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ZL2VAL > SPIRIT 05.01.04 21:09l 100 Lines 5477 Bytes #999 (0) @ WW
BID : 32770_ZL2AB
Read: GUEST
Subj: How Spirit navigated Mars journey
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Sent: 040105/1845Z @:ZL2AB.#46.NZL.OC #:32770 [New Plymouth] FBB7.00g $:32770_Z
From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SPIRIT@WW
How Did Navigators Hit Their Precise Landing Target on Mars?
Dr. Michael Watkins, Navigation and Mission Design Manager
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" 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.
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73 de Alan
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Points to ponder
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