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Darren
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 Posted: Wed Jan 26, 2005 1:58 pm Post subject: |
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| Sniff wrote: | | The satellites are in geostationary orbit...ie they are in fixed positions and their location as seen from Earth does not vary. |
That is incorrect, GPS Satellites are in a Geosynchronous orbit not a Geostationary orbit, their positions remain fixed at a predetermined altitude and position above the earth which rotates below it. Therefore they appear to 'orbit' the earth once every 12hours.
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Sniff
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Location: Munich, Germany
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| Posted: Wed Jan 26, 2005 2:22 pm Post subject: |
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| Darren wrote: | | Sniff wrote: | | The satellites are in geostationary orbit...ie they are in fixed positions and their location as seen from Earth does not vary. |
That is incorrect, GPS Satellites are in a Geosynchronous orbit not a Geostationary orbit, their positions remain fixed at a predetermined altitude and position above the earth which rotates below it. Therefore they appear to 'orbit' the earth once every 12hours. |
My apologies...I meant geosynchronous...hence my statement of 'fixed positions'!
This is from the Harvard web pages, a fairly good description I think:
The Global Positioning System (GPS) is a constellation of 24 artificial satellites. The GPS satellites are uniformly distributed in a total of six orbits such that there are four satellites per orbit. This number of satellites and spatial distribution of orbits insures that at least eight satellites can be simultaneously seen at any time from almost anywhere on Earth. The GPS satellites circle the Earth at an altitude of about 20,000 km (13,000 miles) and complete two full orbits every day. The GPS satellites are not in a geostationary orbit, but rise and set two times per day. Each satellite broadcasts radio waves towards Earth that contain information regarding its position and time. We can receive this information by using special receivers, called GPS receivers, which can detect and decode this information. By combining signals transmitted by several satellites and received simultaneously, a GPS receiver can calculate its position on the Earth (i.e., its latitude and longitude) with an accuracy of approximately 10 m. There are more sophisticated receivers that can be used to determine position with an accuracy of a few millimeters. |
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sifi
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Joined: Nov 19, 2003
Posts: 125
Location: Lincolnshire
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| Posted: Wed Jan 26, 2005 2:57 pm Post subject: |
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| I thought geostationary and geosynchronous were the same thing - just different names. |
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Darren
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| Posted: Wed Jan 26, 2005 3:15 pm Post subject: |
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No, Geostationary orbits cause a satellite to appear stationary with respect to a fixed point on the rotating Earth, this is different to a satellite in a Geosynchronous orbit.
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lbendlin
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Joined: 02/11/2002 22:41:59
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Location: Massachusetts, USA
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| Posted: Wed Jan 26, 2005 3:16 pm Post subject: |
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| Sniff wrote: | | The satellites are in geostationary orbit.. |
Nope. The GPS sats are moving around the earth every 12 hours (roughly). The SBAS satellites (WAAS/EGNOS) are geostationary.
By the way, very interesting article here:
http://www.metaresearch.org/cosmology/gps-relativity.asp
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Lutz
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Sniff
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Location: Munich, Germany
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| Posted: Wed Jan 26, 2005 3:23 pm Post subject: |
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| lbendlin wrote: | | Sniff wrote: | | The satellites are in geostationary orbit.. |
Nope. The GPS sats are moving around the earth every 12 hours (roughly). The SBAS satellites (WAAS/EGNOS) are geostationary. |
Yep...already had that pointed out to me! 
sifi....see the definition below:
Geosynchronous - A term applied to any equatorial satellite with an orbital velocity equal to the rotational velocity of the Earth. The geosynchronous altitude is near 6.6 Earth radii (approximately 36000 km above the Earth's surface). To be geostationary as well, the satellite must satisfy the additional restriction that its orbital inclination be exactly zero degrees. The net effect is that a geostationary satellite is virtually motionless with respect to an observer on the ground. |
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lbendlin
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| Posted: Wed Jan 26, 2005 3:27 pm Post subject: |
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The SBAS satellites sit in the same lane as the TV satellites - in the ring around the equator. Must be pretty crowded in that belt...
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Lutz
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Sniff
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| Posted: Wed Jan 26, 2005 3:54 pm Post subject: |
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| lbendlin wrote: | | The SBAS satellites sit in the same lane as the TV satellites - in the ring around the equator. Must be pretty crowded in that belt... |
I guess it's a pretty big area! |
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Oldboy
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Joined: Dec 08, 2004
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| Posted: Wed Jan 26, 2005 4:04 pm Post subject: |
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| As long as thet are going in the same direction! |
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lbendlin
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| Posted: Wed Jan 26, 2005 4:25 pm Post subject: |
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you wanted to say: as long as they don''t wiggle around. Being geostationary they don''t go anywhere, unless you watch the whole system from outer space - but then GPS is of no use to you
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Lutz
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Oldboy
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| Posted: Wed Jan 26, 2005 4:49 pm Post subject: |
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| 'Touche' |
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gingernut777
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| Posted: Fri Jan 28, 2005 10:13 am Post subject: |
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| Darren wrote: | | Sniff wrote: | | The satellites are in geostationary orbit...ie they are in fixed positions and their location as seen from Earth does not vary. |
That is incorrect, GPS Satellites are in a Geosynchronous orbit not a Geostationary orbit, their positions remain fixed at a predetermined altitude and position above the earth which rotates below it. Therefore they appear to 'orbit' the earth once every 12hours. |
I think you'll find they do orbit the earth.
http://www.nasm.si.edu/galleries/gps/
or here
http://www.offroaders.com/info/tech-corner/reading/GPS.htm
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Darren
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| Posted: Fri Jan 28, 2005 10:36 am Post subject: |
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If you read my response again you'll see that I said exactly that. However the term 'orbit' is a misnomer, it is the earth that rotates beneath a space object which gives the appearance of that object orbiting.
If you were observing the earth from space at an altitude above the GPS satellites you would see that they are stationary whilts the earth rotates beneath them, that is a geosynchronous orbit.
In contrast, a geostationary orbit is one where the satellite has velocity equivalent to the rotational speed of the earth itself, positioned above the equator these SV's therefore remain in a fixed location above the earth.
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Oldboy
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| Posted: Fri Jan 28, 2005 11:33 am Post subject: |
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| Surely if the earth was the only 'moving' object, and rotates every 24 hours how can each satellite be seen every 12 hours (have a 12 hour 'orbit')? Unless it moves! |
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Darren
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| Posted: Fri Jan 28, 2005 11:37 am Post subject: |
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Yup, you are of course quite correct. As they 'orbit' once every 12hrs they do effectively have a velocity equivalent to twice the speed of the Earth's rotation. My mistake 
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