Bush to consider shutting down GPS in extreme emergency

Tunnels are not always straight.

It works on the surface, and that's where it was used.

What good is checking the alignment _after_ digging the tunnel?

That's a bit late in the game to make any corrections, isn't it?

There are several ways.

The Chunnel engineers must have felt differently, since they still used
GPS.

 

After you've finished digging them, yes.

I don't think he used lasers.

 

The important datum is WGS-84, the GPS datum. The line can be moved to
coincide with GPS, if need be.

 

The 0° Meridian and the Equator are the basis of the whole mapping done
of the planet ever since. All other systems are based on that.


Juergen Nieveler

 

You don't dig a tunnel in a single day. Plenty of time to do checking
and make corrections.

But you can bet he had surveyors with "ancient" tools like theodolites.


Juergen Nieveler

 

So why are you claiming they used GPS? The drilling machines were lined
up using lasers.


Juergen Nieveler

 

So? You know how far away from straight it's supposed to be, so you can
measure it.

The only points of the tunnel where it comes to the surface are the
exits.

You check at regular intervals. Alignment can't be THAT bad without
being spotted by normal eyesight, and checking with the lasers means
that you notice it soon enough so that corrections still are possible.

No. A deviation of 5cm over 100m of tunnel can easily be spotted, and
that's something you can correct.

Of course, back then the joke went "What do you do if the two crews
miss each other? Then we'll have two tunnels!"

Not for this kind of tunnel. You can only drill through the rock -
there was no alternative.

What for? Please provide some proof on that claim.

GPS could have been used to map the start points on both ends, and it
could have been used on the ships that did geological surveys before
work started - but that's it. GPS cannot be used underground, even you
have acknowledged that.


Juergen Nieveler

 

Because that's what the references claimed.

 

The tunnel doesn't have to come to the surface.

To see what mistakes you've already made?

You said tunnelS.

To make sure the tunnels were meeting each other.

Lasers can't be used through solid rock, even you have acknowledged
that.

 

There are multiple systems, and they aren't necessarily aligned.

 

They used GPS indeed to set up ground reference points and controls. The
drilling under the channel was laser referenced.

You and I agree on this one.

 

Juergen Nieveler wrote:

In a nutshell:

You need control reference points outside the tunnels: GPS
You need control of the path while drilling: Lasers/Theodelites

The GPS networks at one end of the tunnel v. the networks at the other end will
place the laser references (at each end) with an end to end error of less than
5mm per km between the two networks. ... about 25 cm all up.

(Possibly less, some survey systems in use at the time may have been better than
5mm per 1 km baseline).

Cheers,
Alan

 

Simple geometry. Ask your maths teacher. Hint: Right-angle-triangles
are involved.

It would be kind of useless if it wouldn't come to the surface at
some point...

Yes. And then you correct the mistakes. We're not talking about
meters here, but centimeter-size mistakes at the worst.

We were talking about the Eurotunnel.

Yes. But the parts of the tunnel that already have been finished
aren't solid rock. All you need to do is make sure that the drilling
machine goes on straight - and that's easy.

Juergen Nieveler

 

The cocky maay not use it himself, but almost every Ag plane does, and
many fire bombers. It is rapidly becoming standard on many broard acre
ag operations.

The critical thing will be how many handle the sudden unexpected loss
of nav data. History already says it will not be pretty...

If you KNOW that GPS is out, you can cope. A Supertanker losing nav
signals while all half dozen or so crew are at lunch is another story.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

 

That gets you a good rate, distributing the time is more dificult. NTP
and GPS have made it unimaginably easy to do that. And low cost!

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

 

Want to buy a slightly used C90 or Apache? Very economical, only
uses 60% of the fuel of others


--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

 

Not really. They are in intermediate orbits, way below the
geostationary orbit.

The other factor is that you cannot use a GPS satellite if it is
just straddling the horizon. It needs to be up perhaps 5 to 10
degrees to be useful.

I guess somebody will have to do the calculation and tell us
exactly how many percent of the earth's surface are in useful
view of one GPS satellite. I guess it's something less than 40%.

Hans-Georg

 

On 17 Dec 2004 20:09:52 -0000,

Switching off the GPS system will likely also cost some lives,
more or less directly. I could imagine a whole variety of
scenarios in which lack of GPS service threatens lives or
impedes life-saving operations.

Also, a certain amount of money can always save lives. The
question is which ones to save. Saving just any life, no matter
what, is not possible. People are dying unnecessarily every day.

Hans-Georg

 

Stan, Alan,

thanks for the precise information. I was aware of some of it
and have to stick to what I wrote above, "Flying a twin while
knowing that it wouldn't be able to continue after an engine
failure seems undesirable to me."

I would think that the only sensible solution when you're higher
or hotter is to load the plane only so much that you can still
climb on any one engine.

Hans-Georg

 

Small knives - which as someone pointed out, is a technology that has
been available since the Bronze Age!

Brian Rumary, England

http://freespace.virgin.net/brian.rumary/homepage.htm