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The lunar surface presents two basic terrain types; relatively flat maria, and the rougher lunar highlands; which present further considerations for the lunar navigator [Buratti, 1989]. The lunar maria exist mostly on the near side of the Moon so the navigator traversing these lunar "seas" will usually have access to the Earth for navigation information. The flat featurelessness of the maria mean that there will be little to block lines of sight, either for celestial measurements, radio transmission or reception, or for identifying and measuring locations of surrounding landmarks such as mountains or crater rims. On the other hand, many areas of maria are, for all purposes, devoid of unique landmarks for use in navigation, as seen in Figure 3.

The lunar highlands are characterized by heavily cratered, mountainous terrain which may provide more landmarks than the maria (see Figure 4). But a navigator working in rough highland areas could find lines of sight blocked by mountains or crater walls and hence be somewhat limited in external sources of information. Also, difficulty in identifying landmarks due to the gently rolling nature of lunar highlands has proven a significant problem for Apollo astronauts [Apollo 14, 1971].


Figure 3. Typical Mare region viewed by Lunar Orbiter. The few available landmarks are usually ambiguous and would be difficult to identify from the surface [from Gutschewski et al., 1971, Plate 150-1].


Figure 4. Typical highland region located between the crater Lade and Sinus Medii [from Musgrove, 1971].

Finally, while the Moon exhibits some magnetism, it is quite weak and relatively poorly charted. Magnetic compasses would not function, and other direction references must be used [Buratti, 1989].

Many navigation schemes require the measurement of local vertical. The only way of accomplishing this measurement without knowing current position is to use a vertical sensing device. this may be some type of spirit level or pendulum, which uses gravity as a vertical reference [Hung, 1970]. Unfortunately, lunar mass-concentrations (Mascons) can slightly disrupt these measurements and thereby induce error. This could be compensated for with knowledge of the vehicle's approximate location and the locations of lunar mascons [Albee, 1989].

Finally, the lunar coordinate system is essentially the same as that of Earth; lines of longitude and latitude referenced to the axis of rotation. Using the right-hand-rule for both Earth and Moon, "north" is in the direction of the vector of angular momentum, and "east" is in the direction of spin. The lunar prime meridian is centered on the near side of the Moon.

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