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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
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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
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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
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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. |
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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
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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
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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
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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
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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
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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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