# LCNS Positioning of a Lunar Surface Rover Using a DEM-Based Altitude Constraint
###### Abstract  
With the renewed interest in lunar surface exploration, the European Space Agency envisions to stimulate the creation of lunar communications and navigation services (LCNS) to enable, among others, autonomous navigation capabilities for lunar rovers. As the number of satellites foreseen in such a service is much smaller compared to Earth-based global navigation satellite systems, different complementary technologies are pursued to improve the attainable navigation accuracy for lunar rovers. One way to improve the position accuracy provided by the LCNS satellites is to constrain their vertical position using a high resolution digital elevation model (DEM). This article presents the results of a variance covariance analysis of an extended Kalman filter implementation in which the LCNS ranging measurements are used together with the altitude provided by a DEM from the Lunar Orbiter Laser Altimeter instrument of the Lunar Reconnaissance Orbiter. Assuming a realistic orbit determination and time synchronization accuracy of the LCNS satellites, the usage of a navigation-grade inertial measurement unit and an oven-controlled crystal oscillator, a 3-sigma position accuracy of less than 10 m can be obtained. Furthermore, the availability is substantially improved as the DEM-aided solution enables a position solution in case of only 3 visible satellites. D 2022143942202214394221439422143942  
initiatives, NASA has proposed the LunaNet framework to enable interoperability among different lunar communication and navigation service providers [8]. The challenge of accurate navigation for lunar surface users (i.e., rovers) has been of interest since the early robotic explorations of the Moon [9] and typically relies on technologies such as dead-reckoning, using, among others, inertial navigation systems (INS) [10], wheel odometry and visual odometry (VO) [11; 12; 13] for terrain relative localization. The advantage of visual navigation is its passive nature [14] (as it uses visual light) and the imagery can be used for hazard avoidance and traverse planning [15]. Dead-reckoning technologies, however, are not suitable for long traverses as the error grows without bounds [16]. For absolute positioning, planetary rovers relied on \"ground-in-the-loop\" processing of down-link images in which on-board images where interactively registered (i.e., mapping between local and global maps) to orbital imagery [17]. Such an approach, however, is impractical for autonomously operated vehicles. Furthermore, the attainable absolute accuracy is determined by the resolution of the global (orbital) maps [18]. Nevertheless, recent developments may allow autonomous processing and registering of global and local imagery or 3D point clouds [15]. Rover images can also be used to derive local digital elevation models (DEM). When the global DEMs are available on-board the rover, they can be correlated against local DEMs to provide a global position [19]. However, variation in the solar illumination angle may lead to a change in the terrain appearance which complicates the image registration [17]. LIDAR (light detection and ranging) can be used to remove the dependency on surface illumination conditions [16]. Tests on Earth in a Mars-Moon analogue environment when using 3D LIDAR scans for matching against a 3D orbital map for global navigation (complemented by visual odometry and an inclinometer/Sun-sensor) resulted in position errors smaller than 100 m [16]. Finally, craters can be used as landmarks to yield a global navigation solution [17] obtaining high positioning accuracies (5 to 10 m). Nevertheless, the position accuracy is determined by, among other things, the size of the crater and illumination conditions. Alternatively, rovers can use radio frequency (RF) tracking techniques [20]. Current radiometric tracking of surface users relies mostly on a direct-to-Earth link, which has some intrinsic limitations. First of all is the impossibility to serve far-side missions.