [10]. In addition to real observations, several simulation studies were conducted to assess the potential of VLBI observations of Earth-orbiting satellites. For example, [PERSON] et al. [11], [PERSON] [12], and [PERSON] et al. [13] estimated the VLBI station positions from simulated observations of GNSS satellite constellations utilizing the satellites' navigation signals in L-band. Other studies assume a VLBI transmitter on satellites, as is the case for GENESIS. [PERSON] et al. [14] estimated VLBI station positions from simulated observations of a co-location s  
Figure 1: Visualization of the GENESIS satellite.