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Unlike the more famous Apollo Guidance Computer I can't find any information about the computer used in the manned lunar rover. It seemed to track three values and display them on analog gauges: the total distance traveled (an odometer), a bearing pointing towards the starting point (the lunar lander) and the calculated distance back to that starting point if traveled in a straight line.

Was it a digital computer like the AGC or did it have more in common with a mechanical calculator? If it was digitial did it use integrated circuits or only discrete components? Or if it was mechanical in nature then did it still require power to operate or did it only need the wheels to spin?

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    $\begingroup$ Not a duplicate of space.stackexchange.com/q/13162/6944 which does not ask about the computer. $\endgroup$
    – Organic Marble
    Oct 9, 2020 at 0:40
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    $\begingroup$ Great question. Fast forwarding, these days do we have any minimal GPS (LPS ?!) sats in orbit around the Moon? $\endgroup$
    – Fattie
    Oct 9, 2020 at 20:31

1 Answer 1

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The rover computer was an analog/digital hybrid system called the Signal Processor Unit (SPU). As explained in answers to this question How did astronauts navigate the Lunar Roving Vehicle (LRV) over the surface of the Moon? the inputs to the computer came from wheel pulse odometers and a directional gyro unit. Here is a block diagram of the system.

enter image description here

The SPU selects the distance increment detected by the third fastest wheel and resolves this increment into northings and eastings (in meters) using the heading input from the DGU synchro transmitter. These resolved increments are accumulated to yield Cartesian coordinates of the vehicle position with respect to the starting point. A Cartesian to polar coordinate transformation is then effected which produces the range and bearing of the vehicle with respect to its starting point.

The analog side of the computer is shown here: enter image description here

And the digital side here:

enter image description here

The system was initialized before the start of a drive by consulting with the ground to determine the true heading based on a "sun shadow device" and attitude angles. The gyro was torqued to the true heading and the system reset, and was then ready for operations.

enter image description here

The system was powered by 36 volts dc from the rover batteries, and internally converted this to 5, 16, and 28 volts dc, and 115 V, single phase, 400 Hz alternating current.

The paper does not describe the type of components used in the digital part of the computer as far as I could tell. Perhaps someone more knowledgeable than I about electronics (which is basically anyone) could tell by looking at the schematic.

Reference: LUNAR ROVING VEHICLE NAVIGATION SYSTEM PERFORMANCE REVIEW

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    $\begingroup$ This is exactly the kind of answer I was looking for. I had assumed it must dead reckoning based on some form of constantly adding a movement vector to a position vector. I also didn't realize that the sun shadow instrument played an active role in initializing it, the information I was able to find only hinted at it being some sort of backup. $\endgroup$
    – David
    Oct 9, 2020 at 16:48
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    $\begingroup$ The "Vectoring Timing & Control" box sure looks like a CPU sequencer, but with no branching in its sequence. It just loops over its micro-program over and over, 36 times per second. I'd love to see the schematic for that sequencer (but not suggesting it belongs in this excellent answer). $\endgroup$
    – Wayne Conrad
    Oct 9, 2020 at 17:39
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    $\begingroup$ An amazing answer here. $\endgroup$
    – Fattie
    Oct 9, 2020 at 20:30
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    $\begingroup$ SSD device .. but what about if it was cloudy ? $\endgroup$
    – Fattie
    Oct 9, 2020 at 20:32
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    $\begingroup$ The mission planners relied on predictions by the Lunar Weather Bureau, a subdivision of the Spaceflight Meteorology Group, to forecast the weather at the chosen landing locations. The launches would have been scrubbed if the forecast was for overcast or even partly-cloudy conditions. $\endgroup$
    – db48x
    Oct 10, 2020 at 12:36

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