From: Dave Emery <die@pig.die.com>
Date: Mon, 7 Jul 1997 11:24:52 +0800
To: azur@netcom.com (Steve Schear)
Subject: Re: Hack the Mars rover (fwd)
In-Reply-To: <v03102803afe5e6c84462@[10.0.2.15]>
Message-ID: <199707070257.WAA26483@pig.die.com>
MIME-Version: 1.0
Content-Type: text




Steve Schear wrote :

> You're right, its beyond imagination that any amateur would have the
> resources at their disposal to override NASA's uplink (unless ther's
> another Capt'n Midnight lurking at a commercial uplink station ;-).

	Different frequency bands.  And much less power for a commercial
uplink to illuminate a geo satellite with significant antenna gain
toward it's footprint.   Commercial uplinks are usually in the 20 to
500 watt power class going into the feed, whereas the DSN has 5 and
10 kw capability into much larger dishes (30 meter versus 6 to 9 meter).

> 
> Rather than trying to seize control of lander just do a DOS hack by keeping
> the ground stations from hearing the lander signal.  You said yourself that
> the path loss to Mars is very large (maybe around 200 dB), this means that
> even with those huge antennas their link margins can't be too high.

	Greater than 200 db.  But indeed one could certainly come up
with enough rf power from some point on the ground in line of sight to a
DSN dish to  completely overwhelm the signal from the bird.   

	However, such a signal would be instantly spotted and identified
and probably DF'd fairly rapidly.  It would be unlikely one could knock
out the downlink for very long without being located (and vigorously
prosecuted).

	But most of the time Mars is visible from more than one DSN
earth station and given the high priority of the mission the most
likely thing would just be to switch stations to one a third of
the way around the globe or more.   Would obviously be a nuisance
and get some people very mad, but since the ground stations fail
from natural causes from time to time such a handover would be
fairly routine.

> 
> I'll assume that in order to improve the margins they're using spread
> spectrum techniques, trading bandwidth for spectral efficiency.

	Spectral efficiency is usually bits/hertz of bandwidth.  They do
use QPSK or BPSK (mostly QPSK) which is about as power efficient - 
using FEC, vitirbi soft decision detection  and non differential coding
- as any possible modulation would be irrespective of bandwidth.
That is to say for a given data rate and carrier power to noise temp
ratio there is no modulation that would yield a better BER irrespective
of bandwith used.


  Without
> getting into the specifics of jamming technology, unless they have a very
> large process gain (like the 63 dB claimed for GPS)

	Process gain is a measure of the ratio of the spreading sequence
bit rate to the underlying data bit rate for a direct sequence spread
spectrum signal. There is very little to be gained by using spread
signals rather than  non spread signals in this application except
perhaps very accurate ranging information. They do not make sending k
bits per second with BER less than e bits second any easier.   There has
been some use of spreading sequences for ranging in the DSN, but I do
not know whether the pathfinder mission used that mode.

	Obviously a spread signal would require lots more power to 
jam with noise or cw carriers, but even assuming side lobes -80db down
from the main lobe (really hard to do) a jammer working from nearby would
not need to be putting out a lot of power to overload the receiver
and correllators.

, which is very unlikely
> for a number of reasons, that a properly designed transmitter located near
> their downlink stations would spill into the passband of their very
> senstive receivers (probably liquid-He cooled LNAs) making receiption
> difficult to impossible.  Of course, such transmitters would be relatively
> easy to find so only intermittent operation might be practical.
> >
	And would be spotted almost instantly on spectrum analyzers and
other monitors.


> >	And finally, demodulating the downlinks and recovering
> >information from them is relatively easily accomplished once the hard
> >part  (obtaining the G/T required) is somehow handled.  NASA tends to
> >use very straightforward modulations and FEC and does not encrypt the
> >downlinks.   And a fair amount of detail about the data formats is
> >publicly available.
> 
> If the data formats and coding techniques are public and well documented
> the task is simplified many fold.
> 
	Yes it is, although making educated guesses and going from there
is certainly possible.

							Dave Emery
							die@die.com
							Weston, Mass.