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6. DTN for the Mars Rover Program

EXAMPLE of a STORE and FORWARD NETWORK

This is the script of a radio show broadcasting soon on KFJC in Los Altos, California. It continues a discussion about InterPlanetary Networking …

Konstantin:  Welcome back for our second show highlighting InterPlanetary Networking. Our first show focused upon the special problems that an interplanetary network needed to address—especially those related to disrupted connectivity and the long signal delay inherent in traversing interplanetary distances. Mike Snell from the San Francisco Bay Area Chapter of the Internet Society joins us again. This time he wants to talk about some of the data networking problems experienced by the Mars Rover program, and how those were addressed using the basic principles involved in InterPlanetary Networking. Welcome back, Mike.

Mike:  Thanks, Konstantine. It’s good to be back.

Konstantin: Last time, you closed by promising to talk about how the Mars Rover program encountered some pretty serious problems early on, and how employing some of the basic principles of Interplanetary Networking solved that problem.

Mike: Right you are, Konstantin. Back when the first Mars rover landed on the surface of Mars, NASA encountered an unexpected—and very serious—problem. The first rovers were designed to communicate directly with NASA’s deep-space network—arrays of 240 ft. dia. Antennae located in Spain, Australia and the California desert so that there is always an antennae area pointed at any location in the plane of the solar system at any given moment. This antenna had two problems: 1) it could only communicate at 28kb/sec when it worked right AND 2) it wasn’t working right: it was overheating, and it could burn out—unless the duty cycle was significantly reduced.

Konstantin: So let me get this straight: you had a really bad transmission rate to begin with—like downloading photos and video over mid 1990’s dial-up speeds. And now the data throughput to earth would be choked down even further. Scientists at NASA must have been really ticked off about that.

Mike: If you think about it, the success of any exploratory space mission *is* the data transmitted back to earth, Konstantin.

Konstantin: Well the mission obviously didn’t fail. What happened?

Mike: Somebody at NASA realized that the rover had two antennae on it. One for communicating directly with earth, and a UHF antenna as well.

Konstantin: How did that help?

Mike: Well, the Mars Orbiter, which was now orbiting the planet.

Konstantin: Wasn’t that the space vehicle sent to survey the surface of Mars to determine where to land the rovers?

Mike: The very vehicle. It’s mission was over, and it also had a UHF antenna onboard.

Konstantin: So they what—beamed the data up to the Orbiter using the UHF antenna? How could that help?

Mike: Well they worked out a system kind of like the old Pony Express—the reprogrammed some software in the Mars Orbiter vehicle so that it would receive bundles of data from the rover, STORE it locally on the spacecraft, and then FORWARD it to the Deep Space Network the next time it could see the DSN.

Konstantin: Cool. But it was still pretty crummy bandwidth. At least by today’s standards.

Mike: Bonus!  TheUHF antennae could transmit data up to the satellite almost six times faster! 128K!

Konstantin: Wow… late 1990’s dial-up speeds….

Mike: It was so successful that since about 2005 ALL the Mars rovers have been designed to use this same basic store and forwarding architecture that folks at NASA and elsewhere are contemplating as the emerging standard for all interplanetary networking communications.

Konstantin: Go on! There’s dozens of space agencies. They’d never agree.

Mike: Ah but they are already starting to… more on that next time.

Konstantin: Okay, okay. You’re right—we’re out of time—but I’m going to hold you to that promise. That’s it for today. If you want to find out more about InterPlanetary Networking—be sure to check out their website at www.ipnsig.org.

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