# LCNS Positioning of a Lunar Surface Rover Using a DEM-Based Altitude Constraint
## 6 Conclusions  
The global space exploration roadmap assessed the gaps of robotic teleoperation in a report called \"Telerobotic control of systems with time delay-Gap assessment report\" [48]. One of the challenges identified is the location accuracy. In the report, the authors refer to Earth-based GNSS as one of the options to overcome this gap but indicate that such a service is not available in the lunar environment. The proposed LCNS system aims to cover this gap, providing accurate ranging signals than can be exploited, in real time, targeting both teleoperated or autonomous rovers as discussed this publication. In this paper, we have shown that by constraining the height of the rover, using highly accurate digital elevation models that are available for the lunar South Pole, both the availability and accuracy of a lunar surface rover can be substantially improved. Furthermore, the proposed navigation solution operates under all illumination conditions and allows for a very repeatable and autonomous traverse: after the first traverse is performed, all subsequent traverses can be done fairly autonomously following the same path (as a path free of obstacles is found). Using the available DEMs, the rover is able to continue to estimate its position while only 3 satellites are in view (contrary to the 4 satellites that are required without such a height constraint). This increases the availability of the navigation solution from 52% to 82%. Furthermore, by substantially reducing the HDOP by providing an additional measurement in the vertical direction (i.e., a direction which is typically poorly constrained for a satellite-based navigation system), the 3-sigma horizontal covariance is reduced from more than 800 m (considering the 99.7 th percentile of the 3-sigma horizontal covariance) to less than 100 m, with very conservative assumptions for the IMU grade and the ODTS uncertainty. A sensitivity analysis showed that an improvement of the LCNS ODTS accuracy does not have a major impact on the final performances (i.e., an improvement of the 3-sigma horizontal covariance by 8 m reducing the ODTS error by a factor of 3). An improvement of the IMU equipment, however, has a significant impact on the attainable performances, in particular when less than 4 satellites are tracked by the rover. This is expected as the IMU governs the process noise and the capability of the EKF to tighten the predictions and thus the final performances. Considering both the improved ODTS accuracy and a navigation-grade IMU, the 99.7 th percentile of the 3-sigma horizontal covariance can be reduced below 10 m (i.e., 8.6 m), allowing compliance with very stringent user requirements for surface-moving rovers (e.g., the NASA Lunar Communication Relay and Navigation services requirement LCRNS.3.0570, [49]). Note that the accuracy provided in this publication (3-sigma horizontal covariance) provides a first-order estimation on the type of accuracy that can be expected for a lunar rover using LCNS and DEMs to determine its position. Considering the navigation-grade IMU, the final user position is mostly limited by the uncertainty on the DEM data component. This is intrinsic to the DEM data, and in principle, LCNS cannot provide a direct support to reduce it. However, LCNS can support future LOLA-like instruments, providing precise ranging data to improve the precise orbit determination of the satellites hosting the altimeter instrument and iteratively improve the DEM accuracy. Even if variance-covariance analysis has some limitations (e.g., biases are not modelled), the use of 3-sigma in all error contributions allows to overbound stochastically the user accuracy capable of including a certain margin of biases coupled with the corresponding 1-sigma standard deviation. This publication has shown that a simple 4-satellite constellation in lunar orbit and a user LCNS receiver that account for DEM and IMU measurements allow to achieve performances below 10 m 99.7 percent of the time over the traverse considered in this publication.