QUESTION:
What is the transmission delay from the time a command is sent to the
Pathfinder and its execution?

ANSWER from Bridget Landry on July 21, 1997:
That depends.  Some commands are built to execute as soon as the spacecraft
receives them, which is currently a little over 11 minutes, due to the
distance.  These are called realtime or immediate commands.  We also have
stored sequence, or absolute timed commands, which carry a time-tag, and
execute at that time.

Bridget Landry
Deputy Uplink System Engineer

ANSWER from David Mittman on July 12, 1997:
Right now, it takes a radio signal about 11 minutes and four seconds to
travel from Earth to Mars. That's called the one-way light time (or OWLT).
The OWLT is increasing about four seconds each day. The time it takes a
command to travel to the spacecraft, and for ground controllers to see the
effects of the command back on Earth is a round-trip light time (RTLT) or 22
minutes and eight seconds, plus some computer processing time, usually about
2-5 minutes.

ANSWER from Bridget Landry on July 21, 1997:
When the Pathfinder landing site is on the far side of Mars, we can't send
or receive transmissions to or from it.  The rotation of the Earth doesn't
trouble us, as we have three communication stations, called the Deep Space
Network (DSN), which are located in Madrid, Spain, Canberra, Australia, and
Goldstone, California.  As the Earth rotates, at least one of the stations
is visible from Mars at all times, so as long as Pathfinder is on this side
of Mars, we can get signals to and from.

I haven't heard of a rock causing problems with communications, though we
could lose contact with the rover Sojourner, if it were to go behind a
sufficiently tall rock, which is why we don't do that.

As far as controlling the spacecraft, since it takes over 11 minutes,
currently, for a signal to reach Mars, we have to plan a whole series of
actions for the rover and the lander to do, then send them up for execution,
without more input from the ground.  This also allows us to command
activities during the Martian night, when Pathfinder is on the other side of
Mars, so that it can take pictures of stars and the rover can make analyses
of rocks.

Hope that answers your questions!

Bridget Landry
Deputy Uplink System Engineer