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THE SPECIAL CASE OF LUNAR SURFACE NAVIGATION NEAR THE POLES

Polar navigation on Earth presents special difficulties for the navigator. These difficulties are largely a result of the unreliability of magnetic compasses near the Earth's poles. Since the weak lunar magnetic field requires the use of gyroscopes as a heading reference rather than magnetic compasses, this cannot be a factor in lunar polar navigation. However, as one approaches the geographic poles, the longitude lines converge rapidly to a point, and this causes problems on both Earth and Moon. If the lunar navigator's heading reference is referenced to lunar coordinates, then problems develop when traversing near either pole. A vehicle traveling at a constant course of, for example, 060¿, will not travel in a straight line, but in intersecting each longitude line at 60¿, will spiral in towards the pole as seen in Figure 13. Note that the desired "straight" course is a great circle, which intersects longitude lines at different angles along its length. A course which intersects longitude lines at a constant angle is known as a rhum line course or loxodrome [Comeaux, 1983].


Figure 13: Rhum Line course vs. Great Circle course near the poles. Notice that the rhum line crosses all longitude lines at a constant angle. [after Comeaux, 1983].

Special methods and techniques, known collectively as grid navigation, help alleviate the problem of converging longitude lines by establishing an arbitrary heading reference set at an infinite distance. All lines pointing to grid north are therefore parallel, and a vehicle following a constant grid heading will travel in a straight line as depicted in Figure 14. A directional gyro aligned to this reference is used to maintain azimuth information. The direction of grid "north" is an arbitrary selection, what matters most is that the same directional reference is used consistently during dead reckoning. On Earth, a common orientation of grid lines is parallel to the Greenwich Meridian, though any consistently applied standard may be used. On the Moon, the 0¿ longitude line bisects the near side of the Moon, and could be used as a standard reference for grid north at the lunar poles.


Figure 14: Polar grid reference system. Note that the course line intersects the grid reference lines at a constant angle.

It is here that the similarity between grid navigation and the low-fidelity model of lunar navigation is strongly apparent, and makes the low-fidelity model very attractive as a polar navigation scheme. Grid lines, like the north-south longitude lines in the low-fidelity scheme, are everywhere parallel. Conceptually, one may consider the low-fidelity navigation scheme described earlier as a form of grid navigation, only using true north as a directional reference rather than grid north.

A major factor to consider in adapting the low-fidelity scheme to polar navigation is the position fixing technique. used. Landmark fixing would still work in polar regions, but low sun angles and long shadows will make many landmarks invisible for long periods. In fact, the interiors of some polar craters are believed to be permanently shadowed [Arnold, 1979], so another fixing scheme, such as celestial, must be adapted to lunar polar navigation to ensure the continued ability to obtain fixes.

The possible existence of water ice in these permanently shadowed lunar regions has excited great interest for planners of future lunar missions. Lunar water deposits would greatly simplify the logistics of large scale lunar bases and are a high-priority exploration subject. Furthermore, the polar regions are a place where access to solar energy is continuously available for lunar operations. We can anticipate, therefore, a significant demand for viable lunar polar navigation techniques.

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