While reading about the Chinese lunar rover Yutu wandering the lunar surface, I heard a comment about the moon’s magnetic field being weak, variable and lacking a north/south orientation and because of this a magnetic compass would be unreliable.
I wondered, without the ability to use a compass, how did astronauts not get lost when driving the Lunar Rover on the moon? While nowadays there are many solutions to this problem, they simply didn’t exist in a lightweight, portable package in the 1960′s. Very indicative of the space race era, the rover’s designers found an elegant solution using old school visual sky measurements, basic geometry, and computers.
Apollo Lunar Roving Vehicle
The Lunar Roving Vehicle (LRV) was a sporty, two seat, four-wheeled, electric dune buggy that was folded into a triangular storage bay on the exterior of the Apollo 15, 16, and 17 Lunar Modules (LM). Once on the moon, the astronauts deployed and unfolded the LRV and then roved off into the lunar sunset. The LRV greatly expanded the exploratory range of the astronauts, increasing the variability of lunar rock samples brought back to Earth. It’s not surprising then that some of the most important geological information we learned about the moon came from those three missions.
LRV Navigation System
The LRV navigation system consisted of three main components working together: a directional gyro (DG), odometers, and a mini-computer known as an analog/digital signal processing unit (SPU).
The directional gyro was a Lear Siegler Model 9010, similar to the kind found in high-end aircraft of that time, that displayed heading information on a rotating compass rose. The odometer system consisted of nine magnets attached inside each wheel rim that sent rotational data to the SPU.
The SPU, after negating any slipping wheel information, calculated and displayed total distance traveled. The SPU also used signals from both the directional gyro and odometer to determine range and bearing back to the LM. All of this information was visible on a single compact display called the Integrated Position Indicator found on the center control console (see above pic).
But wait. The directional gyro displays heading information on a rotating compass rose? I thought you couldn’t use a compass on the moon? DG’s don’t rely directly on Earth’s magnetic properties, but maintains which direction you are headed due to principles of angular momentum. Electricity (or suction in some aircraft models) keeps the gyroscope inside it spinning and when you turn, angular momentum keeps the reference arrow or airplane on whatever you have in the center of the DG, pointing upward as the compass rose revolves around it. This property allows you to constantly see which direction you are traveling in.
The trick is: the DG doesn’t naturally know which way is north. You have to set it manually (see the Block Diagram above where it says “Manual Torquing” above the DG block), usually in reference to a compass showing magnetic north. Argh. But there is no lunar magnetic north. This is where low tech meets high-tech with the Sun Shadow Device (SSD) mounted on the center console. Before wandering off into the lunar highlands, the astronauts positioned the LRV with the sun directly behind them.
The SSD was then unfolded, casting its shadow on a horizontal linear scale found on the console. The astros reported where the SSD shadow was on the scale, along with LRV roll and pitch information, to the mission control center (MCC) in Houston. Using those values, along with sun and star positions reported earlier, MCC calculated which direction was lunar north. MCC then reported back to the astronauts what to dial in on the DG compass rose and they were on their roving, rock collecting way.
Well, not really. While the navigation system was tested on Earth, it had never been used on the moon, so several safety precautions were built-in to the LRV missions: landing areas were chosen to have large, identifiable landmarks and the crew trained to identify them; traverses never went so far that they couldn’t see the top of the LM from atop a landmark (calculated to be maximum of ~9.5 km); and always making the first leg of the traverse the longest. This way, if the LRV failed, the astronauts could always locate the LM and walk back with ample life support.
Lastly, there were cameras and antennas on the LRVs that allowed controllers in the MCC to act as backseat drivers and constantly monitor the astronaut’s lunar location. Fortunately, the LRVs performed relatively flawlessly on all three missions and while not super precise (see ‘Maximum Position Error’ on the table below), the astronauts found the NAV system to be dependable.
The LRV carried astronauts on all the three missions for a total of 89 km (~55 miles) with a max range of 7.8 km (4.8 mi) on Apollo 17. Compare that to the total distance walked for Apollo 11, 12, and 14 at 5.4 km (3.3 miles) with a maximum range of 1.4 km (4,600 ft).
Why Not Just Use a Map, Fancy Pants?
You’re probably asking yourself, “Self, why didn’t they just use a map?” Well self, they did have maps. A whole slew of laminated contour, photo, and emergency bearing maps with prospective routes drawn on them. The map scales ranged from 1:5,000 to 1:25,000 and all packaged nicely in a handy spiral bound manual.
But the maps seemed to be only helpful for identifying large lunar features. While on foot during Apollo 12, astronauts Alan Shepard and Ed Mitchell had a notoriously bad time with map navigation. It was mentioned that the moon’s “rolling terrain, high contrast lighting and monochromatic nature” made it hard to see landmarks and judge distance. As explained by Ed Mitchell in “On the Moon: The Apollo Journals,”
“Large craters, which we expected to be able to see standing out on a reasonably flat plain, were not on a flat plain. They were hidden behind other craters, ridges, and old worn-down mounds. You could not get enough perspective from any one spot to precisely see where you were. The undulations over the neighborhood were 10 to 15 feet [high].”
I imagine that this effect was complicated further when moving along at rover speeds of 8-10 km/h (5-6 miles/h) and bouncing around in 1/6 gravity. During the Apollo 16 debrief, it was mentioned that the “maps did not reflect the topography very well” with Gene Cernan, Commander of Apollo 17, adding that he would often just choose a large landmark in the distance, note its bearing and head in that general direction.
So, while the Apollo astronauts had maps, they certainly weren’t comfortable relying on them. The LRV NAV system seemed like a much more trustworthy and safer solution for the longer distances they would be traveling.
Gene Cernan did eventually put his maps to good use. Amid loading the back of the LRV, he accidentally ripped of a fender extension with a lunar hammer that was sticking out of his shin pocket. Driving without the fender caused the LRV to rooster tailed large amounts of sticky lunar soil everywhere. After some thought, the solution was to duct tape a few maps to it to act as a make shift fender. It worked like a charm. Cernan was so proud of his solution, he brought the maps and fender extension back to Earth and they are currently on display at the Smithsonian Air & Space Museum in Washington, D.C..
Where Do We Go From Here?
While current un-crewed rovers don’t have to return to the comfort of a lunar module, some aspects of the Apollo systems live on in their design. Four U.S. Martian rovers have used wheel odometers that account for slippage to calculate distance traveled. They’ve also employed gyroscopes (in the form of an inertial measurement units) to determine heading and pitch/roll information. For navigation, after scientists on earth select an area of interest, stereo image navigational cameras (NAVCAM) on the rover map the surrounding area, calculate the safest possible route, then navigate autonomously to the target. This is all done computationally with the rover’s on board computers similarly to the LRV’s simple SPU.
The rovers can also use their cameras to scan the sky for the sun, record it’s path and compute which way the rover is orientated. Martian rovers have even used a SSD in the form of a ‘MarsDial’ to tell sun position and act as a camera calibration target.
NASA’s next generation crew transport is the prototype Lunar Electric Rover (LEV), resembling a hybrid of the ascent stage of the Apollo Lunar Module and LRV. A pressurized crew cabin will allow for two astronauts to live for up to 180 days, while twelve wheels with independent turning ability gives the LEV 360 degrees of steering control for traversing difficult terrain.
The LEV is still in the concept/development stage, so a final navigation system hasn’t been committed to, but I’m willing to bet that the legacy of the Apollo LRV will live on in it. A view of the LCD display (below) shows a digital version of a compass rose, with roll, pitch and velocity information, resembling the Integrated Position Indicator of the LRV.
Regardless of where humans explore in the future, we’ll probably need some sort of wheeled vehicle for surface transportation. The Apollo Lunar Roving Vehicle was the first extraterrestrial human driven transport ever designed and offered navigation challenges we had never encountered. As we push further outward, those demands will continue to test our abilities to know where we are in the universe.