Navigation

Measuring a spacecraft's distance and velocity from its radio signal — range from the round-trip signal time, and line-of-sight velocity from the Doppler shift. Combined over time from the ground stations, these fix a spacecraft's trajectory across the Solar System.

A precise angular-position technique: two widely separated stations record the spacecraft's signal and that of a distant quasar, and the tiny difference in arrival times pins down the spacecraft's position on the sky to a few nanoradians — vital for accurate planetary arrivals.

Using a spacecraft's own camera images of a target body against the background stars to determine its position relative to that body — essential in the final approach to an asteroid, comet, or moon, where ground tracking alone is not precise enough.

Onboard software that processes optical-navigation images in real time and steers the spacecraft itself, without waiting for commands from Earth. It was pioneered by Deep Space 1 and used for terminal guidance in fast flybys and the DART impact.

A small camera that identifies star patterns to determine which way a spacecraft is pointed, to a fraction of an arcsecond. Star trackers give the precise attitude a spacecraft needs to aim its high-gain antenna at Earth and its instruments at a target.

Gyroscopes and accelerometers in an inertial measurement unit that sense a spacecraft's rotation and acceleration, propagating its orientation between star-tracker updates and through manoeuvres when other references are unavailable.

A miniaturised, ultra-stable atomic clock small enough to fly on a spacecraft. By putting precise timekeeping onboard, it enables one-way radiometric navigation — the spacecraft can determine its own position without the round-trip to a ground clock — a technology demonstrated in Earth orbit.