# Design of CubeSat-Based Multi-Regional Positioning Navigation and Timing System in Low Earth Orbit
## 2 Materials and Methods
### Mission Statement  
The purpose of the mission is to design a LEO CubeSat Constellation that has the capability of providing persistent PNT signals in specific areas of interest, with the size of a country, around the globe. This shall be achieved by the CubeSats relying on the MEO GNSS for orbit determination and time synchronization (ODTS), facilitating a multi-tier architecture [6]. Time and frequency synchronization to existing GNSS constellations ensures compatibility and interoperability. No dedicated ground segment is needed for timekeeping, while the requirements for space infrastructure are kept at a minimum. Based on the heritage from the ERMIS CubeSat mission, the spacecraft (S/C) shall mostly consist of GOMspace components, while the ADCS shall be from CubeSpace. The ERMIS mission, led by the UOA (University of Athens), is Greece's first small satellite constellation and is part of the Greek CubeSat in-orbit validation projects. It consists of one 8U and two 6U CubeSats and aims to demonstrate technologies like 5G IoT connectivity, inter-satellite links, Earth observation applications, on-board data compression, and optical laser communications [7]. Following the philosophy described above, the PNT payload housed onboard shall consist of a multi-frequency GNSS receiver along with an antenna, a CSAC, a navigation computer, a transmitter, and an additional antenna to transmit the PNT signals. Like the ATOMIC payload, this architecture allows for the GNSS data to be processed through a real-time navigation filter to achieve accurate ODTS, while the S/C's predicted trajectory is used to create its ephemeris. The CSAC plays a critical role as the onboard time and frequency reference. Its phase and frequency are consistently adjusted to align with the GNSS broadcast time, using the onboard system. A calibration signal ensures that the signal generator's phase is fully synchronized with GNSS. This enables the generation of GNSS-synchronized navigation and ranging signals, which can be used alongside GNSS signals for single point positioning [4]. This kind of system can provide services to an unlimited number of users. Additionally, Satellite Laser Ranging (SLR) shall be used to confirm the satellite's orbit when it passes over an SLR-capable ground control station (GCS). Furthermore, COTS components with publicly available datasheets that contain the necessary information for the system design shall be preferred. The mission's ground segment shall consist of UOA's GCS, along with other GCSs in Greece that are available as a service. The constellation shall be deployed through multiple launches, requiring consideration of a range of different launch providers. Small launch  
Figure 1: Architecture of a CubeSat-based PNT payload in line with New Space. vehicles should be an option to consider in case the desired orbit is rarely available through Rideshare launches. Also, separate launches are considered necessary for S/C scheduled for different orbital planes. The number of S/C in the constellation shall be kept as low as possible. Each S/C shall undergo LEOP (Launch and Early Orbit Phase), a Commissioning phase, and a Payload validation phase before starting its Nominal operations phase. Each satellite shall have an operational lifetime of at least 3 years. After the end of the mission, the S/C shall be deorbited according to the relevant regulations and laws in place. As stated above, the system is aimed to provide persistent PNT services to specific areas/countries. This design allows for service in multiple areas in similar latitudes while minimizing the payload duty factor. In this paper, the focus is the European region; thus, the area is selected to be Germany since it is a Central European country of substantial area (\(\sim 357,600\) km\({}^{2}\)) where many major metropolitan districts are located. Overall, such a satellite system would enhance current PNT services over Germany by implementing strong signals from LEO and new geometries that are not achievable by the MEO GNSS.