Source: Tooley et al (2010). Lunar Reconnaissance Orbiter Mission and Spacecraft Design. Link: https://link.springer.com/content/pdf/10.1007/s11214-009-9624-4.pdf
Information on LOLA: https://lola.gsfc.nasa.gov/
Kasprak et al. (2019). The effects of topographic surveying technique and data resolution on the detection and interpretation of geomorphic change.
Determination of Lunar Topography Map
Topographic maps are used for a variety of reasons on earth: selecting regions for construction, knowledge of the surrounding region for boots-on-the-ground exploration, route planning, and potentially understanding where mineral deposits might be. Though many topographic maps have been made of the lunar surface, the finest resolution they currently offer is 25 m in latitude, 1 km in the longitude, and 10 cm vertical (Figure 1). While this is sufficient for most scientific analysis and even rough landing, the tasks that humans will need to perform to actually develop a sustainable lunar infrastructure (such as constructing large pressure modulated structures, creating lunar freeways for long distance traveling, or even simply understanding how much leveling will need to occur to build) will require resolutions on the order of a few meters. Therefore, to develop long term sustainable lunar infrastructure, high resolution lunar topography data is imperative. However, generating the resolution necessary is particularly challenging because of the large data sets associated with a finer resolution map, as well as actually collecting and processing the data (which either comes at the expense of time or requiring data collection across the surface of the moon, rather than orbiting).
One of the first attempts to facilitate the creation of high-resolution topography maps of the lunar surface began in 2009 by National Aeronautics and Space Administration (NASA) with the launch of the Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO) spacecraft (Tooley). LOLA contains a laser transmitter, a receiver, detectors, and a passive radiator (Figure 2). It operates by propagating a single laser pulse at 1064 nm, 28 Hz through a micro-structured pattern that is specifically designed to split the single laser pulse into five beams. The beams travel to the surface from LRO in a 50 km orbit and returns to LRO. LRO interpolates time of flight (ie. Range), pulse spread, and transmit/return energy. This raw data is transmitted back to earth where scientists determined information on surface brightness, surface slopes, surface roughness, and geodetic topography. Though, in theory, it would be possible to generate higher resolution graphs using a device such as LOLA; however, it would require large amounts of fuel and time for orbiting and a large amount of computer processing and software capabilities that are not presently available.
In order to achieve more insight into alternative ways of generating such topography maps, the methods used to generate high fidelity topography maps of earth are explored. There are many tools and techniques for capturing high resolution topography of the earth including: real-time kinematic global positioning systems (RTK-GPS), multibeam sonar, and terrestrial laser scanning (TLS). These methods have allowed for characterization at submeter resolution in both the latitude and longitudinal directions (Kasprak).
RTK-GPS is a satellite-based positioning system that uses signal phase information, as well as information from a reference station, to provide corrections to a time-delay based estimate of location. Using RTK-GPS, centimeter scale resolution has been achieved on the earth. To translate this method to the lunar environment, not only would a satellite need to be orbiting the moon, but a base station would need to be built in a well-defined, well-known location that is robust to the environment on the lunar surface.
As mentioned, another method widely used to generate high resolution topography is multibeam sonar. These systems operate by transmitting a sound pulse at a known frequency and measuring the time between transmission and detection in a receiver. Multibeam sonars use several transmitter and receiver pairs to cover large areas. While this method has worked well at generating topography of the sea floor, it likely would not work on the moon as the density of gas particles is not high enough to propagate sound.
The final method mentioned is terrestrial and airborne laser scanning (TLS). TLS, also known as ground-based lidar, is able to achieve sub centimeter accuracy by using light detection and ranging (LiDAR). TLS works by emitting laser pulses towards distances of interest and generating a point-map from the data. The greatest downside to TLS is the weight of the instrument, energy, and the required proximity to the target. Implementation of such a system would likely require a system that can hover close to the surface of the moon and collect data.
Though equipment currently exists that could be used for RTK-GPS and TLS, the biggest limitations will be in equipping the devices to survive the solar winds and micrometerite impacts. Further, isolating the electronics to protect against the regolith on the surface.
Figure 1. Current High-Resolution Lunar Topography Maps (https://moon.nasa.gov/resources/87/high-resolution-topographic-map-of-the-moon/)
Figure 2. LOLA (source: https://lola.gsfc.nasa.gov/)
NASA’s goals for returning to the lunar surface are two-fold: Answer scientific questions and enable sustainable human dwelling. One of NASA’s strategic objectives directly supports both goals: SCI-A-8 calls for the determination of the stratigraphy, structure, and geological history of the moon. One of the beginning steps towards achieving this objective is developing fine resolution topography, geodetic, and resource distribution maps (< 1 m resolution). Such an objective, and the high-resolution maps that would help achieve this objective, are crucial to the goal of sustainable human inhabitance of the lunar surface because they can help inform hypotheses about weather patterns, resource distributions, desirable locations for building, and pre-processing of the lab before building can take place. With the strict goal of developing a Permanent Lunar Outpost, knowledge of the resources near the site, as well as how much leveling will need to take place are crucial first steps to developing the plans.
Some of the key scientific questions that would be addressed by the development of fine resolution topography, geodetic, and resource distribution maps are mainly related to increasing the information about distributions on the lunar surface and informing potential hypotheses on weather patterns and historical development of the surface. For example, what is the distribution of resources, topography, and roughness on the surface of the moon? Using this information, the following questions might also be able to be answered: what does the distribution of resources, topography, and roughness tell us about the history of the moon? What can be interpolated about current weather patterns on the moon? How consistent are the changes to lunar topography over time?
Determination of high-resolution maps of the lunar surface could be achieved using an orbiter or a rover. When choosing which will best address the need, the trade-off between resolution versus time arises. With a rover, resolution on the order of micrometers can likely be achieved, but such a high-resolution map will require large data storage, time, and algorithms for accurately stitching together the individually obtained maps. On the other hand, an orbiter will likely only be able to achieve resolutions of 1 m (estimated value, depending on how close of an orbit can be achieved), but can do so in a fraction of the time. Further, adopting the use of an orbiter means that Lunar Orbiter Laser Altimeter (LOLA) on Lunar Reconnaissance Orbiter (LRO) can be utilized, which further cuts down cost and time.
As suggested, LOLA on LRO was one of the first technologies to attain high-resolution topography and surface roughness maps of the lunar surface. Though a totally new orbiter could be launched to achieve this goal, the mission concept here proposes a transfer orbit on LRO from its usual 50 km, near-circular, polar orbit to a much closer orbit such as 10-15 km. LRO is equipped to perform station-keeping maneuvers to maintain its orbit, however the technology can also be used to change the altitude of orbit. Such a maneuver has been performed in the past, and a mean altitude of 21 km above the lunar surface was achieved. From this 20 km orbit, LRO was able to achieve 40 m pixel resolution1. Depending on the dynamics of orbit, if a closer altitude orbit could be achieved, such as 10 km, then the tools aboard LRO in LOLA, such as surface roughness, time of flight distance measurements, and reflectance, would greatly enhance the resolution of the currently existing maps. Finally, refraction data from the LOLA might be able to be used to determine patches where water and other resources with known refractive indices are.
The final orbit maneuver that would be necessary to achieve continuous resolution in the map would be to change the plane of the orbit. Such a maneuver would take special consider on the effects of inclination and swivel2.
Feedback: Incorporate Anish’s mission concept as a potential user for the information hoping to be attained by this mission. Further, how will the mission plan to boost the resolution of the lunar topography maps without going down to unrealistically low elevations?
As shown in the science traceability matrix, the main goals of this mission are to aid the development of sustainable infrastructure capabilities and operations on the moon, investigate and mitigate exploration risks to humans, and finally give insight into the impact history of the Earth-Moon system. Focusing on the development of sustainable lunar infrastructure, building lunar infrastructure requires knowledge about the amount of preprocessing on the surface that is required, and therefore the fine resolution localized topography. Such knowledge of lunar topography also proves beneficial to astronauts navigating on the lunar surface, as well as validating hypotheses about the impact history of the Earth-Moon system. Therefore, the first science objective is to obtain topographical resolution maps of the lunar surface to 1 m, with the corresponding observable being topography. The chosen physical parameters (based on the infrastructure available using the lunar reconnaissance orbiter (LRO)) is using time-of-flight data from the lunar orbiter laser altimeter (LOLA) and image data from the lunar reconnaissance orbiter camera (LROC). The corresponding mission requirements are predominantly utilizing multiple revolutions around the Moon to achieve the desired resolution and utilizing the existing framework. Combining data streams from LOLA and LROC will help achieve the mission requirement of 1-10 m resolution topography maps.
In addition to knowledge about the topography for building infrastructure, knowledge about the roughness of a surface that infrastructure will be built on is also crucial. The corresponding science objective is to obtain surface roughness maps down to (but not limited to) 10 m resolution. Similar to the topography map goal, the physical parameters that will be used to observe surface roughness is limited by what is on LRO. LOLA utilizes light scatter to achieve surface roughness data. Like topography mapping, the corresponding mission requirements are utilizing multiple revolutions to enhance certainty in the data and utilizing previous framework.
Finally, for building infrastructure on the lunar surface, knowledge about how far away from necessary resources, such as water, will dictate where the bases will be built. Similar to the previous objectives, the corresponding objective is to obtain surface reflectance maps to 10 m resolution. The observable that is aimed to achieve here is resource distributions, and this data will hopefully be determined using returning light intensity from LOLA.
The top level mission requirement that dictates the feasibility and success of all of these goals is the time-sensitive orbit transfer of LRO.