Subject: [NEXTEL-1] How the Mars Rovers Phone Home

From: Marcelrf <[email-masked]>
Date: Thu, 04 Mar 2004 03:22:14 -0500

How the Mars Rovers Phone Home

By Philip Chien

02:00 AM Mar. 02, 2004 PT

How do you get giant amounts of data from a spacecraft sitting on the
surface of Mars? For NASA's Spirit and Opportunity rovers, the indirect
way is the best way -- using relay satellites that orbit Mars.

Each rover has a high-gain microwave antenna that can transmit
scientific data directly to Earth, but more often than not uses a
low-gain UHF antenna to transmit the data to either Mars Odyssey or Mars
Global Surveyor, spacecraft in orbit above Mars.
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"This transfer of data is much more efficient because the orbiters have
much bigger solar arrays and bigger antennas, so they're not so much
energy- and bandwidth-constrained as the rovers are," Jet Propulsion
Laboratory telecom engineer Andrea Barbieri said. Think of it as a
low-power cell phone talking to a nearby transmitter tower, which has
far more power. Using spacecraft in orbit, the relays can transmit data
10 times faster than the direct link.

For the 1976 Viking mission, NASA used brute force -- a 20-watt
transmitter with a nuclear power source. The current Mars missions have
to be much smaller and less expensive. So they use low-power
transmitters and rely on solar power. But they have the advantage of
three decades of improvements in electronics, both on the spacecraft and
on the ground.

Each spacecraft to Mars builds on previous missions and provides
information that helps future missions. Every Mars orbiter has a Mars
Relay UHF radio that can communicate with probes on the Martian surface.
That can include base stations, rovers or even balloons or airplanes
flying above the surface.

Mars Global Surveyor arrived at Mars in 1997, and Mars Odyssey arrived
in 2001. They've accomplished their main missions and continue to
provide scientific data while also working as relay platforms. "With
each spacecraft orbiting Mars, we're building up a 'telephone
switchboard' to enable more and more data to be returned from missions
on the Martian surface," NASA planetary exploration manager Ed Weiler
said.

Here's how it works. The rover's computer has a timetable that
determines when the orbiter is going to be overhead. The rover transmits
its data at 250 Kbps, and it's received by a spacecraft orbiting 250
miles above the Martian surface.

"Typically each rover has been using Odyssey for two passes per sol
(Martian day), and Mars Global Surveyor for one pass per sol," Barbieri
said. The orbiters save the data from the rovers along with their own
scientific data. When the orbiter is in the proper position, it aims its
antenna at Earth and transmits all of the data.

Even at the speed of light, it takes more than 10 minutes for a signal
from Mars to arrive at Earth. Giant 230-foot-diameter satellite dishes
with super-sensitive, low-noise electronics receive the signal.

The Deep Space Network has dishes in California, Australia and Spain.
Besides the Mars missions, the Deep Space Network also tracks missions
headed toward Saturn, comets and asteroids, so the astronomers have to
do some careful scheduling to ensure that spacecraft don't get busy
signals.

Once on Earth, the data is transmitted to the Jet Propulsion Laboratory
in Pasadena, California, and then distributed to science teams around
the world. As of Feb. 18, the two rovers have sent 10 billion bits of
data from Mars to Earth. Only 18 percent of that data was transmitted
directly to Earth; the rest was sent through the Mars Odyssey and Mars
Global Surveyor relays.
The intra-Mars wireless network has an additional node in orbit.
Europe's Mars Express also has a UHF relay, and it was tested with
Spirit to verify that the international spacecraft could communicate
with each other, providing an additional path back to Earth. NASA plans
additional Mars orbiting spacecraft in 2005 and 2009.

Selected pictures and other scientific data are presented at regular
press conferences and put on the Web to share the experience with the
world. NASA says hits to the Mars rover websites have recently topped
6.53 billion, greater than the population of the Earth.

http://www.wired.com/news/technology/0,1282,62409,00.html?tw=wn_tophead_1

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