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 knob.
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.