Title /
Abstract /
Contents /
Part 1 /
Part 2 /
Part 3 /
Prev./
Next
SYSTEM PERFORMANCE
The LRV navigation system was field tested at Merrium Crater in Flagstaff, Arizona, using a truck mounted with magnetic switches on the wheels to simulate the inputs of the LRV wheel pulse odometers. As in the actual LRV, the gyro heading was initialized with a Sun Shadow Device and a solar ephemeris. Five sorties were made with this truck, each about 20 km long and each satisfied the requirements given in the original NASA contract [Smith, 1973].
An anecdotal description of these tests was given by Paul R. Norris, a Boeing employee of the time who worked on the development of the SPU:
"One of the tests of the nav system was to put it on a jeep down in the Arizona desert. Someone drove a stake in the ground, then they drove the jeep nearly 10 miles away, turned it over to some other men and told them to find the stake. They found it." [Norris, 1971].
Operational performance:
The LRV navigation system was evaluated after each flight, data from the system was tabulated and traverse routes were plotted. As an example, the Apollo 15 traverse II plot is shown in Figure 18. The plot is based on verbal readouts of range and bearing to the initialization point (the Lunar Module) made every few minutes, combined with actual positions as determined by the astronauts. The astronauts are referred to in the figure as the "lunar geology investigation team". Though the final report does not specify techniques or methods, presumably the actual positions were determined by postflight discussion of surrounding features and analysis of in-situ photographs. A summary of navigation system performance is given in Table 1.
Figure 18. Apollo 15 traverse II plot [from Smith, et al., 1973].
Table 1. Apollo 15, 16, and 17 LRV navigation system performance summary [From Smith et al., 1973].
The Apollo 15 traverse II was started with approximately 1�
of heading misalignment. Why this occurred has not been clearly stated. Regardless, the system completed traverse II with a maximum position error of <350m. More seriously, Apollo 16 experienced a failure of the navigation system on its second traverse, after the crew changed the power switch configuration on the LRV. The failure manifest itself by no change in the range and bearing indicators for the last segment of the traverse. Fortunately the system operated normally for traverse III after the power switches were returned to their normal configuration [Smith et al., 1973].
The failure of the Apollo 16 navigation system, while only a temporary problem, points out the need for redundant navigation systems, or perhaps even emergency navigation techniques. This need was perceived prior to the launch of the Apollo 15 mission, and an emergency technique for measuring azimuth to lunar landmarks was developed [Blucker, 1971]. The importance of a reliable lunar navigation system is emphasized further by the difficulty of the Apollo 14 astronauts in finding their way around the rolling hills of Fra Mauro.
The Apollo LRV demonstrated that a relatively simple dead reckoning system can operate in the lunar environment on local traverses with remarkable accuracy. This despite using a low-fidelity model of lunar navigation and only occasional updates to the heading reference. Gyro drift, gyro misalignment, case torquing, and wheel slippage were all listed as contributors to position error, but could not be evaluated quantitatively because of insufficient data [Smith et al., 1973].
Prev./
Next