Re: Speeding


"chippy" <chippy@xxxxxxxxxx> wrote in message
news:4d0655F18asm7U1@xxxxxxxxxxxxxxxxx
M.I.53 wrote:

My understanding was that the GPS system used the data in a way to
stop that drift happening.
http://www.nasm.si.edu/gps/spheres.html
GPS 10-20m accuracy, DGPS 1m accuracy.
I have not found a site that gives the error due to one sat.
dropping out although if 5 sats. are available one dropping out
would still leave 4 to determine the 3d location, and only 3 are
required for 2d.


This drift does indeed occur. It is more likely when there is
relatively few satellites from which to triangulate the position and
can be made worse if the geometry of those satellites is poor. No
amount of software can correct for this. If it could then rather
less than 24 satellites would be necessary to make the sytem work.
If there are 12 satellites in view (usually the maximum a receiver
can deal with), and the geometry is good, then 1 drop off does have
minimal effect. But if 3 or 4 drop off, such as when a vehicle
changes orientation, then drift occurs as the positional correction
is assuming 12 satellites until the clock is recalculated (around 20
seconds or so depending on the speed of the processor).


There are only 24 sat. in operation at any one time, I find it hard to
believe 12 would ever be available. I think the most I have ever had is
6.


There are often more than 24 as replacements are often launched before an
existing satellite fails or runs out of fuel. This allows testing to take
place. The system allows for a maximum of 32 operational satellites, but I
am not aware that number has ever been in service at once. Although much of
the time there may well be less than 12 satellites overhead, good GPS units
with a decent antenna are able to work from satellites well down to the
horizon. There can often be 12 (or even one or two more) above the horizon,
just as there is often less the 12 above the horizon. Having said that, it
is the satellites' varying height in the sky that actually degrades the
accuracy as atmospheric effects distort the timing of the signals. Military
grade GPS units have the ability to measure the distortion and correct it,
hence their greater accuracy.


DGPS acuuracy of 1 metre is only applicable if you are standing right
next to a DGPS beacon and that beacon is basing its fix on more or
less the same satellites that you are. The accuracy degrades with
increasing distance. 3 metres is an average, degrading up to 10 as
the limit. All this assumes a full 12 satellite fix with perfect
geometry.

The above is not correct according to the GPS websites.


The officially claimed average accuracy for a civilian DGPS unit is 3 metres
degrading with distance from the beacon. Obviously it will only degrade to
the accuracy of a non DGPS system. Different accuracies can be claimed
under ideal or far from ideal positions.

Although 3 satellites are required for a 2d fix, the user is required
to provide the altitude. Either that or the unit will assume the
last good altitude, which may or may not be true.

You're talking to the right person about GPS units. I have one here
that is accurate to 10cm (under ideal conditions) degrading to 2
metres if no DGPS beacon is available.

To be honest, what you say is contrary to all I have read, but it may
be due to much of that being sales talk. I have no depth of knowledge
about GPS and am only an infrequent user.


The sales people do tend to claim the sort of accuracy available under
perfect conditions. These rarely occur, and in practice, the odd time
occurs when a position is impossible to determine due to lousy geometry. As
the satellites move across the sky, once a position becomes possible, the
accuracy is awful, but rapidly improves as the satellites move. I once
found myself in the Gulf of Mexico in spite of the road signs insisting that
I was approaching Gatwick Airport. My GPS unit indicate a speed of 999 mph
(the maximum) for a while as the position quickly altered.

What you may not have found on the sites that you have visited, is that
there are GPS units that operate on only the civilian signals that are
accurate to 5 *centimetres* (under good conditions). Such units are used
for surveying, but they suffer from the disadvantage that because they work
on a principle that was unforseen when the GPS system was developed, the
processing power in order to derive a fix is phenomenal and usually carried
out post survey. Instead of using the transit time of the satellite
signals, they employ the convenient side effect of the interference patterns
created by them because all the satellites transmit on exactly the same
frequency.

As a further nugget: when GPS was first deployed, it didn't work at all,
because the Americans overlooked the point that time passes very very
slightly faster on the satellites than it does on the ground (because
Einstein's Special Theory of Relativity states that the passage of time is
dependant on gravitational force). GPS receivers have to take this into
account.


.

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