###### Abstract
Space-compliant GNSS receivers are also utilized to conduct space-based GNSS sounding projects, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission [8]. The key technique implemented by the COSMIC mission is radio occultation, where the bending effect of the GNSS signals propagating through the ionosphere is utilized to retrieve the electron density (_Ne_) of the ionosphere, measure GNSS scintillation, and produce other atmospheric sounding data. Some recent CubeSat missions also apply this radio occultation technique to measure ionospheric scintillation and sense ionospheric irregularities as well, such as the Compact Total Electron Content Sensor (CTECS) [9] and the Scintillation Prediction Observations Research Task (SPORT) [10]. The Coherent Electromagnetic Radio Tomography (CERTO) constellation of radio beacons were developed to fly on LEO satellites to measure TEC, scintillations, and plasma irregularities below the satellite orbit together with ground-based beacon receivers [11]. A pioneering project, called the Ionospheric Observation Nanosatellite Formation (ION-F), proposed and discussed the potential application of spacecraft formation flying to ionospheric measurement [12]. GNSS-based LEO SFF opens new doors for ionospheric remote sensing. For satellite missions, a simulated platform to test the functionality of all the key hardware and software systems is required to validate the mission feasibility before launch. Hardware-in-the-loop (HIL) simulation testbeds are an ideal platform during research and development phase to prototype GNSS algorithms and assess GNSS receiver(s) performance in various emulated scenarios of GNSS-based SFF. The first of this kind of simulation testbed was established by United States' National Aeronautics and Space Administration (NASA) at 2001 [13], and later on being used to support several formation flying missions (e.g., MMS) development. The German Aerospace Center (DRL) [14] and a few other universities (e.g., Stanford University [15], Massachusetts Institute of Technology [5], University of Toronto [16], and Yonsei University [17]) have also been involved in this type of GNSS-based SFF simulation testbed development. However, most of these simulation testbeds are primarily applied to GNSS algorithm development rather than ionospheric remote sensing missions. A GPS-based simulation testbed for two-satellite formation flying, the Virginia Tech Formation Flying Testbed (VTFTFB), was recently developed with applications to ionospheric remote sensing [18]. An ionospheric plasma bubble observation scenario was incubated and verified on the VTFTFB by running HIL simulations. Vertical _Ne_ profiles can be retrieved with differential GPS vertical TEC, and simulated ionospheric irregularities can be investigated by analyzing _Ne_, TEC, and amplitude scintillation measurements. The purpose of this study is to describe the extension of the VTFTFBinto a multi-constellation simulation testbed by adding Galileo. Various benefits come with the addition of Galileo, which includes, but is not limited to, more signal and data types, better spatial and temporal coverage, higher navigation, and remote sensing accuracy. This is advantageous to multi-scale observation and multi-constellation GNSS comparisons. Compared to a GPS-only scenario, applying multi-constellation GNSS will enhance the capability of both ionospheric remote sensing and navigation with the additional Galileo satellites and signals. Twenty-two (22) Galileo satellites were operational as of March 2019 with more to be launched. The historical improvement on ionospheric remote sensing due to the growing Galileo constellation can be quantitatively evaluated by simulating the Galileo almanac at different times. The hardware and software infrastructure of the new VTFTFB are overviewed in Section 2. The ionospheric measurement and navigation results from HIL simulations are presented, analyzed, and discussed in Section 3.