Bush to consider shutting down GPS in extreme emergency

[Mxsmanic]

Unimportant for a tunnel. You plan which direction you dig in, and
check wether you are digging straight.

Tunnels are not always straight.

No. GPS doesn't work underground - period.

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

Because the lasers were used to sight through the tunnel, not through
the rock.

What good is checking the alignment _after_ digging the tunnel?

You place a laser at the start of the tunnel, and if you can
still see it at your end of the tunnel the tunnel is more or less
straight.

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

Just how do you think people built tunnels like that?

There are several ways.

Do you think they dig a trench and drop the concrete tubes
in from above? Some tunnels ARE built like that, and in that
case GPS would be usefull, but that's not how the Eurotunnel
was built, that method only works for small tunnels that cross
shallow rivers.

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

 

[Mxsmanic]

But you do have line of sight along the tunnels.

After you've finished digging them, yes.

Do you imagine that Brunel used GPS when building the zillions of rail
tunnels he made ?

I don't think he used lasers.

 

[Mxsmanic]

Take a trip to Greenwich. Stand on the Meridian. Turn on your GPS
receiver. Note that it won't show exactly 0°, even though you're
standing on the exact line that DEFINES the 0° Meridian.

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

 

[Juergen Nieveler]

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

 

[Juergen Nieveler]

After you've finished digging them, yes.

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

I don't think he used lasers.

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


Juergen Nieveler

 

[Juergen Nieveler]

I read up on how the Chunnel was surveyed and constructed, and thus
avoided making a fool of myself in advance.

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


Juergen Nieveler

 

[Juergen Nieveler]

Tunnels are not always straight.

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

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

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

What good is checking the alignment _after_ digging the tunnel?

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.

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

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!"

There are several ways.

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

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

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

 

[Mxsmanic]

So why are you claiming they used GPS?

Because that's what the references claimed.

 

[Mxsmanic]

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

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

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

You check at regular intervals.

To see what mistakes you've already made?

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

You said tunnelS.

What for?

To make sure the tunnels were meeting each other.

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.

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

 

[Mxsmanic]

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.

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

 

[Alan Browne]

There are thousands of references that say they used GPS, so unless
there is a vast conspiracy to conceal the real methods used, GPS was it.

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.

 

[Alan Browne]

Juergen Nieveler wrote:

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

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

 

[Juergen Nieveler]

How?

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

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

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

To see what mistakes you've already made?

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

You said tunnelS.

We were talking about the Eurotunnel.

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

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

 

[prep]

Slightly reduced effectiveness without GPS. And we're talking about
a minor share of farmers who actualy DO use GPS already - most
don't.

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.

 

[prep]

From a vial of caesium, usually - at least the servers NTPx.PTB.DE
do.

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.

 

[prep]

All certified twin engine aircraft can fly, indeed climb at gross
weight, on one fully functional engine and pilots train for the
eventuality regularly.

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.

 

[Hans-Georg Michna]

Hans-Georg Michna writes:

Only slightly less than half. The satellites are in very high orbits.

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

 

[Hans-Georg Michna]

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

In mine, the saving of the life of a single individual is enough for me.

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

 

[Hans-Georg Michna]

A twin has to be able to climb, with the gear and flaps retracted, with
the critical engine inoperative. But not at gross weight, and not at all
temperatures. It just has to be able to do it at some weight and
temperature. The flight manual has a WAT chart (weight, altitude,
temperature) that shows under what conditions the aircraft can climb
single-engine, and how fast the climb or descent will be. Few light
twins can climb single-engine at much above sea level and much above
standard temperature. Installing more powerful engines means more fuel
consumption, higher costs, etc. Everything is a compromise of some sort.

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

 

[B.Rumary]

Nope, they used box-cutters.

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