NASA's Mars Curiosity rover does not have built-in GPS. The only way to track Curiosity's whereabouts and how far it has traveled is by following the six explorer's wheel marks.
For this reason, engineers put holes in Curiosity's treads so that
every time the wheels turn, they leave a unique imprint on Mars.
Orbiters photograph the print and scientists can determine how far
the rover has moved.
The track pattern spells out "JPL" in Morse code through a series of
"dots" and "dashes." JPL is an acronym for NASA's Jet Propulsion
Laboratory, the agency arm in charge of Curiosity.