###### Abstract  
The Virginia Tech Formation Flying Testbed (VTFTFB) is a global navigation satellite system (GNSS)-based hardware-in-the-loop (HIL) simulation testbed for spacecraft formation flying with ionospheric remote sensing applications. Past applications considered only the Global Positioning System (GPS) constellation. The rapid GNSS modernization offers more signals from other constellations, including the growing European system--Galileo. This study presents an upgrade of VTFTFB with the incorporation of Galileo and the associated enhanced capabilities. By simulating an ionospheric plasma bubble scenario with a pair of LEO satellites flying in formation, the GPS-based simulations are compared to multi-constellation GNSS simulations including the Galileo constellation. A comparison between multi-constellation (GPS and Galileo) and single-constellation (GPS) shows the absolute mean and standard deviation of vertical electron density measurement errors for a specific Equatorial Spread F (ESF) scenario are decreased by 32.83% and 46.12% with the additional Galileo constellation using the 13 July 2018 almanac. Another comparison based on a simulation using the 8 March 2019 almanac shows the mean and standard deviation of vertical electron density measurement errors were decreased further to 43.34% and 49.92% by combining both GPS and Galileo data. A sensitivity study shows that the Galileo electron density measurements are correlated with the vertical separation of the formation configuration. Lower C/N\({}_{0}\) level increases the measurement errors and scattering level of vertical electron density retrieval. Relative state estimation errors are decreased, as well by utilizing GPS L1 plus Galileo E1 carrier phase instead of GPS L1 only. Overall, superior performance on both remote sensing and relative navigation applications is observed by adding Galileo to the VTFTFB. GNSS; GPS; Galileo; satellite formation flying; ionospheric remote sensing; hardware-in-the-loop simulation 2019  
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the Gravity Recovery and Climate Experiment (GRACE) mission [3], and the European Space Agency (ESA) Swarm mission [4]. The performance of guidance, navigation, and control (GNC) systems is very critical in SFF missions. As the most ubiquitous modern positioning, navigation, and timing (PNT) technology, the global navigation satellite system (GNSS) is commonly used in absolute navigation and relative navigation for SFF. The navigation accuracy and reliability continuously increase rapidly using multiple global GNSS systems including the American Global Positioning System (GPS), the Russian GLONASS constellation, the European Union's Galileo constellation and the Chinese BeiDou (or COMPASS) constellation. The modernization of these GNSS constellations plus the development of newer generation GNSS receivers with more advanced software algorithms have brought GNSS users into a golden era of multi-constellation GNSS with unprecedented quality of PNT services. Precise relative navigation can be accomplished by using differential GNSS, where centimeter or sub-centimeter level accuracy of short baseline (\(\sim\)1 km) relative position determination in low Earth orbit (LEO) can be achieved by utilizing the single or double differential carrier phase technique [5; 6]. Beside navigation, GNSS is also widely used for ionospheric remote sensing as the Earth's ionosphere dynamically impacts GNSS signal propagation. Multi-frequency GNSS receivers are widely used for ground-based ionospheric remote sensing, such as total electron content (TEC) measurements and GNSS scintillation observations. MIT Madrigal database gathers thousands of GPS stations to create a global TEC map database [7], which greatly benefits the community for ionospheric space weather monitoring and studies.