GPS in space

There are two options:

ISS and JV *are* in lower orbits than the GPS constellation.

or

The GPS satellites don't transmit unidirectionally.

Re: GPS in space uh?

Having a quick scan of the web, GPS satellites orbit at 12000 km, which is a geostationary orbit, ISS is at approximately 340km (max of 440km). Therefore, probably with a little adjustment, GPS could be used.

GPS in Space

Considering the altitude of a GPS satellite (which is geostationary) is some 35,786km from Earth, and the altitude of the ISS is 350-460 km, you would appear to be very very wrong. It is most definitely orbiting lower than GPS satellites, about 35,300km lower.

Jules Verne?

One of the greatest imaginations ever and his name is given to a cargo truck?

Couldn't we call it the L Ron Hubbard and celebrate each time it's consigned to fiery oblivion?

@GPS in space

Ermm no!!! The GPS satellites are not in geostationary orbit, otherwise they would always be in the same place (as, say the orbital television satellites) and all the satellites would lie in the same plane, rather than covering the sky.

NOT geostationary

Although much higher than the ISS (~12,500 statute miles or ~20000km) the GPS satelites are not geostationary. Call up the satellite diagram of a GPS receiver and you can see them move (one orbit in 12 hours, IIRC).

White Van Man

Sorry Mike, can't call it the 'El Ron'* as it learning to drive it would involve having to do several meaningless courses at great expense.

But it does come with a free 'personality check' for all at the ISS.

(*El Ron - legendary bandit who stole from the poor and gave to the really rich)

GPS basic facts

GPS constellation altitude is 20,200km. This is below geosyncronous orbit level; in fact, GPS sats make exactly 2 orbits a day. Furthermore, they are in highly inclined orbits, as opposed to communications satellites, which are in equatorial orbits.

GPS sats beam their signals in fairly tight beams down to the earth's surface (for efficiency purposes), but there is enough spillover that it can be used without problems in low earth orbit. The major modification that is needed to a standard GPS unit to work on the ISS is to remove the "sanity checking" code in the GPS ("gee, I'm several hundred kilometers high, and moving along at over 10,000km/hr. I must be broken!").

Ham radio enthusiasts put a GPS in one of their recent "Oscar" amateur satellites. They found it was even possible to get GPS to work above the level of the constellation, there is enough leakage of signal outwards (and you can detect GPS signals from sats near the edge of the earth) to get good readings.

With a decent antenna, you could get GPS to work on the moon -- well, the parts facing the earth, anyway.

Oh dear

GPS orbital radius = 26,600 km (distance from the centre of the Earth)

Altitude = 20,200 km (distance from the surface of the Earth)

Geostationary orbital radius ~42,000 km

Quick check - orbital period at the surface of the Earth ~90 minutes, geostationary means an orbital period of 24 hrs, i.e. 16x greater. Kepler's law gives period squared proportional to radius (semi-major axis of you insist) cubed. 16^(2/3) ~ 6.4 Earth radii (~6,400 km) which gives 41,000 km (good enough).

Mines the anorak, ta.

Differential GPS

And just to comment on differential GPS, as opposed to standard GPS.

differential, you receive the same set of satellite on both receivers, one on the ISS one on Jules Verne. GPS gives you errors in location, but both the receivers should be getting the same error.

There for although you don't know your exact place, you very precisely know the difference between you in 3D, conceptually to a few mm, and so can navigate.

Three.

You do only need three to get a 3D fix. More than 3 is better. It depends on the position & separation of the 3 signals as to how good your fix is. But the nearby ISS will have same error, so differential position really very good with 3.

@The pedant's pedant

Oh dear. Wherever there is a pedant, there is usually another pedant standing ready to correct them. It's nuclear, not nucular.

Unless you are George W Bush.

/awaits minor punctuation mistake correction by previously unknown new species of super-pedant.

@@The pedant's pedant('s pedant ?)

"/awaits minor punctuation mistake correction by previously unknown new species of super-pedant."

Wouldn't that be a hyper-pedant, since the reply you are being pedantic about is already one being pedantic about someone else pedantry ?

Is this meta-pedantry ? My latin-prefix-fu is weak today.

@@@The pedants' pedants' pedants' pedant

As you are now categorising pedants as a group and super-pedants another and so on, isn't it now appropriate to refer to them as groups of pedants, therefore requiring that the apostrophe follow the group?

BTW The Other Steve, does this make me an uber-pedant?

I'm confused, could you all be more pacific? :~)

Geostationary

"(35,786km. Really, that specific eh? Above what? The perfectly smooth, flat surface of the Earth?)"

It's an altitude above mean sea level. Which yes, is a perfectly smooth flat sphere, even if it is 'imaginary'.

http://en.wikipedia.org/wiki/Geostationary

And there are ~3,281ft in a km & exactly 5,280ft in a mile, meaning 1 mile ~= 1.609...km. Nautical miles are different again, as was mentioned...