Re: SPS/Pulsars

Quoting Dances With Rocks <kochte@stsci.edu>:

> 
> On Mon, 25 Feb 2002, K.H.Ranitzsch wrote:
> 
> > > > ----- Original Message -----
> > > > From: "Indy" <kochte@stsci.edu>
> > > > Well, if the beacons imitate specific real pulsars in their
> frequencies
> > and
> > > > repeat patterns, they could, at least over a limited volume, be
> made
> > > > indistinguishable from the real ones and make navigation
> difficult. Lots
> > of
> > > > similar cases from fake lighthouse to electronic warfare.
> > > >
> > > > But the time lag seems a serious problem.
> > >
> > > That and how do you disguise the real pulsar so it isn't picked
> up?
> > 
> > Not really neccessary. If you have one fake, he has 50% chance of
> picking
> > the wrong one. Make you beacon appear stronger than the pulsar, and
> the
> > electronics may be dumb enough to go for the wrong one 90% of the
> time.
> 
> I'll give you that. However...you have to make some assumptions as to
> how space nav algorithms would work (unless you do the "cheapest
> bidder
> did your nav design", then you are only going to pick 3 known pulsars
> ;-).
> I would imagine (however wrong this may be) that space nav systems
> would
> (should!) take in as many pulsars as possible for a firm fix of
> position,
> not just three (that leaves you more vulnerable to potential spoofing
> attempts). But maybe it's just me. Maybe the NAC sold out to the
> cheapest
> bidder and shoot for 3.  ;-)
> 
> > > Moreover, I would think that unless you have rearranged where the
> pulsars
> > > are in space completely, a savvy nav officer is going to pick up
> on
> > > any discrepencies when correlating to other known pulsars. With
> hundreds
> > > to choose from, how do you know which ones will be picked for
> spatial
> > > location purposes?
> > 
> > Right, this would make spoofing hard.
> > On the other hand, if he doesn't know where he is, how much
> correlation can
> > he do to find out where he is ?

It's not /just/ looking at the pulsars. You also know /which one/ you're

looking at because of the pulse rate. So you actually have to get their 
positions and frequencies right. And then there's the fact you're
observing 
from a moving starship -- at interplanetary speeds you /ought/ to be
able to 
get a distance to the pulsar from it's parallax in a couple of hours.
Certainly 
if its parallax is high (close object) you're going to notice.

Using pulsars wouldn't get you very accurate anyway - they're probably
not 
dense enough... but you would get an approximate area. Knowing that and
having 
a star catalogue means you can spot the nearby bright stars, you can
work out 
their approximate apparent magnitude, approximate positions, find them
(and 
check their absorbtion spectrums, just to be sure) and get a more
accurate fix. 
You ought to be able to get within a cubic AU fairly easily from a few
pulsars 
and some local stars.

Of course: if you fake the pulsars, the local stars aren't going to
match... 
the chances of there being similar stars in the right places to match
your 
faked location is pretty minimal.