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Appendix 5B:
LMF Positional Transponder System

According to the baseline mission for a growing, self-replicating Lunar Manufacturing Facility (LMF) presented in section 5.3.4, a 100-ton seed is dropped to the lunar surface and thereupon unpacks itself, sets up the initial factory complex, and then proceeds to produce more of itself (or any other desired output). Clearly, the level of automation and machine intelligence required lies beyond current state-of-the-art, though not beyond the projected state-of-the-art two or three decades hence. Because of the already challenging design problem, it is highly desirable to keep all seed systems as simple as possible in both structure and function. This should help reduce the risk of partial or total system failure and make closure less difficult to achieve at all levels.

One of the more complicated pieces of hardware from the Al standpoint is the "camera eyes" and pattern recognition routines (visual sensing) that may be needed. Although it is possible that standardized robot camera eyes may be developed, it is more likely that each particular application will demand its own unique set of requirements, thus greatly reducing or eliminating any gains in simplicity of camera design. The pragmatic industrial approach (Kincaid et al., 1980) and design philosophy in these cases, especially in the area of computer vision, is to: (1) simplify, (2) use unconventional solutions, and (3) "cheat (i.e., solve another problem). It may be that the best way to handle the problem of computer vision is to find a way to largely avoid it altogether.

When the seed unpacks itself it opens into a rather wild environment full of hills, bumps, ledges, crevasses, boulders, craters, and rocks. Surface navigation by mobile robots will be a serious challenge to Al technology. How will a machine know where it is, what the terrain ahead may be like, or how to get home? Laser tracking is one possibility, but probably too complicated when out of line of sight. Pattern recognition of geological and geographical landmarks is another possibility, but there are at least three serious deficiencies associated with this solution. First, the pattern recognition routines must be extremely sophisticated and the sensor very high in resolution and in the ranges of illumination that may be accommodated. Second, to recall how to get home after a lengthy perambulation across the lunar surface may require vast amounts of onboard computer memory. Every turn, every detour, every move the robot makes must be recorded, analyzed for spatial displacement geometry, and the present-position pointer augmented against the stored features maps and correlated with the geographic images received through the vision sensors to plot the shortest route home to avoid the inefficiency of retracing the original physical path. Third, since exploration, development, and construction operations are always in progress around the site, each robot would need a memory capacity sufficient to recall in detail all changes in the landscape between the last series of explorations and the present one - the view is always changing. It may not be practical to design this much Al into each mobile robot, nor to require the central computer to exercise full teleoperator control of a large fleet of nonautonomous mobile robots.

5B.1 The Transponder Network

One way to achieve accurate positioning of all mobile robots while retaining their navigational autonomy is to employ a transponder system operating in the gigahertz frequency range. Much like the LORAN and NAVSTAR systems on Earth, these radar beacons would permit the accurate determination of position by simple triangulation for mobile robot devices located anywhere in the vicinity of the seed. A frequency of perhaps 30 GHz, easily within the range of current technology, would be required for 1-cm positioning accuracy. The transponder system could be orbital-based, but for the present design a ground-based system has been assumed with at most a single satellite for purposes of initial calibration.

When the seed unpacks, its first task is to unfurl the "home base" transponder. Power consumption has not been examined in detail but should not exceed 100 W, the amount supplied by a 1 m2 solar panel. The next step is to establish an accurate navigational baseline between the home transponder and a reference transponder some distance away, perhaps using a relatively simple nonlaser surveyor's transit. A second baseline is similarly established in some other direction, and the whole system then calibrated and synchronized to coherence. Thus deployed, a local radio navigation grid exists which can fix the position of any appropriately equipped receiver to within 1-cm accuracy, horizontally or vertically, anywhere near the seed.

Since the transponder operates on line-of-sight, each transmitter must be placed a certain distance above the ground in order to "see" the entire area for which it is responsible. The general horizon distance formula is X = (h2 + 2hR)1/2, where X is the distance to the horizon, R is lunar radius, and h is height of the observer/transmitter above ground. Horizon distances for the Moon are given in table 5.8, neglecting surface irregularities.

Table 5.8.- Horizon Distances For The Moon

Observer height h, m Horizon distance X, km
1.0 1.9
2.0 2.6
3.0 3.2
4.0 3.7
5.0 4.2
10.0 5.9
15.0 7.2
20.0 8.3

As the original facility grows the transponder network also must be expanded. At the very minimum, a mobile robot should remain in communication with at least three noncollinear beacons to accurately fix its location. (The problems of feature shadowing and unit downtime may require the use of four or five stations. The exact number and layout can only be determined after the specific landing site has been selected and mapped from orbit. One possible deployment geometry is a grid of equilateral triangles with sides roughly equal to the desired horizon distance, with transmitters at the vertices. For example, the triangle pattern edges should be roughly 2.6 km if 2-m high antennas are used. This ensures that the range circle of any mobile robot receiver always will encompass at least three transponder units, thus permitting high-accuracy triangulation. (See fig. 5.33.) Depending on the maximum size of the mature LMF and the maximum feasible height for transponder antennae, the number of transmitters necessary to support the growing seed may range from the tens up into the thousands.

Figure 5.33. -
Range circles for mobile robots using LMF transponder network for navigation.

In any case, the main seed computer may be presumed to carry lunar topographical maps of the landing locale, assembled prior to landing and accurate to l-m resolution, in hard memory. This knowledge, plus the accurate positional information provided by the transponder network, should help to eliminate surprises at the expanding LMF site and lessen the need for a highly sophisticated "intelligent" vision-based surface navigation capability.

5B.2 References

Kincaid, William et al.: Summer Study Background Briefing on Computer Vision, Fault-Tolerant Systems, Large Space Structures and Antennas. Lockheed Missiles & Space Company, 7 July 1980.