Viking's Radio Science Experiment

The two Viking landers (July 1976 - the first successful landings on Mars) and the radio-science experiment that rode with them.

The landers' orbiters relayed their S-band radio signal back to Earth. As the signal passed behind Mars, the atmosphere it had to cross changed the signal's Doppler shift in a measurable way - and the DSN on Earth read that change like a gauge. The result was the first in-situ measurement of another planet's atmosphere: a thin carbon-dioxide world with a surface pressure of only ~6-7 millibars - about 1% of Earth's. No instrument on the surface was needed to measure it; the radio wave did the measuring.

Built on the Doppler principle (the same physics your phone uses to correct GPS) and on the DSN's tracking precision. Enabled radio science on every mission since - Magellan mapped Venus's gravity field the same way, and the Mars Reconnaissance Orbiter still profiles Mars's atmosphere with its radio.