Steve: Phones are the the main way that people
access satellite navigation data nowadays.
We have all sorts of other
receivers but this is what we'll tend to
have and so that's the illustration I'm
going to be using today.
Brady: There are satellites up there your phone works out
where's one and it goes "how far am I from it."
Is it something like that?
Steve: It's
pretty much like that and but the
details are a little bit more
interesting.
We have a series of satellites traveling
above our heads about 22,000 kilometers
away, broadcasting a signal; and the main
part of that signal is the time; so if
you think of the satellite as a giant
clock in space about the size of a
double wardrobe with huge great wings on
either side with solar panels, that's what
our satellites look like, and they're
broadcasting 24-7 the time and with that
time signal it's also broadcasting a little
bit of information about the health of
the satellite, whether it is on the right
orbit, whether it's clock is as accurate
it could be and they're constantly
comparing the time on the satellite
clock with a much more accurate, I say much
more accurate, and another atomic clock
on on the ground which it will tell the
satellite that it's clocks are running
either slightly slow or slightly fast or
they're gaining X number of nanoseconds a
week or a year or whatever, and
that will be broadcast down to the
receiver and it will tell the receiver
all the information it needs to know to
go to calculate the time precisely. It
will also add a little bit extra
information it will tell the the
receiver where to see and where to find
the other satellites in the
constellation, so that once you found one,
that will tell you how far it is away
from one particular satellite.
Imagine that in the middle of here is a
satellite, and it's broadcasting the time.
The surface of the balloon is an
equidistant time from the satellite.
We know cause we've picked up the signal at somewhere
on the surface of this balloon is us; but
without a little bit more information we
could be anywhere within the radius of tens of
thousands of kilometers perhaps.
So, this is where it all gets complicated, but I need
about three hands, but if we did it take
a second satellite broadcasting the time.
Brady: Do you want me to hold one for you?
Steve: I think I might just be able to manage it.
But, we then have a circle of
intersection, and I now know the time i'm
somewhere along that circle. That's still
an awful lot of room to play with so I
ask another satellite to tell me
the time, and that satellite we interact
on a circle like that, so now i know that
I could be either of those two places.
Still not very good because being in two
places at once doesn't really help
anyone.
So we take information from another
satellite, and that will give us one
single point where I could be. This is
the point is probably thousands of miles
in outer space. That is the way in which
we use the time signal from the
satellites to tell us exactly where we
are.
If only it was as easy as that, it
would be great. Brady:  I was going to say so
surely you know, I'm driving on the
motorway that's going to be out-of-date
quite quickly Steve: That is refreshed
several times every second, then the
refresh rate is about 50 times a second,
so even if you're traveling along in a
motorway the jet fighter it will keep up
with you, don't worry. I've mentioned we
have an accurate time coming from the
satellite. The satellite is broadcasting
on a radio wave which is traveling at
the speed of light; over 22,000
kilometers it's going to hit you about
0.07 of a second after it's released from
the satellite. The satellite has accurate
clocks, you don't. If you were able to
stick an atomic clock for around about
50k into your mobile phone you will be
able to
compete with satellites for accuracy of
timing. The trick is to make the
satellites share the time and make this
into an atomic accuracy clock. If we want
to know the height, the distances West
the distant North sounds, that's three
unknowns, two unknowns, three civil
taneous equations can solve it by
putting the fourth one and we get four
dimensions, the fourth dimension being
time; and so all of a sudden my gps
receiver, my phone is showing the
same level of nanosecond precision and
timing that the satellites are.
We're pretty good at working out what an orbit looks like, and even though the
satellite is traveling at about seven
kilometers a second, we know where they
are; but even though we think of space is
fairly empty there are influences that
might not that satellite off its exact
orbit
