RE: Re: [FH] FTL, astronomy

Some numbers regarding GPS and using pulsars in the same way.

A good GPS can fix your position to about 1 meter.  Assuming most
satellites are 11,000 miles (17.6 thousand klicks) then you get an
accuracy of plus/minus 0.006% (1 meter of error per 17,600,000 meters). 
If a pulsar is 100,000 LY away, your error is going to be 568.2 LY. If
it's 10,000 LY then the error is only 56.8 LY.

GPS gets this accuracy from three sources - the receiving station is
kept on time by auto-correcting from 4 signals.  If there is error in
the receiving unit, the apparent location of the receiver will not form
a single point, but show 4 possible points.  The receiver tries to find
a single correction value that makes the 4 points combine into one. 
This internal, self-correction keeps the receiver on "atomic time"
without having to have an actual atomic clock.

The second source is that it knows the exact position of the satellite
at any given time.  An almanac is generated to predict the exact
position of the satellite at any given time.  Due to small disturbances
due to the Sun, Moon and other large bodies, this acutally varies
slighty.  Ground stations using highly accurate radar to constantly
monitor the position of each satellite.  The actual position is then
transmitted up to the satellite and incorporated into the signal that
each one sends out so that each receiver can make the appropriate
corrections.

The third source is signal delay correction.  Light/radio waves travel
at 186,000 miles per second - IN A VACUUM - interference with the
atmosphere (ionizing radiation and such) can reduce the signal speed by
a few nanoseconds.  Signals from different satellites might pass through
different conditions, which means there is no single correction factor. 
Ground stations are used that are relatively close, and thus would be
likely to see the same errors from the satellites.  The ground stations
operate in reverse - they have a known position, they know what the
timing of the signal should be, the measure the incoming signal and
determine how much off it is.  They then send this correction to the GPS
receiver (through normal radio channels, not the GPS signal)

One other issue is that the signal from GPS satellites is highly coded
into a pseudo-random signal.  It is a complex signal with no repeating
sequences (which allows you to know that the signal you are getting now
is not the same one that occured a second ago) and it also encodes
infomation about the satellite's position.

>From these details, I find it hard to believe that using pulsars will
be anywhere near as effective as GPS since I don't think we are going to
be able to train pulsars to transmit a pseudo-random signal.  In
addition, the time length is huge - the signal you are getting from the
pulsar might be tens or hundreds of thousands of years old.  The
measurements taken to ID the pulsar a century before might be from a
wave that occured after you measured yours (i.e. another weird event
with FTL where you traveled 200LY farther away from the pulsar and
passed the earlier wave).

Some food for thought.