Comms Track: Spacecraft Links and Flight Software Basics · Module 1 of 4 · about 15 min

The link: how spacecraft talk to Earth

Every mission has two directions of traffic. The uplink carries commands from the ground to the spacecraft. The downlink carries telemetry back: health readings like temperatures, voltages and battery charge, plus the science or imaging data the mission exists to collect. When you hear an operator say they "have downlink", they mean the ground is hearing the spacecraft.

For missions far from Earth, NASA uses the Deep Space Network (DSN). It has three antenna complexes: Goldstone near Barstow, California; one near Madrid, Spain; and one near Canberra, Australia. They sit roughly 120 degrees of longitude apart, so as Earth turns, a distant spacecraft rises over one site before it sets at another. The largest dishes are 70 meters (230 feet) across. The DSN is run by JPL for NASA's Space Communications and Navigation (SCaN) program.

Distance is the problem that shapes everything else. Radio travels at the speed of light, and the gap between Earth and Mars swings from about 54.6 million km to about 400 million km. NASA puts the one-way delay at about 3 minutes at closest and about 22 minutes at farthest. Nobody joysticks a Mars rover. Teams send a planned sequence of commands, and the vehicle carries enough autonomy to handle the minutes in between.

Mars surface missions rarely talk straight to Earth for the bulk of their data. A rover has limited power and a small antenna, and it loses line of sight to Earth for about 12 hours a day as Mars rotates. Instead it hands data to orbiters overhead at rates up to 2 megabits per second, and the orbiters, with bigger antennas and more solar power, relay it to Earth. NASA calls this the Mars Relay Network, and says every image seen from the Martian surface since 2004 has come home through it.

NASA also counts link modes. If the ground is only receiving, the link is one-way. If the ground is sending an uplink that the spacecraft receives while its downlink is coming in, the link is two-way. Those words show up in operations jobs, so it helps to know them before an interview.

Sources

• NASA: Deep Space Network
• JPL: Deep Space Network
• NASA Science: Mars Relay Network
• NASA Basics of Space Flight, Chapter 10: Telecommunications

Next: Decibels, antennas and the link budget →