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Why aren't GPS satellites geosynchronous?


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A new Geocacher here.

 

My first surprise when geocaching was that the GPS satellites aren't geosynchronous. I thought that any satellites that are lower than a geosynchronous orbit eventually come down. This means that the satellites have to use power occasionally to stay in a correct orbit. This seems a risky thing to do for such precisely positioned satellites.

 

Am I misunderstanding the physics here?

 

Thanks,

 

tgspidell

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The only place where you can have a geosynchronous orbit is over the equator, and if all the satellites were over the equator, you would not be able to tell north from south.

 

As it stands, the satellites are in orbits which give good fixes as much of the time as possible (I assume it's at least an NP-complete problem, eh). The orbits are monitored by ground stations, and each satellite broadcasts the almanac every 15 minutes, IIRC. (In other words, if you can flawlessly receive one satellite for 15 minutes, you should have a complete, updated almanac.)

 

Anyway, even with satellites in geosync orbits, there are still variables to be dealt with. There is no panacea to do away with all of chaos theory and orbital mechanics; so they'd have to monitor the satellites anyway if they wanted to have such precise fixes. (And since I'm really tired and fading fast, I'll let someone else take it from here. icon_smile.gif)

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The only place where you can have a geosynchronous orbit is over the equator, and if all the satellites were over the equator, you would not be able to tell north from south.

 

As it stands, the satellites are in orbits which give good fixes as much of the time as possible (I assume it's at least an NP-complete problem, eh). The orbits are monitored by ground stations, and each satellite broadcasts the almanac every 15 minutes, IIRC. (In other words, if you can flawlessly receive one satellite for 15 minutes, you should have a complete, updated almanac.)

 

Anyway, even with satellites in geosync orbits, there are still variables to be dealt with. There is no panacea to do away with all of chaos theory and orbital mechanics; so they'd have to monitor the satellites anyway if they wanted to have such precise fixes. (And since I'm really tired and fading fast, I'll let someone else take it from here. icon_smile.gif)

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No extra work is required just to keep them in non-geosychronous orbits -- they are closer to Earth so they orbit at a faster rate, but their orbits are still nominally stable orbits.

 

Geosynchronous orbits are simply orbits that are placed directly over the equator and at a distance that allows the orbit speed to balance out at the same revolution rate that Earth happens to spin. From an orbital mechanics point of view, there is nothing special about geosychronous orbits.

 

It's just like the planets around the sun. Like Earth, Mercury is in a stable orbit around the sun, but it is much closer so must travel must faster to stay in orbital balance.

 

Not having the GPS constallation confined to the equator line, as the previous poster pointed out, helps increase accuracy and reception everywhere in the world.

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There is another very good reason not to use geosnychronous orbits for navigation satellites... it leaves a large part of the earth without coverage.

 

"How can that be?" I hear you ask? Well, the footprint (the area on the ground that can "see" a satellite) of a satellite in geosynchronus orbit doesn't extend form pole-to-pole. I would have to redo the calculation to know the exact answer, I believe it only extends from about 60d South ot 60d north. This isn't a huge issue with most geosynch. sats because not many direct-TV users live that far north or south, but there is a lot of navigation users up there.. from airline flights that take great circle routes from SF to London or Tokyo or fishing fleets in the waters off Alaska or Greenland.

 

Also, as others have pointed out, if all the sats line up in a line, then you lose one hemisphere of information. And while a string of geosynch sats don't quite make a line here in the northern hemisphere (it's a gentle curve actually), it's close enough to cause a serious DOP (dilution of position).

 

However, the WAAS sats are in geosynch orbits, but they're not used for positional information directly.

 

-- Mitch

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Not only would geosynch orbits give limited coverage of the globe and poorer geometry for coordinate calculation, it would also potentially cause permanent 'dark spots' of coverage.

 

If you happen to be in a situation where you can't get a lock on enough satellites to caclulate a position it is possible that waiting until the satellites are in a different position will solve the problem. As the satellites move around they're constantly changing how their signals reach any given position. So what is blocked from reception at one time may not be blocked at another. If the satellites are geosynchronous they never change how the signals reach a spot so if reception is bad at a point it will always be bad at that point.

 

I'm not lost!

I just don't know where I am.

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quote:
Originally posted by Pneumatic:

However, the WAAS sats are in geosynch orbits, but they're not used for positional information directly.


 

Actually the WAAS GEO's can be/are used as a GPS ranging source from ranging data from the WAAS type 9 message. WAAS is required to maintain WAAS network time within spec to GPS time but there are some ranging problems at times to these 2 GEO's.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by Pneumatic:

However, the WAAS sats are in geosynch orbits, but they're not used for positional information directly.


 

Actually the WAAS GEO's can be/are used as a GPS ranging source from ranging data from the WAAS type 9 message. WAAS is required to maintain WAAS network time within spec to GPS time but there are some ranging problems at times to these 2 GEO's.

 

Cheers, Kerry.

 

I never get lost icon_smile.gif everybody keeps telling me where to go icon_wink.gif

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quote:
Originally posted by smithcon:

-- they are closer to Earth so they ortsbit at a faster rate,

 

.


 

Not really, check out http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

Goto "J-Track 3D" A window will pop open showing the Earth and a bunch of white dots. The dots are satellites.

Select "Satellite" in the upper left hand corner of the screen, then click on "Select" and it will open a drop down menu. Make sure you have "ALL" selected as "TYPE" and "View". Now scroll down untill you get to "GPS BII-02" and it will show you witch one of those tiny dots is that particular satellite. Now tell me, does that orbit look close to the Earth compaired to the other sats that are out there?

Sorry smithcon, did mean to sound like an arse.

 

but their orbits are still nominally stable orbits.

 

.

 

"My gps say's it RIGHT HERE".

http://www.geogadgets.com

 

[This message was edited by Steak N Eggs on June 16, 2002 at 04:54 PM.]

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S&E,

 

Cool site!

 

One thing I though was cool.. check out the HST satellite, which of course is the Hubble Space Telescope. Looking at that gives you an idea of the orbit of the shuttles when they orbit the earth.

 

The HST orbits somewhere around 300 miles above the surface, which is somewhere near the limit of the shuttles.

 

Jamie

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quote:
Originally posted by Steak N Eggs:

Originally posted by smithcon:

-- they are closer to Earth so they ortsbit at a faster rate,

 

.

 

Not really, check out http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

Goto "J-Track 3D" A window will pop open showing the Earth and a bunch of white dots. The dots are satellites.

Select "Satellite" in the upper left hand corner of the screen, then click on "Select" and it will open a drop down menu. Make sure you have "ALL" selected as "TYPE" and "View". Now scroll down untill you get to "GPS BII-02" and it will show you witch one of those tiny dots is that particular satellite. Now tell me, does that orbit look close to the Earth compaired to the other sats that are out there?

 

.


 

Actually if you click on one of the GPS satellites to show its orbit and then spin the diagram a bit you will be able to tell that the GPS satellite orbits have a smaller radius than the geosynchronous sats (about 2/3 to 3/4 of the geosynch radius) and therefore orbit faster around the earth.

 

And to get back to the original question, all satellite orbits decay to one degree or another, even geosynchronous ones. This is due to the friction of the satellite with the minute amount of molecules and atoms flying around in space (it is not a true vaccuum). However, closer to earth (<1000km radius) the amount of stuff in space is significantly higher than further out and therefore orbits decay on time scales noticable to us. As someone else mentioned geosynch orbits are not particularly special, except that the angular speed of the satellite happens to match the angular speed of the earth and the satellite appears to sit still above a particular spot on earth. The satellite itself doesn't know that though, and still follows all laws of physics.

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quote:
Imagine the precision you could get if we could convert all those over to GPS! I bet you could get signals inside of buildings, too. Although I wonder how much fun Geocaching would be if you could locate a cache within inches?

 

Yeah, but you could attach a GPS to your keys and find them much more quickly everytime.

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Fixed, highly-directional earth station dishes rely on, stabily-positioned geosyncronous-orbiting satellites to maintain optimum signal acquisition.

 

Mobile navigation and communication devices receive signals broadcast from multiple, asyncronous orgiting satellites. Satellite siginals are received within the broadcast pattern without need for highly-directional receiving antennas or fixed position geosyncrnous orbiting satellites.

 

"When you find it, its always in the last place you look."

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quote:
What about the Grange point?

 

Nice catch GypsyMoth. Satellites (man made or other) do hold "stable" at Lagrange points and do not decay in the manner that I stated before. They are still subject to friction and the associated force which would normally cause a loss in velocity as I mentioned before. However due to the gravitational effects of the three body system defining the Lagrange point the friction is being counteracted and they remain at the same location. Lagrange points do require two large bodies relative to the size of the satellite such as the moon and the earth or the earth and the sun. Objects at Lagrange points orbit (around the common center of mass) at the same rate as the other two large objects, that being about 28 days or one year in my previous examples. These points are being used for some specific research satellites such as the Advanced Composition Explorer (ACE) studying the sun and the solar wind at the L1 Lagrange point between the earth and the sun.

 

See this web page for more info on Lagrange points and diagrams.

Also

http://map.gsfc.nasa.gov/m_mm/ob_techorbit1.html

http://www-istp.gsfc.nasa.gov/Education/wlagran.html

 

[This message was edited by chloew on July 02, 2002 at 08:01 AM.]

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This discussion about the GPS sats would lead one to believe they are in a lower orbit. Frankly, I have no idea. icon_razz.gif But, I did read about how the GPS works at howstuffworks.com and below is an excerpt. It says the GPS sats are over 12,600 miles up!!!! dadgum. Is that for real? Here's what it said:

 

===============

From this discussion you have learned several important facts about the Global Positioning System:

 

The Global Positioning System needs 24 operational satellites so it can guarantee that there are at least four of them above the horizon for any point on Earth at any time. In general, there are usually eight or so satellites "visible" to a GPS receiver at any given moment.

Each satellite contains an atomic clock.

The satellites send radio signals to GPS receivers so that the receivers can find out how far away each satellite is. Because the satellites are orbiting at a distance of 12,660 miles (20,370 km) overhead, the signals are fairly weak by the time they reach your receiver. That means you have to be outside in a fairly open area for your GPS receiver to work.

======================

 

Anyway, it is a very interesting article. However, I don't thing we will be seeing GPS sat flares the way we see Iridium flares!

 

6826_500.gif

When you leave this earth, the cache stays.

 

dove.gif

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This discussion about the GPS sats would lead one to believe they are in a lower orbit. Frankly, I have no idea. icon_razz.gif But, I did read about how the GPS works at howstuffworks.com and below is an excerpt. It says the GPS sats are over 12,600 miles up!!!! dadgum. Is that for real? Here's what it said:

 

===============

From this discussion you have learned several important facts about the Global Positioning System:

 

The Global Positioning System needs 24 operational satellites so it can guarantee that there are at least four of them above the horizon for any point on Earth at any time. In general, there are usually eight or so satellites "visible" to a GPS receiver at any given moment.

Each satellite contains an atomic clock.

The satellites send radio signals to GPS receivers so that the receivers can find out how far away each satellite is. Because the satellites are orbiting at a distance of 12,660 miles (20,370 km) overhead, the signals are fairly weak by the time they reach your receiver. That means you have to be outside in a fairly open area for your GPS receiver to work.

======================

 

Anyway, it is a very interesting article. However, I don't thing we will be seeing GPS sat flares the way we see Iridium flares!

 

6826_500.gif

When you leave this earth, the cache stays.

 

dove.gif

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