Re: GPS + Pulsars + an apology/rebuttal

On Sat, Feb 23, 2002 at 10:52:47AM -0500, Thomas Barclay wrote:
> Question:
> Can we duplicate or fake a pulsar? If so, could 
> we mess up people's navigation? Could we 
> interfere with signals from a pulsar? Or cancel 
> them via some form of interference? 

Unlikely. A pulsar is a rather powerful star flashing rather quickly.
Blocking it would require at least a stellar sized
object. Interference /could/ be possible - plonk a blackhole between
the star and the ship and bend the light away from the ship?

Kind of tricky to organise...

> Question:
> When establishing our own space beacons (SPS 
> - GPS in space), would we use some sort of 
> pulsar like signal, but with more uniqueness?

Pulsars are a) VERY bright and b) very unique. Their flash rates are
VERY precise. From bearings on half a dozen you could fix position
reasonably well. Knowing a rough position (like: which system you're
in) you could take fixes on nearer stars and get positions down fairly
well. That's how the Hubble orients itself - it knows about "key"
stars and fixes on them - obviously you need to know the rough
apparent magnitude of them before you can identify them in the sky to
take bearings. Pulsar light comes with the encoded flashing to tell
them apart.

> Normally I don't respond to allegations*[1] of 
> veiled US bashing, but in this case I must make 
> an exception. I am aware the US owns, built, 
> and (AFAIK) entirely paid for the GPS satellite 
> constellation. I am aware of the limits of the 
> accuracy and several techniques of getting 
> around (or attempting to) the inaccuracies that 
> can be introduced into the data feed. Anyone 
> who uses this system other than the Americans 
> (and maybe a Redcoat or two) are in the 
> 'buyer beware' (and given you paid nothing, 
> that's a capital B in beware!). 
> 
> You did not, in fairness, mention that the US can 
> also (you did build, and did pay for, and do 
> own - AFAIK - the constellation) turn off the 
> feeds from some satellites entirely or probably 
> even deactivate the whole shebang if it really 
> wants to black out an area. 

ISTR the precise timing information for sub-100m positioning comes in
a secondary databand which was (until recently) encrypted. It's the
unencrypting of this band that's made GPS as a traffic navigation aid