## 1 Introduction  
Global navigation satellite system-reflectometry (GNSS-R) is a rapidly developing approach to Earth observation that makes use of signals of opportunity from global navigation satellite systems (GNSS), which have been reflected from the Earth's surface. GNSS-R using global positioning system (GPS) signals has already proven capable of accurate retrieval of ocean surface roughness and wind speed [1; 2; 3; 4] as well as of sea-ice presence [5; 6; 7]. The technology is particularly promising as it does not require a dedicated transmitter, thereby reducing cost, mass, and power requirements of the receivers. Past and present spaceborne GNSS-R missions (UK Disaster Monitoring Constellation (UK-DMC) [8], TechDemoSat-1 (TDS-1) [9], CYclone Global Navigation Satellite System (CYGNSS) [10], BuFeng-1 [11]) have been focused on collecting reflected GPS signals. In principle, however, the existing receivers can be adapted to the acquisition of navigation signals within a similar frequency spectrum transmitted by other navigation systems (including the European Galileo and the Chinese BeiDou), which potentially allows for a significantly larger number of simultaneous reflections available within the footprint of the GNSS-R sensor, which should translate into higher sampling. The combination of a large number of simultaneous reflections alongside the potential for affordable multi-satellite constellations [12] offers the opportunity to build up an Earth observation system delivering global coverage and high revisit frequency on a relatively low budget. However, enabling GNSS-R sensors to use signals incoming from multiple navigation systems requires extensive adaptation work including signal processing optimization to account for different signal characteristics.  
Geophysical inversion using GNSS signals from constellations other than GPS has already been achieved in the past, including ground experiments exploiting GLObal NAvigation Satellite System (GLONASS) and Galileo [13; 14] and airborne experiments using BeiDou, GPS, GLONASS, and Galileo [15; 16; 17]. A limited number of spaceborne collections of reflected signals transmitted by the Galileo In-Orbit Validation Experiment (GIOVE) were collected by the UK-DMC mission, although hardware limitations led to significant signal attenuation [18]. Multi-constellation sea surface altimetry using both Galileo and GPS has been conducted using raw data from the CYGNSS mission [19].  
During the lifetime of the TDS-1 mission (2014-2019), the receiver aboard TDS-1 was periodically operated in raw acquisition mode (i.e., bypassing on-board signal processing) and raw sampled data were collected and relayed to the ground to be processed later. Some of these collections were timed to include non-GPS reflected signals such as Galileo signals. In this study we present data from seven Galileo-reflected tracks, as well as one BeiDou-3 track. To the author's knowledge, this possibly represents the largest satellite Galileo reflectometry record at the time of writing. An initial analysis of available signal-to-noise ratio (SNR) is presented here, together with a preliminary assessment of the sensitivity of Galileo reflections to surface characteristics.