QUESTION:
What happens to the rover if the communication between the rover and the
lander fails (for instance out of range)? Is the rover lost or can it
navigate without communication and move closer to the lander?

ANSWER from Andrew Mishkin:
Good question. There are several answers, depending on when and for how long
lander-rover communications fails.

Whenever the rover is driving, it periodically stops and sends a "heartbeat"
message to the lander. If the lander responds, communications is normal, and
the rover continues its traverse. If the rover doesn't get a response, it
backs up to the last location at which it successfully communicated, and
tries again. This should take care of problems caused by driving
unexpectedly out of range. (It's very unlikely that we would drive out of
communications range during our primary mission, because the range of our
radio is several hundred meters. In the primary mission, we will probably
operate within 10 meters or so of the lander, keeping the rover in good view
from the lander's camera. Later in the mission we may get more bold.)

Under some circumstances the lander may have turned off its radio modem
during a period when the rover is attempting to communicate. This may occur
overnight when the rover wakes up to do a health check or read out the
results of its APXS instrument. If the lander does not respond to the rover,
then the rover will temporarily buffer its data into non-volatile memory. As
soon as communications is reestablished (probably the next morning) the
rover will transmit all of its buffered telemetry to the lander.

If the communications failure is more permanent (more than a day), the rover
will eventually activate one of its on-board contingency sequences. The
contingency sequences enable the rover to perform a version of its mission
on its own. The rover attempts to send telemetry back to the lander, under
the assumption that the communications problem is only from the lander to
the rover, and that the lander may still be listening. (If the lander is
still receiving, then we still get telemetry on the ground. If not, it
doesn't hurt to try.) The rover will drive to preset locations, deploy and
operate the APXS instrument, perform other experiments, and take pictures.
If communications betweem rover and lander starts working again, the rover
will accept the new command sequence from the lander and exit contingency
mode.

During the rover's "extended" mission, we could decide to drive the rover
out of communications range. If so, we would temporarily disable the
"heartbeat" function, and design the traverse sequence to bring the rover
back into range before the sequence ends. The contingency sequence in the
extended mission would command the rover to drive in the direction of the
lander.

We have carefully examined the types of problems that could occur in
rover-lander communications, and designed the rover to recover from these
problems under most circumstances.

--Andrew Mishkin, Sojourner Rover Mission Operations Engineer