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How Did Navigators Hit Their Precise
Landing Target on Mars?
Anyone who's been blindfolded and spun
around knows how hard it is to "pin the
tail on the donkey," even though players
are pointed in the right direction when
they last look at their target. To land in
a precise location on Mars after traveling
over 300 million miles, navigators at the
Jet Propulsion Laboratory (JPL) had to
overcome the head-spinning challenges of
calculating the exact speeds of a rotating
Earth, a rotating Mars, and a rotating
spacecraft, while they all simultaneously
are spinning in their own radical orbits
around the Sun.
All the hard work paid off January 3
when navigators hit their target at the
top of the martian atmosphere to within
about 200 meters (660 feet), setting a new
standard for navigation accuracy for all
future interplanetary missions. "The
trajectory was so perfect that not only
was it within 200 meters, we also didn't
need to adjust course in the final eight
days of cruise," said Dr. Michael Watkins,
navigation and mission design manager at
JPL.
Navigators canceled two trajectory
correction maneuvers that were scheduled
to correct the flight path by firing a
series of small engine thrusters. The
navigation team researched the exact
performance of the engine thrusters to a
tiny fraction of a millimeter per second
to ensure flawless aiming for the four
previous maneuvers. "The Mars Exploration
Rover spacecraft design team helped our
ability to navigate precisely in the sense
that they created a dynamically quiet
spacecraft. Spirit didn't thrust much
during prior trajectory maneuvers because
the spacecraft was spinning for stability,
and when it did thrust, it did so in a way
that was easy for navigators to predict
movement," said Watkins. Spacecraft
thruster firings are a significant effect
navigators have to deal with, but even the
seemingly insignificant solar radiation
pressure and thermal radiation forces
acting on the spacecraft to a level equal
to less than a billionth of the
acceleration of gravity one feels on the
Earth need to be taken into account.
Without knowing the acceleration error to
that degree, the spacecraft would have
moved off course by 3.7 km (2.3 miles)
over 10 days.
"We had to know everything from how the
iron molten lava in the center of the
Earth was churning to how plate tectonic
movements were affecting the wobble of the
Earth to how the plasma in the atmosphere
delayed the radio signals to and from the
Deep
Space Network stations," explained Dr.
Louis D'Amario, Mars Exploration Rover
navigation team chief. "We assembled the
best navigation team in the world with
experts in orbit determination, propulsive
maneuver design, and entry, descent, and
landing trajectory analysis," said
D'Amario. The navigation team has been
working extremely hard on this mission for
three years - they even sacrificed their
holidays this December and New Year's Eve,
and they have essentially worked around
the clock for the last two weeks.
Navigators use radio signals sent and
received by the Deep Space Network (DSN)
antennas on Earth to compute spacecraft
position and velocity. Three DSN sites are
roughly equally spread around Earth's
globe at 120-degree intervals, so that
antennas are pointed toward Mars at any
given time as the Earth turns. If the
exact location of any of these antennas is
incorrect by just 5 centimeters (2 inches)
on the surface of Earth, that math error
builds over the 150 million kilometers (90
million miles) distance between Earth and
Mars, creating a 1500-foot (0.3-mile)
location error at Mars. So hitting a
precise landing site target that is
scientifically interesting on Mars is
impossible unless the calculations of how
fast Earth is rotating on its own axis is
known to the timing of 0.2 milliseconds.
At the other end of the journey,
navigators must also know the location of
Mars to the level of accuracy of several
hundred meters. Using recent measurements
with Mars Global Surveyor and Mars
Odyssey, navigators know the location of
Mars relative to the Earth to half a mile
or less.
The navigation team's intense attention
to detail was focused on ensuring that
this mission would be the most accurately
navigated in history. Navigators ran up to
1,000 different location accuracy
solutions several times every day to cover
the full range of possible answers. The
navigation team also used a tongue-tying
tracking technique called
spacecraft-quasar delta differential
one-way range or DDOR (pronounced "Delta
Door"), which utilized their knowledge of
locations of quasars to a few billionths
of a degree to help locate the
spacecraft's motion in the "up or down"
direction in the sky. "Even though it was
seemingly impossible to reach the small
science-rich landing site inside Gusev
Crater, the dedicated navigation team hit
the bulls-eye tonight to put us in
position for a winning science mission,"
Watkins said.
///
Respectfully
Submitted
Josie
Cory
Publisher/Editor
TVI Magazine
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