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PART II: LUNAR SURFACE NAVIGATION IN PRACTICE
Experiments in landmark navigation for lunar applications were performed in 1969 by the Jet Propulsion Laboratory. In two separate studies, the researchers performed landmark navigation exercises in the western Mojave Desert. A truck was fitted with television cameras and monitors to display the surrounding landmarks to the navigators inside the truck. The experiments were fairly extensive, involving both navigation and geological reconnaissance. The navigators were told to determine their position based on landmark identification and manually plotting azimuths to the landmarks on a chart. In the first study, fixes of relatively high accuracy were accomplished, but accurate identification of landmarks was found to be a crucial factor. The second study was careful to point out that tests on Earth do not imply feasibility on the Moon, since lunar features may not be "adequately cooperative". These conclusions proved prophetic.
Apollo 14 astronauts Alan Shepard and Edgar Mitchell were the first to experience difficulty in identifying lunar landmarks. Mitchell noted soon after landing that the terrain upon which the lunar module sat was gently undulating, exhibiting more vertical relief than had been anticipated. During their second Extra Vehicular Activity (EVA), a traverse up the gentle slope of Cone Crater approximately 1,000 meters from the lunar module (LM), the astronauts had great difficulty determining where they were based on the maps they were using. This was attributed to the rolling terrain obscuring landmarks, and also to their line of sight relative to the sun. Outbound from the LM, the sun was at the astronaut's backs; shadows were hidden by the objects producing them, and salient features such as craters were difficult to see. Ultimately, Shepard and Mitchell never found Cone Crater, and had to return to the LM. Later investigation showed that they had actually come within a few meters of the rim of the crater, but had never actually seen it.
A higher vantage point would likely have allowed Shepard and Mitchell to see their objective clearly, but does not guarantee landmark identification in all instances, as the following account of the landing of the Apollo 15 LM, Falcon, illustrates:
"At about 9,000 feet (2,750 m) above the surface Scott [David Scott, Apollo 15 Commander piloting the LM] noted the peak of Hadley Delta to his left; until he reached 5,000 feet (1,500 m) the only other landmark he could spot was Hadley Rille. The terrain was less sharply defined than he had expected from simulations." [Compton, 1989].
As in Apollo 14, the lunar surface features did not exhibit the sharp angularity we are accustomed to seeing on Earth. Rather, the surface is more rounded due to constant erosion by micrometerorites.
Apollo 15 was the first mission to carry a lunar roving vehicle (LRV) to the Moon. The LRV navigation system is described in detail in following pages. Suffice here to say that the navigation system provided the astronauts with continuous range and bearing to the LM throughout their traverse. Astronaut Scott and Jim Irwin, like Shepard and Mitchell, found landmarks not always easy to identify, but with knowledge of range and bearing to the LM supplied by the rover's navigation system, were able to maintain a continuous position reference and achieve their various destinations.
The conclusions of scientific experiment and the experience of astronauts navigating on the Moon show significant evidence that lunar landmarks cannot be depended upon as the sole source of navigation information. Correct identification of landmarks is critical to this technique, but lunar surface features are generally not sharply defined. This is due to the gently rolling nature of the lunar terrain and the lack of shadows when observing the lunar surface under certain Sun angles.
THE APOLLO LUNAR ROVING VEHICLE
Smith, et al. , provides the most comprehensive review and evaluation of the Apollo Lunar Roving Vehicle (LRV). In October of 1968, NASA awarded the Boeing company a contract for LRV construction. The LRV, shown in Figure 15, was intended to enhance the exploration of the Moon by allowing astronauts to examine a wider area than possible on foot in the limited EVA time available. The LRV's navigation subsystem was required to provide the astronauts information necessary to return to the Lunar Module (LM) by the shortest route, the total distance traveled, the vehicle speed, and the ability to navigate to a predetermined site.
Figure 15. The Apollo 17 Lunar Roving Vehicle (LRV) [from Cortright, 1975]
The navigation display electronics, shown in Figure 16, consisted of an Integrated Position Indicator (IPI), and a speed indicator. Information necessary to initialize the navigation system was given by a Sun Shadow Device (SSD) and a combined roll and pitch attitude indicator. The SSD was hinged at the top and included a needle on the free end to cast a shadow on the indicator.
Figure 16. LRV navigation display [from Smith, et al., 1973].