How does NASA navigate a Mars rover’s direction and determine its location with the planet having no global magnetic field?

To steer a rover, we need a good base map. The Mariner 9 orbiter expanded maps to cover the Red Planet in the early 1970s. The imaging has only gotten better since then. Depending on where you are, the global map has a resolution anywhere from less than a meter to more than 100 meters. For the Mars Science Laboratory (aka Curiosity), we collected high-resolution images from orbit covering the landing and main science areas on lower Aeolis Mons. NASA and Jet Propulsion Laboratory engineers used this base map and mathematical techniques to point the rover on its way to Mars and land it within a few kilometers of the target. Descent imaging pinpointed the rover within a meter or two on the surface.

For driving, Curiosity takes a series of stereo images around the rover with its navigational cameras (NAVCAM) when it finishes moving for the day. The rover team makes a mosaic from the overlapping images and projects it onto the ground. We then compare this ground-projected image, called an orthophoto, with the base map. We look for similar rocks and ridges in each image and adjust the rover center to a point where the features overlap. The science team locates all other features, like rocks or outcrops that they’re interested in, relative to this fixed position. We can calculate these features’ positions down to the accuracy of the NAVCAM images, which can reach millimeter precision within a few meters of the rover.

Curiosity also carries an inertial measurement unit (IMU) that gives positional information to help locate the rover, both distance traveled and roll, pitch, and yaw just like an airplane. However, we use “ground in the loop,” i.e. humans, to verify and correct errors in drive position after long drives or in cases where we have lots of slip due to sand or skid from steep slopes. While we have many sophisticated instruments aboard the rover, visual triangulation serves us well to keep the rover on the “straight and narrow” as we head toward our science destinations.