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How does my satnav know where I am? A cheap off the peg sat nav makes you a better
navigator than anyone in the world fifty years ago. It can tell you exactly where you are,
which was a subject of uncertainty for many thousands of years, but how?
Although there are rival systems in various stages of development, including the European
GALILEO project, all modern sat nav systems rely on the Global Positioning System known
as NAVSTAR to tell them where they are. NAVSTAR is a system of 24 satellites that
orbit the earth at altitudes of around 11,500 miles, and which have been organised and paid
for (thank you) by the US military. Originally NAVSTAR was a top-secret cold war
project, designed to guide intercontinental ballistic missiles accurately onto targets
in the Warsaw Pact countries, rather than onto a nearby Chinese Embassy. But as the
cold war ended the world defrosted, so the decision was taken to open up NAVSTAR to civilians;
the US military stopped encrypting the signals sent by these satellites so that anybody could
read them. And although the early GPS receivers were
both bulky and expensive, size and cost have fallen almost exponentially. These days you'll
find GPS receivers inside mobile phones and even some of the sort of expensive watches,
enabling you to tell the time and look like a ***.
GPS itself works very simply. Presuming you've got somebody to launch and maintain the network
of orbiting satellites it relies on, then actually working out where you are is pretty
easy, sortof. The secret is one of time. Each GPS satellite contains a very accurate
clock, which continually broadcasts both its exact time, and also its location -- known
as the ephemeris data. The receiver works out how far it is from
the satellite by comparing the message it receives to its own clock and then calculating
distance according to how long its taken the signal to arrive travelling at the speed of
light. With one satellite, this information is pretty
useless: the receiver just knows how far it is from the signal. But with two satellites
the distance radii should intersect to give a 'fix', with more satellites increasing the
accuracy further. The minimum number of satellites needed for acceptable accuracy is four, but
there should be a minimum of 10 above each part of the world at any time.
What the system is doing is really just a very long range version of a navigator of
old taking fixes from say a church steeple, a lighthouse and a hilltop, and drawing some
triangles on a map. How does the receiver know what time it is
to the same accuracy as the atomic clocks carried by the satellites? By a very clever
fudge: rather than having a super-accurate clock on board, the receiver can effectively
reverse the equation it uses to plot position to work out what the time is based on the
distances from different satellites. Like position, the more satellites it can see,
the more accurate its time will be. Originally, the 'civilian' signal from the
satellites was deliberately degraded to reduce accuracy -- early GPS receivers could only
tell you where you were to the nearest 100 metres, which made it tricky to use them for
precise navigation. But, in 2000, this 'selective availability' was switched off, meaning that
GPS is now accurate to about 15 metres for all of us.
The US military can still turn GPS off or scramble it at will, if your satnav suddenly
packs up it's possible that some form of covert black op is being conducted in your vicinity.
Of course, modern navigation devices often need better accuracy than pure GPS can give
them -- like knowing which side of a motorway you're driving down. Which is why they feature
a load of other sensors: compasses, accelerometers to measure velocity and -- in the case of
mobile phones -- the ability to do some triangulation of local phone masts. Car navigation systems
will also presume they are travelling on a road, and will move their position to correspond
if the reading is slightly out. Of course, just because you and your satnav
know where you are, that doesn't mean you have any clue where you're going.
A point made every time another 44-ton truck comes up your driveway because that's what
the sat nav told the driver to do. It is clever but it isn't a substitute for
intelligence. Now we think we're terribly clever we have
orbiting satellites we have talking maps inside a box but the big science question of the
future is will we ever be able to build the starship enterprise? Well the answer is
in this SciGuide.