http://mars.jpl.nasa.gov/odyssey/spo.../20051024.html

Ice Beneath Mars Is Asking, "Can You Hear Me Now?"
Jet Propulsion Laboratory
October 24, 2005

In August 2003, as the twin Mars Exploration Rovers were barreling
toward Mars in their flying saucers, scientists and engineers sent a
radio signal disguised as the rovers? "voice" to the Odyssey orbiter
at Mars. The call to Odyssey was what Dr. John Callas, Mars
Exploration Rover Science Manager, defines as a "can-you-hear-me-now?"
test. Scientists and engineers wanted to ensure the UHF (ultra-high
frequency) radio system on Odyssey, a primary communications relay
between the rovers and Earth, would work. Odyssey responded with a
resounding yes, and something else from Mars responded too?.

Hearing Unexpected Echoes In The Noise

When the first, clear "I-can-hear-you" reply beamed back from Odyssey,
modest high-fives and conservative cheers were exchanged amongst the
small team of PhDs huddled around a computer near a 46-meter (150-foot)
antenna at Stanford University known locally as the "Dish." SRI
International manages the radio telescope, the only deep space antenna
near the Jet Propulsion Laboratory that can send UHF radio waves from
Earth to Mars. As each new line of relay test data streamed down to the
computer screen at the Dish, Stanford University's Dr. Ivan Linscott
began to mutter, "Huh? what's that? - that's strange."

A peculiar stripe on the data-return-screen was arcing underneath the
straight line that represented the primary communications from Odyssey.
The mysterious curve then intersected the primary line, and stopped
sending data at the same time the main signal disappeared. The team
initially dismissed the strange line as signal noise that engineers
term
"radio frequency interference" (RFI). But the curved line of data has
now earned the title, "Lucky Stripe," and the so-called static has
become the subject of the "Mars Bi-Static UHF Radar Experiment."

Experiment Is Like A Fun House Mirror

After a week of studying the stripe, the team discovered that the extra
data was actually a reflection from the surface of Mars. "Anyone who's
used rabbit ears to pick up a television signal has probably seen a
ghosting effect - a signal echo," explains Callas.

The ghosted image arrives off-center and is more transparent because
the
source signal hit some neighboring wall or structure and then bounced
back to the TV receiver a little later and more scattered. The Lucky
Stripe is a reflection of the martian surface, and the stripe is curved
because the Odyssey spacecraft was traveling in an arcing orbit over
the
planet as the echo from the surface of Mars reverberated back to Earth.

"Just like a distorted reflection in a fun house mirror tells you
something about the mirror's shape, radar reads an echo of the surface
of Mars that tells us about the shape of the surface it's reflecting,"
explains Callas. The team later confirmed multiple reflections, which
suggests they are seeing "echoes" of material beneath the surface of
Mars.

Expanding Science Using Current Resources

Fast-forward to October, 2005. The bi-static radar experiment team, led
by Callas, is now using the Odyssey UHF radio system and SRI's antenna
to hunt for subsurface water ice - a key component to understanding the
history or future of life on Mars. Timing is best now as Earth and Mars
merge toward a close approach on October 29, 2005 and UHF radio signals

become stronger and take less time to travel between the planets.
Scientists are hoping to find unexpected treats in the form of "noisy"
radar tricks from now until just past Halloween.

This bonus science is possible because the communications team didn't
disregard the "noise" from the original test and later NASA support for
the discovery. "This simple, inexpensive technique utilizes resources
that are already in place," explains Callas. The NASA-funded experiment
is a collaboration between JPL-Caltech, Stanford University, and SRI,
and uses antenna-scheduling resources and communication protocols
already in place to support the rover mission.

Rover Mission Planner Byron Jones reflects the mindset of various Mars
experts who have helped the experiment succeed. "There's a real sense
of
community during our Mars Relay meetings as various missions work to
share memory space on Odyssey so the bi-static experiment can operate,"
says Jones. When Mars is whispering, "Can you hear me now?" through
cryptic radio echoes, all of the mission teams can't help but eagerly
listen.

To learn more about two other radar experiments for Mars, please see:

MARSIS
http://mars.jpl.nasa.gov/express/mission/sc_science_marsis01.html on
Mars Express

SHARAD http://mars.jpl.nasa.gov/mro/mission/sc_instru_sharad.html on
Mars Reconnaissance Orbiter