# Laser Observations of GALILEO Satellites at the CBK PAN Astroegodynamic Observatory in Borowiec
## 3 GALILEO Constellation  
The GALILEO system currently has 28 satellites (Figure 3). The constellation creates two different types of satellites based on their operational nature. The first type is the In-orbit Validation (IOV) satellites, which consist of 4 satellites: E11 (G101), E12 (G102), E19 (G103), and E20 (G104). The second type comprises the remaining 24 satellites, known as the Full Operational Capability (FOC) satellites. Figure 4 depicts an artist's representation of the FOC satellite. The GALILEO constellation was divided into three groups due to orbital parameters, orbiting the Earth in circular MEO orbits at an altitude of 23,222 km with an inclination of 56 degrees. Out of the entire GALILEO constellation, two objects failed to reach their intended orbits and instead ended up orbiting the Earth in eccentric orbits with an inclination of 50 degrees. These are the first pair of the FOC GALILEO satellites, E14 and E18, which were launched in 2014 aboard a Soyuz ST rocket, also known as GALILEO objects GSAT-201 (GALILEO 5, Doresa) and GSAT-202 (GALILEO 6, Milena). All GALILEO satellites are equipped with retroreflectors [12], but the IOV and FOC groups differ in the number, size, and material of the retroreflectors they are made of. IOV satellites have 84 Corner Cube Retroreflectors (CCRs) measuring 33 mm \(\times\) 23.3 mm, made of doped fused silica (Suprasil 311). Satellites from the FOC group have 60 CCRs measuring 28.2 mm \(\times\) 19.1 mm, made of fused silica. Figure 2: Number of tracked objects by BORL station in years 1992–2023. It has been shown in a paper [19] that kHz laser ranging measurements are important for determining the attitude of GALILEO satellites. By measuring the angle at which the laser beam hits the satellite's panels to the tracking station, the satellite's attitude can be determined. It has been found that the FOC GALILEO satellite panels have irregular reflection patterns because of the uneven distribution of retroreflectors in this group of GALILEO satellites [20]. The GALILEO constellation is managed by the EUSPA based in Prague. The GALILEO system offers various high-performance services:  
* Open Service (OS);
* High-Accuracy Service (HAS);
* Public Regulated Service (PRS);
* Search and Rescue Service (SAR);
* Open Service Navigation Message Authentication (OSNMA);
* Commercial Authentication Service (CAS);  
Figure 4: Artist’s view of a GALILEO Full Operational Capability (FOC). The green circle marks the panel with laser retroreflectors [18]. Figure 3: The constellation of GALILEO satellites [17]. GALILEO Emergency Warning Satellite Service (EWSS). The GNSS market is growing as more devices and services are being developed using signals from GNSS satellites. According to the EUSPA report, global revenues from GNSS operations are projected to increase from EUR 260 billion in 2023 to around EUR 580 billion [21]. These figures demonstrate the significant impact of GNSS technology on the global economy. Additionally, the GNSS technique and the GALILEO constellation play a crucial role in scientific development by supporting the implementation of the International Terrestrial Reference Frame [22] and the determination of Earth Rotation Parameters (ERPs), which necessitate highly precise satellite orbits [23; 24; 25]. However, there is inconsistency in determining the orbits of GNSS satellites using single techniques. Therefore, combined solutions (GNSS + SLR) are being proposed. According to a study by [26], the bias of the Length-of-Day parameter is 20% lower in the combined solution compared to the microwave technique. Another significant scientific aspect related to the operation of the GALILEO constellation is the study of general relativity. While these satellites may not be suitable for satellite navigation applications, they are effectively utilized in other scientific research and advanced physical experiments related to general relativity [27; 28].