On the phone, tones can signal a connection. On paper, they can add
shape and dimension. On Mars, they can do both. This is why members of
the Mars Exploration Rover Entry Descent and Landing team at NASA’s
Jet Propulsion Laboratory will be on the lookout for a series of tones
during the mission’s landings in January 2004.

“Tones are a simple way to send information directly to Earth about
what the rover thinks it is doing as it enters the Martian atmosphere
and prepares to land on Mars,” said Mars Exploration Rover Entry,
Descent and Landing Telecommunications Lead Dr. Polly Estabrook.

After the Mars Climate Orbiter and Mars Polar Lander missions were
lost in 1999, mission engineers began looking at ways to strengthen
communication during future landings. The Mars Exploration Rover team
plans to use tones in conjunction with other methods to assess the
state of the rovers shortly before, during and after landing. These
tones cannot be heard, but they can be detected by special equipment
located at NASA’s Deep Space Network.

The Mars Exploration Rover mission uses a radio called the Small Deep
Space Transponder, which can generate tones at up to 256 different
frequencies-more than enough to cover the possible states of the
spacecraft, engineers said. Using data analysis equipment specially
designed by a JPL team for the Mars Exploration Rover mission, the
tones are presented as colored bars displayed according to frequency
and received time. In addition, the detection software decodes the
meaning of the tones and displays the names of various events
associated with the tones. However, due to the extreme conditions of
heat and speed during landing, there is no guarantee the tones will be
detected-even if the mission is going exactly as planned.

The process of entry, descent and landing on Mars is no walk in the
park. It entails getting a 827-kilogram (nearly a ton) spacecraft,
entering the martian atmosphere at 19,300 kilometers (12,000 miles)
per hour, to safely slow to a stop on the surface in six nail-biting
minutes. Complicating matters is the martian surface, which is plagued
with unpredictable winds and obstacles: massive impact craters,
cliffs, cracks and jagged boulders.

During the first four minutes of descent, friction with the atmosphere
slows the spacecraft to 1,600 kilometers (1,000 miles) per hour. With
only 100 seconds left, and at the altitude a commercial airliner
typically flies, a parachute opens to further slow the spacecraft to
321 kilometers per hour (200 miles per hour). With 6 seconds left, and
at 91 meters (100 yards) above ground, the retro rockets fire to bring
the spacecraft to zero velocity. Seconds later, the lander freefalls
from a height of about four stories, cocooned in airbags to cushion
the hard blow as it hits the ground at 48 kilometers (30 miles) per
hour, or more if it is windy. The lander then bounces approximately 30
times from as high as a four-story building and down to a rolling

If the tones show up, they will keep controllers from being locked in
limbo, unable to learn the status of the rovers until after the
complete landing. While many events will occur during the landing
process, engineers hope about 15 of those events to be signaled to
Earth. Each rover is programmed to transmit a tone every 10 seconds to
tell engineers on Earth about its progress. The first tones sent by
the spacecraft will signal its deceleration as it enters the Martian
atmosphere. Engineers could also receive a tone after important events
such as parachute deploy, heat shield jettison and lander separation.
Fault tones are also sent if the spacecraft thinks one of its
subsystems is performing unusually. Once the lander has touched down,
it will send five tones every 30 seconds to keep engineers informed
about its health.

As with any form of radio communication, many factors– such as
spacecraft antenna motion and atmospheric conditions– can affect the
quality and reliability of radio signals or render them undetectable.
This is also true for the tones, which are also difficult to detect
because of the spacecraft’s motion during descent.

“We engineers all understand that because these signals are so
difficult to detect, an absence of them during or after landing does
not necessarily mean that the rover has had a bad day,” Estabrook

In addition to the tones, rover engineers will strengthen their
chances of receiving information about the spacecraft’s descent onto
Mars by waiting for information that has been relayed from the Mars
Exploration Rover to the Mars Global Surveyor spacecraft. The rovers
have been equipped with Ultra High Frequency radios, which they will
use to communicate with the orbiter as they descend onto the Martian
surface. These radios can transmit 8,000 times more information than
the tones. However, due to the unknown geometry between the two
spacecraft, engineers cannot guarantee that they will receive this
data. Within an hour of each rover’s landing, the data should be at
available at JPL.

“The Mars Exploration Rover team has gone to great lengths to improve
the chances of receiving data from the spacecraft during this period
of great activity,” Estabrook said. “But we must be prepared to wait
until at least the next morning, when the rover communicates with us,
to get detailed information on spacecraft health and find out what
exactly happened during descent.”