GPS in space Uh?
How does that work then? Surely the ISS isn't at a lower orbit than the GPS satellites?
Paris is as clueless as me.
The European Space Agency's "Jules Verne" Automated Transfer Vehicle is slowly but surely creeping up on the International Space Station prior to a docking scheduled for 3 April. Artist's impression of the ATV approaching the ISS. Pic: ESA The ATV space truck will today strut its stuff on the second of two "demonstration days …
high earth orbit (HEO)
Geostationary or communications satellites PARKED @ 22,300 miles
medium earth orbit (MEO)
GPS satellites ORBIT at a height of about 12,000 miles
low earth orbit (LEO)
LEO includes orbits having apogees (high points) and perigees (low points) between about 100 km and 1,500 km. The space shuttle and the international space station are in a low earth orbit (LEO) to avoid the Van Allen radiation belts.
Source and more information: http://www.reformation.org/geostationary-satellites.html
As said above the GPS satellites are a lot further out than the ISS
and anyway this is relative GPS - i.e. The Space truck does not care where it is, and does not care where the ISS is, it just cares where it is relative to the ISS - i.e. about to dock or about to crash into it ....
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 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.
Not Geostationary, just high up.
Geo is about 45,000km /22,500Miles.
In theory with a special receiver you could use GPS at an Altitude ABOVE the GPS, as long as you pick up at least 3 of them. Different software in receiver needed.
The height of the orbits is then about 20200 km. Orbits in this height are referred to as MEO – medium earth orbit. In comparison, geostationary satellites like ASTRA or Meteosat – satellites orbit the earth at 42300 km, which is about twice the distance of GPS satellites.
The ISS and Iridium Sat phone are LEO (Low Earth Orbit).
Satellite TV, Thuraya Sat phones and World Space Radio all use Geo or Clarke Belt orbits.
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.
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.
> 45,000km /22,500Miles ???
Unless we're talking about nautical miles, and even then that's really stretching it (1 naut. mile = 1.8km)
> In theory with a special receiver you could use GPS at an Altitude ABOVE the GPS, as long as you pick up at least 3 of them
Nope, you need at least four signals. The reason being, you have 4 degrees of liberty: the conventional 3D coordinates + time (as in : the GPS satellites have a nucular clock onboard, but that's not the case for your receiver, so the exact time is treated as an unknown)
In other words, you sir/madam do not deverse the boffin's icon ! For shame.
The pedant's pedant.
Are you all reading different Wikipedia pages? You can't agree on geostationary orbit (may A.C.C. rest in peace), how many KMs there are in an M, or the altitude at which the GPS constellation orbits. (35,786km. Really, that specific eh? Above what? The perfectly smooth, flat surface of the Earth?)
I can't believe UK Gov's science budget cuts have taken effect so quickly.
"/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.
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? :~)
"(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'.
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...
...is how close the ATV is to being a man-rated spacecraft. If pressurized, it could certainly serve as a taxi to the ISS; only one-way, though as it is not designed to survive re-entry. Speaking as a frustrated US-er, I wish the EU would fund a manned program; after all, you are most of the way there with the ATV, and frankly, I would like to see somebody put the boot up NASA's rear.
As for the sadly non-existent UK manned program, you can have the Earth when the rest of us have finished with it. As far as plans for reviving the Empire go, it's a bit long term, but workable.
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