[meteorite-list] How Did Navigators Hit Their Precise Landing Target on Mars?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:57 2004
Message-ID: <200401041909.LAA21230_at_zagami.jpl.nasa.gov>

http://marsrovers.jpl.nasa.gov/spotlight/navTarget01.html

How Did Navigators Hit Their Precise Landing Target on Mars?
Jet Propulsion Laboratory
January 4, 2004

[Image]
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.
 
[Image]
Dr. Louis D'Amario (left) and part of the navigation team celebrate their
bulls-eye at Mars

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.

[Image]
Navigation team member, Julie Kangas watches as data from Spirit comes
from Mars

"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.

[Image]
Members of the navigation team after seeing Spirit's first signal from Mars

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.
Received on Sun 04 Jan 2004 02:09:38 PM PST