QUESTION:
I am interested in how the interplanetary communication technology is
expected to evolve for future missions.  You must lower bandwidth at greater
distances.  But looking ahead to a manned mission in the 21st century, one
would think a higher data rate would be required for adequate human
communication.  What kinds of technology would enable compressed voice and
video feeds from mars?  Is it a matter of simply having more power?

ANSWER from Leif Harcke on July 22, 1997:
True, the gain in a microwave communications system is inversely
proportional to the square of the distance between the transmitter and
receiver.  This if you double the distance between the transmitting station
and the receiving station, you need 4 times the power to transmit the same
data rate.

More power and bigger antennas.

Compressed voice standards operate around 4 to 5 kbps, and since Mars
Pathfinder currently transmits at 8 kbps, we could do voice with our system.
However, full-motion compressed video takes about 1 to 2 Mega-bps (1 x 10^6
bps) of channel capacity.  We would have to significantly increase the gain
of the communication system to do video.

The easiest way to do this would be to use a large dish antenna at the
spacecraft end of the communications link.  We already have reached the
practical limit for Earth station antennas (Deep Space Network 70 meter
diameter antennas).  By doubling the transmitter power at the spacecraft
(say from 10 watts to 20 watts) and using a large parabolic antenna on the
order of 2 to 3 meters, it would be possible to do a few megabits per second
from Mars.  The Magellan mission used a similar arrangement to return
volumes of images from its orbit around Venus.

Yours,

Leif Harcke
Mars Pathfinder Telecommunications Team