QUESTION:
Since the Mars Pathfinder cruise stage does not carry any imaging
experiments, how is it possible, during the approach, to precisely determine
the point of entry into the Martian atmosphere to ensure a landing at Ares
Vallis?

ANSWER from Dave Spencer:
Your question is the crux of Orbit Determination, which is the discipline of
determining a body's flight path through space, under the influence of
gravity, solar pressure, propulsive events and outgassing, and other small
forces. Basically, through the Deep Space Network tracking stations, we get
two types of data during every tracking pass: Doppler data and Range data.
The Doppler data is a measure of the frequency change of the returned signal
compared to the frequency of the signal uplinked to the spacecraft from the
DSN. Doppler indicates the range rate of the spacecraft relative to the
tracking station. Range data is simply (and approximately) the time it takes
for a signal to reach the spacecraft from the DSN, and be retransmitted and
received on the ground. This "roundtrip light time" is then multiplied by
the speed of light to give the spacecraft range from the tracking station.
Doppler and Range taken together can give a pretty good estimation of the
spacecraft's position and velocity in space.

Doppler and Range information can also be supplemented by optical
navigation, as you suggest. In fact, some sort of imaging of the planet
surface would be necessary to land on a precise landing site (in a
particular crater, for example). For the Pathfinder mission, our landing
site is actually pretty large (200 km x 100 km), so we don't need an optical
navigation system. Sample return missions (likely in 2005, or maybe sooner)
will likely use optical navigation.

-- Dave Spencer, Mars Pathfinder Mission Design and Navigation