GPSr in space...

[Lyra]

Not being much of a techie, I've wondered about this hypothetical situation. Assume I'm an astronaut and I decide to take my eTrex Legend with me on my next space shuttle mission. Once we're launched and establish an orbit, I get near a large window and turn on the GPSr. Assuming that the diffence in orbital distance above earth's surface (can't recall the exact altitude of geostationary orbit and the shuttle's orbit right off hand) isn't so extreme to result in my being out of range of the sattelites, what would the GPSr show?

 

My guess is that it would read normally (e.g., coordinates would reflect the point directly beneath me on earth's surface, with a huge speed reading and an altitude reading that would accurately show my distance above sea level.

 

Any competing theories?

 

Always wear proper caching safety equipment!

[+Uncle Alaska]

I believe that the most recent ham radio satellite (phase 3D or P3D) has several GPSr's on board to test signal reseption during it's highly eliptic orbit...I don't have time to look for links but a Google search might bring up a site with results of the experiment. I know the sat has had some problems but is still up there and working (somewhat)

-UA

 

***********************************************************

...Proudly ranked 620th in the state of California!My Home

[Kerry.]

It's not a theory as GPS as been used in the shuttle for quite some time in conjunction with some "older" type navigation systems.

 

GPS will basically totally replace these older nav systems (for some applications) as these older nav systems are phased out.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Jamie Z]

Lyra,

 

I would venture a guess that since the shuttle (and ISS) are only 200 to 300 miles above the surface of the earth that the GPS would work fine.

 

Now if a guy were 10,000 miles above the surface... I dunno.

 

Jamie

[TriCityGuy]

Since the sattelites are 24,000 km from Earth I find it likely that they have directional antennas. The degree of coverage at that distance is like (and forgive my estimate) 35 degrees or something. That way they use a lot less juice than transmitting 360 degrees. Good question. I'm going to have to do some homework myself!

[+Prime Suspect]

If you acquire enough sats, it should work. Consumer GPSrs use the trick of assuming that your are on or near the surface of the earth in order to calculate your position, when only a few sats are acquired.

 

[Lyra]

To simplify my logic in reaching the conclusion that I did, I "scaled it down" to assuming that there were only three satellites forming a triangle. If I'm on one side of the plane of that triangle, the GPSr would, through triangulation of the directional signals, be able to tell me where I am and how far I am from the plane of the triangle (which I assume is how a GPSr figures out altitude above sea level.

 

As I slowly approach the plane of the triangle, the GPSr would perceive this as an increase in elevation. Once I reach the plane of the triangle, the GPSr would continue to translate this into a continued gain in altitude above sea level, and would continue to do so until once I'm on the other side of the triangle's plane.

 

Based upon that assumption, I reached the conclusion that the GPSr would continue to interpret it as a gain in altitude, even at the point I move outside of the sphere of the satellites.

 

Always wear proper caching safety equipment!

[dave and jaime]

no i doubt if a consumer grade gps unit would work properly in space as the elevation operating limits are about 20,000m.

[+PDOP's]

As Dave and Jaimie said consumer grade GPSr's are limited to 1000 knots and 60,000 feet. Military and NASA units would not have these limits.

 

Interestingly GPS would be more accurate in space as atmospheric errors would be absent. GPS systems are used on both the space shuttles and space station.

[+GEO*Trailblazer 1]

Was that not the mission of the Space Shuttle Endevor on its Earth mapping Mission a couple of years ago? According to Commander Kevin Kriegel the accuracy they have locked on 4 sats is 4 one thousandths of a percent in the center of the bullseye.The Laser shot Feb. 14 of that mission at the asteroid.This is the latest Elevation/3D models for our mapping systems...Yes I watch NASA evertime a mission is launched and also use the Sat. free programs to become a little more educated on the latest technologies. What I believe you are talking about is called,ORBITAL MECHANICS,the Space Grid. http://users.commkey.net/Braeunig/space/orbmech.htm

When all else fails Geotry again.

 

[This message was edited by Trailblazer # 1 on December 10, 2002 at 07:12 PM.]

[+The Cheeseheads]

Considering I can barely get a decent signal inside of my van, I wonder if the shielding of the ISS would keep it from working?

 

- - - - -

Wisconsin Geocaching Association

[Kerry.]

Forgetting about some technical (security) type issues the Standard Positioning Service (SPS) specifications are based on several definitions.

 

One of these is the definition of the "Service Volume", which says the SPS performance supports the terrestrial service volume. The terrestrial service volume covers from the surface of the Earth up to an altitude of 3,000km.

 

So the GPS specifications used on the earth's surface for accuracy, PDOP, reliability, availability, User Range Errors etc are the same out to 3000km.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[dave and jaime]

maybe i should clarify my previous statement. the trimble units we use at work have speed limits of 1000 knot/hr and elevation limits of 100000 ft, when either of these limits are reached the unit shuts down because the manufacturers have designed the software in such a way that values outside this are essentially jibberish. this does not imply that the unit is not capable of working at such altitudes but rather the unit cannot intrepret such data. while the trimbles have higher ceilings than the popular magellan, garmin, and lowerance units the theory is the same, these units normally cannot handle speed greater that 950 knot/hr and elevations greater than 20000m. if someone can explain to me why these specific limits...

[+PDOP's]

Dave and Jaime

 

I'm no expert but from reviewing some threads on this subject in the sci.geo.satellite-nav news group it seems like the limits are there to prevent consumer units from being used in weapons (missiles). To this end consumer units are also built to lower G tolerances.