How Do Scientists and Engineers Find Spirit?

http://marsrovers.jpl.nasa.gov/spotl..._20040105.html

How Do Scientists and Engineers Find Spirit?
Jet Propulsion Laboratory
January 08, 2004

Spirit has been on the surface of Mars for more than 36 hours, and every
minute scientists and engineers are getting closer to determining exactly
where their precious package has landed. Like a jigsaw puzzle, scientists
and engineers must put together at least four complicated pieces before they
have the whole pictu orbital data that shows the overall landing-site
terrain, descent images taken on the rover's ride down, views of the horizon
seen by Spirit's own eyes, and navigation data collected as the spacecraft
entered the atmosphere and during the Mars Odyssey Orbiter's overflights of
the rover.

Puzzle Piece #1: Orbital Images

In the past year, cameras on the Mars Global Surveyor and Mars Odyssey
orbiters have been taking pictures of this "landing strip" every time they
passed overhead. Scientists started off with the knowledge that Spirit was
somewhere within this long oval-shaped ellipse to which navigators guided
the spacecraft.

Puzzle Piece #2: Descent Images

[Image]
This image, taken by the Descent Image Motion Estimation System (DIMES)
camera located on the bottom of the Mars Exploration Rover Spirit's lander,
shows a view of Gusev Crater as the lander descends to Mars.


While the spacecraft was hurtling toward the martian surface, it was able to
take images a little less than a mile above Spirit's landing site.
Scientists compared these images with those taken by a camera on the Mars
Global Surveyor spacecraft. Using the crater features in the descent images
as a guide, scientists were able to find the approximate landing location
within the larger orbital picture. That helped them narrow down the landing
location to a 1.3 kilometer (0.8 mile)in diameter area.

Puzzle Piece #3: Surface Images

[Image]
Tim Parker, geologist

Add some images from the surface of Mars, and geologists like Tim Parker can
eventually tell you where Spirit is hiding. "The real hunt began once we
received the first images from the surface," explained Parker. He is using
hills on the horizon seen by the rover to build a vector plot. Parker's
vector plot has a series of lines that project from the lander to compass
directions on the horizon. Each line is from a single point of view to
landforms that have been identified on the horizon. The map is then placed
over the orbital image (Puzzle Piece 1!) and moved into various positions
until the lines match up to the features in the orbital image. This "match"
will increase the accuracy of the rover's estimated location. "We'll soon
know within 300 meters...or about a third of a mile...where the rover is,"
says Parker. "We're really waiting for additional higher resolution images
and range data to come back from the rover in the next few days so that we
can identify the rover's position more accurately."

Puzzle Piece #4: Navigation Data

The same navigators who worked months to bring Spirit to Mars are assisting
scientists in locating Spirit's exact position. They're contributing their
detailed knowledge and experience of radio signal tracking. It's a little
like listening to the sound of a train whistle - it changes as it
approaches, passes, and continues on. Navigators compared the radio
frequency as it left Spirit with the frequency as it was received by the
Deep Space Network stations here on Earth about ten minutes later.

[Image]
Ralph Roncoli, Spirit navigation team member

Staring on Spirit's third day on Mars, navigators began using another way to
find Spirit's landing site through the Mars Odyssey orbiter. "We know the
orbit of Odyssey around Mars very precisely, so by having Odyssey listen to
the change in the frequency of the radio signal from Spirit as it flies over
the rover, we can pinpoint where the signal came from in relation to where
Odyssey was at that moment," explained Ralph Roncoli, Spirit navigation team
member. "We collect this navigation data from Odyssey during the first week
on Mars, and we hope to nail the location very soon," said Roncoli.

So where exactly is Spirit? Stay tuned as the puzzle pieces come
together....