# LCNS Positioning of a Lunar Surface Rover Using a DEM-Based Altitude Constraint
## 3 Moonlight and LCNS  
The Moonlight navigation service (see Figure 3b for the mission logo) aims to provide a one-way broadcast radio navigation signal to allow a user to compute its position, velocity and time, similarly as done for GNSS (e.g., the signals broadcasted by the Moonlight satellites will be synchronized among them). This concept has been presented in the past (e.g., [30]), but now Moonlight can take advantage of the significant evolution of GNSS technologies both at the satellite/system and user levels. In fact, the Moonlight navigation concept aims to extensively and intentionally reuse GNSS technology in order to achieve fast time-to-market (the current target is to provide the first services in the 2027-2028 time frame) and ease the introduction of the concept within future lunar missions by means of reuse of already existing spaceborne GNSS receivers with minimum modifications. This is achieved by implementing GNSS modulations, navigation techniques and technologies. This concept has been embraced by the NASA LunaNet framework and called lunar augment navigation services (LANS) [8].  
ESA defined 3 phases to establish a lunar radio navigation service:  
* **Phase 1-Use of High-Sensitivity Space Receivers (2022-2025)**: Based on the reception of Earth GNSS signals with high-sensitivity receivers, on-board dynamic filters and high-gain antennas. This phase should provide initial services for Earth-Moon transfers and lunar orbit operations.
* **Phase 2-Lunar Communication and Navigation Services (LCNS) (2025-2035)**: Enhancing the Phase 1 with the transmission of additional ranging signals from both the Moon orbit and surface. This phase should allow a major reduction of the geometric DOP (dilution of precision) and improve the service availability, which, in turn, would translate into enhanced lunar orbit navigation and initial landing/surface PNT (position, navigation and timing) services (e.g., on the Moon's South Pole).
* **Phase 3-Full Lunar PNT System (2035-onwards)**: This phase should provide PNT services on the whole Moon surface and enhance the positioning availability and accuracy in the Moon orbit and on the lunar South Pole. This could be obtained by complementing Phase 2 with additional lunar orbiting satellites and Moon surface beacons. This phase could also allow for the provision of integrity services for safety critical applications, leading to increased autonomy. This contribution focuses on how Phase 2 (LCNS) could support rover surface operations.  
Figure 3: (**a**) Part of the lunar traverse shown in Figure 1 considered in this paper. The terrain topography shown is simulated using the high-resolution DEM provided (5 m/pixel) in the LOLA PDS archive [27]. The left part of the traverse follows the southern rim of the de Gerlache crater, while the right part descends into the direction of the Shackleton crater. (**b**) shows the Moonlight Logo.  
Two parallel Phase A/B1 studies are ongoing at the moment to define the Moonlight system; therefore, this contribution does not aim to provide the definitive constellation or characteristics, but rather to provide an example of what can be achieved with such a system. The results provided in this contribution are considered representative of the future system, even if the actual constellation might not be identical to the one used here.  
As mentioned in previous publications [29; 31; 32], at least during Phase 2 of the Moonlight navigation roadmap, the navigation service will not be able to serve users in every location all the time. Given the limited number of satellites on the initial LCNS constellation (see Section 3.1), the navigation service will be provided in specific time slots for a specific area of the lunar surface. For this reason, user mission analysts will have to plan the critical operations (e.g., final descent of a lunar lander) based on the predicted availability of the service. This process has been described in details in [29; 32], and it is used accordingly in this contribution.