|Using Psuedo Random Code
as an Amplifier
pseudo random code is one of the brilliant ideas behind GPS. It not only acts as a great
timing signal but it also gives us a way to "amplify" the very weak satellite
Here's how that amplification process works:
The world is awash in random electrical noise. If we tuned our receivers to the GPS
frequency and graphed what we picked up, we'd just see a randomly varying line --- the
earth's background noise. The GPS signal would be buried in that noise.
The pseudo random code looks a lot like the background noise but with one important
difference: we know the pattern of its fluctuations.
What if we compare a section of our PRC with the background noise and look for areas
where they're both doing the same thing?
We can divide the signal up into time periods (called "chipping the signal")
and then mark all the periods where they match (i.e. where the background is high when the
PRC is high).
Since both signals are basically random patterns, probability says that about half the
time they'll match and half the time they won't.
If we set up a scoring system and give ourselves a point when they match and take away
a point when they don't, over the long run we'll end up with a score of zero because the
-1's will cancel out the 1's.
But now if a GPS satellite starts transmitting pulses in the same pattern as our pseudo
random code, those signals, even though they're weak, will tend to boost the random
background noise in the same pattern we're using for our comparison.
Background signals that were right on the border of being a "1" will get
boosted over the border and we'll start to see more matches. And our "score"
will start to go up.
Even if that tiny boost only puts one in a hundred background pulses over the line, we
can make our score as high as we want by comparing over a longer time. If we use the 1 in
100 figure, we could run our score up to ten by comparing over a thousand time periods.
If we compared the PRC to pure random noise over a thousand time periods our score
would still be zero, so this represents a ten times amplification.
This explanation is a greatly simplified but the basic concept is significant. It means
that the system can get away with less powerful satellites and our receivers don't need
big antennas like satellite TV.
You may wonder why satellite TV doesn't use the same concept and eliminate those big
dishes. The reason is speed.
The GPS signal has very little information in it. It's basically just a timing pulse,
so we can afford to compare the signal over many time periods. A TV signal carries a lot
of information and changes rapidly. The comparison system would be too slow and