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Ok, I'll Ask The Question A Different Way (was: "Top Gear" Polar



 
 
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  #1  
Old December 30th 09, 01:24 PM posted to uk.misc,uk.sci.astronomy
Fleetie
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Posts: 56
Default Ok, I'll Ask The Question A Different Way (was: "Top Gear" Polar

Forgetting the magnetic pole, then:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?

How would the precession of the Earth affect this?

x-posted to uk.sci.astronomy because there are those there who I think
would know.


Martin
  #2  
Old December 30th 09, 03:06 PM posted to uk.misc,uk.sci.astronomy
Mike Williams
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Posts: 108
Default Ok, I'll Ask The Question A Different Way (was: "Top Gear" Polar Expedition: Their "North Pole" at 78 deg. N)

Wasn't it Fleetie who wrote:
Forgetting the magnetic pole, then:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?

How would the precession of the Earth affect this?


Precession doesn't affect it. There are in fact three types of
astronomical precession that the Earth undergoes, but none of them
affect the point around which the Earth rotates or affect GPS
satellites. For example, the precession of the equinoxes changes the
point in space towards which the rotation axis is pointing, but the axis
retains its position relative to the Earth. Like a tumbling gyroscope,
the axis moves in space, but the gyroscope still spins around that axis.

A totally separate geological effect is polar wobble. The axis of
rotation wobbles by a few metres.

GPS systems operate natively in the WGS84 reference frame. That uses a
fixed point for its North Pole, rather than following the polar wobble.
That has the advantage that my house retains exactly the same lat/lng
values in the WGS84 reference frame. In earlier systems that used the
rotational axis as the reference datum, it was discovered that precision
measurements exhibited measurable variations. Such effects could be
rather awkward if you lived in a country that has a boundary defined by
a particular line of latitude.

When an accurate GPS reads exactly 90 deg, it's within a few metres of
the rotation axis.

--
Mike Williams
Gentleman of Leisure
  #3  
Old December 30th 09, 03:36 PM posted to uk.misc,uk.sci.astronomy
Fleetie
external usenet poster
 
Posts: 56
Default Ok, I'll Ask The Question A Different Way

Mike Williams wrote:
Wasn't it Fleetie who wrote:
Forgetting the magnetic pole, then:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?

How would the precession of the Earth affect this?


Precession doesn't affect it. There are in fact three types of
astronomical precession that the Earth undergoes, but none of them
affect the point around which the Earth rotates or affect GPS
satellites. For example, the precession of the equinoxes changes the
point in space towards which the rotation axis is pointing, but the axis
retains its position relative to the Earth. Like a tumbling gyroscope,
the axis moves in space, but the gyroscope still spins around that axis.

A totally separate geological effect is polar wobble. The axis of
rotation wobbles by a few metres.

GPS systems operate natively in the WGS84 reference frame. That uses a
fixed point for its North Pole, rather than following the polar wobble.
That has the advantage that my house retains exactly the same lat/lng
values in the WGS84 reference frame. In earlier systems that used the
rotational axis as the reference datum, it was discovered that precision
measurements exhibited measurable variations. Such effects could be
rather awkward if you lived in a country that has a boundary defined by
a particular line of latitude.

When an accurate GPS reads exactly 90 deg, it's within a few metres of
the rotation axis.


Thanks, Mike!


Martin
  #4  
Old December 30th 09, 08:21 PM posted to uk.misc,uk.sci.astronomy
Richard Tobin
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Posts: 230
Default Ok, I'll Ask The Question A Different Way (was: "Top Gear" Polar

In article ,
Fleetie wrote:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?


Yes.

How would the precession of the Earth affect this?


Not at all. The coordinate system is fixed in the earth.

-- Richard
--
Please remember to mention me / in tapes you leave behind.
  #5  
Old December 30th 09, 09:38 PM posted to uk.misc,uk.sci.astronomy
Sjouke Burry[_2_]
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Posts: 402
Default Ok, I'll Ask The Question A Different Way

Fleetie wrote:
Forgetting the magnetic pole, then:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?

How would the precession of the Earth affect this?

x-posted to uk.sci.astronomy because there are those there who I think
would know.


Martin

At the true north pole gps is sadly lacking, I dont know the
exact limit, but the gps satelites are not in a polar orbit,
and thats why you start losing gps support.
But as long as you get enough stations, true north and magnetic
north are both available in the nmae output.
  #6  
Old December 31st 09, 12:01 AM posted to uk.misc,uk.sci.astronomy
John Aldridge
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Posts: 5
Default Ok, I'll Ask The Question A Different Way (was: "Top Gear" Polar Expedition: Their "North Pole" at 78 deg. N)

In article ,
says...
In article ,
Fleetie wrote:

At the true North Pole, i.e. that about which the Earth is rotating at a
given time, would a GPS read exactly 90 deg. N?


Yes.


Not by a few metres, I think... see

http://www.ordnancesurvey.co.uk/oswe.../coordinatesys
temsinfo/guidecontents/guide4.html

which says that the Z axis of the WGS84 coordinate system (used by GPS
receivers) points along the 1984 rotation axis, not along the
instantaneous axis of rotation at the time of measurement.

--
Cheers,
John
  #7  
Old December 31st 09, 12:49 AM posted to uk.misc,uk.sci.astronomy
Richard Tobin
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Posts: 230
Default Ok, I'll Ask The Question A Different Way

In article ,
Sjouke Burry wrote:

At the true north pole gps is sadly lacking, I dont know the
exact limit, but the gps satelites are not in a polar orbit,
and thats why you start losing gps support.


There is no need for a GPS satellite to be overhead, so a polar orbit
is irrelevant. There are supposed to be at least 6 satellites visible
from any point on the earth's surface at any time. At least 4 are
required to determine a position.

This page has what purports to be a photo of a GPS receiver at
the north pole:

http://yellowairplane.com/North_Pole...i_Team_54.html

-- Richard
--
Please remember to mention me / in tapes you leave behind.
  #8  
Old December 31st 09, 01:03 AM posted to uk.misc,uk.sci.astronomy
Steve Firth
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Posts: 4
Default Ok, I'll Ask The Question A Different Way

Richard Tobin wrote:

This page has what purports to be a photo of a GPS receiver at
the north pole:

http://yellowairplane.com/North_Pole...i_Team_54.html


boggle

No, not at the photo. At the message on that site.

"We are not *Going* to see Global Warming and Climate Change, We are
SEEING IT NOW. Take a trip to the real Geographic North Pole to see
these events for yourself."

Is it just me that stares at that and thinks that someone really, really
hasn't thought it through?
  #9  
Old December 31st 09, 01:44 AM posted to uk.misc,uk.sci.astronomy
Steve Firth
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Posts: 4
Default Ok, I'll Ask The Question A Different Way

Richard Tobin wrote:

In article ,
Steve Firth wrote:

"We are not *Going* to see Global Warming and Climate Change, We are
SEEING IT NOW. Take a trip to the real Geographic North Pole to see
these events for yourself."


See that ice? See how thin it is? Walk right over there and see
for yourself!


Unsure that CO2 is responsible for global warming? Let's all fly around
the world to find out if it's true!

That makes me groan as much as all those BBC documentaries in which some
knob flies halfway around the world in order to wring his hands about
the stupidity of flying halfway around the world.
  #10  
Old December 31st 09, 01:50 AM posted to uk.misc,uk.sci.astronomy
Richard Tobin
external usenet poster
 
Posts: 230
Default Ok, I'll Ask The Question A Different Way

In article ,
Steve Firth wrote:

"We are not *Going* to see Global Warming and Climate Change, We are
SEEING IT NOW. Take a trip to the real Geographic North Pole to see
these events for yourself."


See that ice? See how thin it is? Walk right over there and see
for yourself!

-- Richard
--
Please remember to mention me / in tapes you leave behind.
 




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