From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Pathfinder/Other space probes we have launched
Date: Wed, 23 Jul 1997 21:11:37 GMT

>: Does our link with the Pathfinder and other probes use secure
>: communication? Does Pathfinder, Voyager, or the other probes have a logon
>: password?

Satellite owners have a tendency not to want to discuss this sort of
thing, for some reason. :-)  It is normal for communications-satellite
command channels to be encrypted nowadays, and you'll probably find that
almost all near-Earth spacecraft do likewise.  Quite apart from possible
malice, it prevents natural and man-made radio noise from accidentally
being taken as commands.

I'm not sure whether interplanetary uplinks are normally encrypted yet
(if not, I'm sure it's coming).  To get into them you'd need a lot of
transmitter power, a big dish, and a pile of technical information that
is not secret but is not widely published either.

>: Could somebody with a large transmitter send commands to Pathfinder and
>: drive Sojurner around? Could somebody point an antenna toward mars and grab
>: the images as there being trasmitted back to earth?

The downlinks generally are not encrypted, so the obstacles there are
purely technical:  a very faint signal with a complex and unpublished data
format.  Transmitter power and dish size on the spacecraft are always at
the expense of something else, so they are made no larger than necessary.
The *smallest* antennas DSN routinely uses for listening to spacecraft are
34m dishes, feeding liquid-helium-cooled maser amplifiers.  The spacecraft
will be designed to put an adequate signal, with a bit of margin, into one
of those at the longest distance expected... and no more.  Depending on
what the spacecraft is doing, it may be designed to need the 70m dishes
instead.

The more modern spacecraft will run at the highest bit rate feasible for
the current distance/DSN configuration, so anybody with much less in the
way of receiving hardware is going to be out of luck.  As others have
noted, it's easy to detect the spacecraft *carriers* with much less
spectacular equipment, but data transmission that's deliberately running
at the limits of DSN is going to be unreadable to most anybody else.  (A
few of the biggest and best radio telescopes can match or slightly beat
DSN's antenna/receiver performance, but they're about the only ones.)
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu