I read that the astronauts who used Lunar rovers used heading indicators, which typically use compasses, for direction. How, on the Moon, did their equipment tell the direction of travel, presuming a compass made for earth would not work on the Moon?
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$\begingroup$ See the answer to this question $\endgroup$– UweJul 30, 2019 at 14:51
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$\begingroup$ See What is a gyrocompass and how might one be used by a planetary rover? as well as During its final descent, how will the InSight lander know cardinal directions in order to land with proper orientation? and also Uwe's link How did astronauts navigate the Lunar Roving Vehicle (LRV) over the surface of the Moon? $\endgroup$– uhohJul 30, 2019 at 21:44
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1$\begingroup$ You might explain why you think they'd need a compass for navigation. They have the sun and the Earth as references, perhaps also the stars - though I don't know how visible the were through the faceplates of their suits. They also have the terrain, which I would assume they'd have studied extensively in preparation for the mission. $\endgroup$– jamesqfJul 31, 2019 at 5:04
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$\begingroup$ Heading indicators (in aircraft) typically use gyroscopes, not compasses. At least in light aircraft, we have to reset them from our magnetic compass each time we start the plane. $\endgroup$– reirabJul 31, 2019 at 21:18
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2 Answers
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The Lunar Roving Vehicle did have a (form of) compass. It was gyroscopic rather than magnetic, thus it needed calibration when first powered up using the sun angle as a reference. It's in the upper left of the console here:
According to Wikipedia:
Navigation was based on continuously recording direction and distance through use of a directional gyro and odometer and feeding this data to a computer that would keep track of the overall direction and distance back to the LM. There was also a Sun-shadow device that could give a manual heading based on the direction of the Sun, using the fact that the Sun moved very slowly in the sky.
I presume the "bearing" display in the middle of the compass is the direction to travel back to the LM. Sun-shadow navigation was helped by the fact that the sun was low in the East during all the lunar EVAs.
answered Jul 30, 2019 at 14:47
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1$\begingroup$ In aviation it says that pilots use the Directional Gyroscope in-flight, but occasionally re-calibrate it (when flying without acceleration at a flat altitude or before/after take-off) to what the compass says (as the compass is prone to errors if not flat/constant velocity). How did they initially calibrate it in the absence of a compass? Just pointed the vehicle towards the LM and used the LM's location as polar North? $\endgroup$ Jul 30, 2019 at 17:40
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2$\begingroup$ I'd guess either the sun-shadow indicator, or the inertial platform of the LM, would be the reference. Pointing at the LM would not be a good choice, because the direction toward the LM changes wildly as you move around near it. Any arbitrary direction you chose would work for navigation, but it would be annoying to correlate your arbitrary direction reference to e.g. lunar orbiter maps later. $\endgroup$ Jul 30, 2019 at 17:59
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9$\begingroup$ Per the document linked in my answer"Gyro heading initialization was accomplished by means of an extremely simple sun shadow device and vehicle attitude indicators" $\endgroup$ Jul 30, 2019 at 18:01
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$\begingroup$ "The Lunar Roving Vehicle did have a compass." I think that's usually called a "Heading Indicator," not a compass. From the picture, it appears to be a standard aircraft Heading Indicator, which is gyroscopic in operation, not magnetic. When we fly an airplane (at least the light GA ones; I can't speak for larger ones with fancier avionics,) we have to reset the HI using the magnetic compass each time before flight. It's part of the Before Takeoff checklist. $\endgroup$– reirabJul 31, 2019 at 21:25
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$\begingroup$ I'll change the wording to "a form of compass", which Wikipedia backs up. $\endgroup$ Aug 1, 2019 at 23:18
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The rovers used a gyroscope-based navigation system.
The Lunar Roving Vehicle (LRV) navigation system consists of a directional gyro, a set of incremental odometers, and a hybrid analog-digital signal processor plus appropriate controls and readouts.
Info from Lunar Roving Vehicle Navigation System Performance Review, (NASA Tech Note D-7469)
answered Jul 30, 2019 at 14:50