[chris (OP)]

Ignoring software issues related to the fact that the longitude and latitude wouldn't fit, if a GPS receiver was taken to the moon, would it still work to pinpoint my position, albeit on the lunar surface?

[syhprum]

The GPS system operates by means of 50 or so satellites in orbit around the Earth they would not be of any use on the Moon even if reception of their signals was possible.
A similar system (at vast expense) could be set up on the moon but is unlikely to be set up there until a war starts up there.

[imatfaal]

Syhprum - you are quite correct about the satellites.  But how do they broadcast their signal?  It would not take much leakage for the signals to be receivable on the moon.  And three or more signals and a decent software algorithm and you have an exact position. 

So - If you were on the dark side of the moon (when you are part of the great gig in the sky) I think you would be unlucky - but on the side that faces the earth I reckon a decent receiver could pick up enough signals to make a fairly reasonable approximation.

[syhprum]

To obtain any useful information about your position on the Earth you have to measure the time delay of the signal from three or more satellites all of which carry very accurate atomic clocks.
Even if you could receive their transmissions on the moon there is no way that this information could be used to work out your postion on the Moon to do this a local system of satellites orbiting the Moon would be required.





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

[imatfaal]

Syhprum - not so.  You have a computer model of orbits around the earth - each GPS signal contains an identifier and a time signal.  Your computer on the moon takes each signal calculate the expected position of the transponder and the relative time; you then have three points in three dimensional space with a time signal from each.  The time delays from the three fixed points in space allow you to work out an exact position. 

I happen to correspond with someone who works intimately with GPS - I will seek his much more knowledgable opinion

[syhprum]

You would need large dish type antennae to receive the weak signals, considerable computing power to allow for the large and variable Earth to Moon distance and the ionospheric delay you are certainly not going to get anywhere with the small devices that we find so useful. 

[SeanB]

GPS works up to the altitude of the satellites that broadcast the signal ( and then a little further with reduced accuracy from the sidelobes of the satellites in view but on the other side of the earth), but most of the transmitted signal is sent down with a very directional antenna to improve the signal on the ground as the satellites have a very limited power source. You can use a GPS on the ISS, but it will be confused very quickly as you are moving too fast, and will be outside the limits hard programmed into the cheap chipsets. If you use a receiver designed for supersonic aircraft you will get a lock, and a velocity of 12km per second as well. On the earth facing side of the moon you would get a GPS signal using a big enough antenna and a lot of low noise amplification before the decoding, but the signal would not be much use as the reference is not a fixed point on the lunar surface, but a point on the earth, and you would have very little usable info as to the relation between the earth and moon rotating, though you can use it to get a good and quite accurate clock.

[syhprum]

Thanks, it is a little embarrassing to be told by correspondents that they will talk to someone who understands the subject

[chris (OP)]

Interesting debate; thank you everyone; I'm also relieved that I gave the same sorts of answers to the question.

Chris

[Madidus_Scientia]

So - If you were on the dark side of the moon (when you are part of the great gig in the sky) I think you would be unlucky - but on the side that faces the earth I reckon a decent receiver could pick up enough signals to make a fairly reasonable approximation.

Why wouldn't it work on the dark side of the moon facing earth?

[Geezer]

Practically, as Syhprum points out, not a snowball's chance in hell. The signals would be far too weak.

Theoretically, maybe. Assuming the guys who wrote the software contemplated the possibility that the GPS receiver could be at a great distance from Earths's surface so that the software does not go mental (which I'm willing to bet will happen), it might actually work, but even if the signals could be received, the accuracy would be lousy.

Navigation systems rely on triangulation, and GPS is no different. The most accurate triangulation occurs when the triangle is close to an equilateral triangle. The less equilateral the triangle, the less accurate the position.

If you were on the Moon, and your GPS system could actually receive the signals from terrestrial GPS satellites, the triangulation would be so far from equilateral that the resolution of your position would be limited to tens of kilometers. In other words, it would be useless.

[syhprum]

Assuming you could receive the GPS signal that would translate to a position on the Earth at that moment to convert that to a position on the moon would be a very complex computation problem allowing for the time delay between the Earth and the Moon, the rotation rate of the Moon, the eccentricity of its orbit and the deviation of its rotational plane from the elliptic.
Chris was wise to ignor software issues.

[Geezer]

Not really. Assuming the system made no assumptions about location, it would simply try to resolve a three dimensional position based on the signals it received. As the distance and angles from those signals increase or decrease, the less accurate the resolution becomes.

[syhprum]

To obtain a position you make a comparison between the time signals transmitted by the satellites and the time you actually receive them, would not a problem arise because the time signals would be Earth time not Moon time.
What the signals you receive would tell you would be where you are relative to the surface of the Earth as though you were flying above the Earth in an aircraft.
I presume what you are interested in is where you are on the moon, no doubt this could be computed but it would be a complex problem.

[Bored chemist]

According to wiki
"The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 kilometers (11 mi) altitude and 515 metres per second (1,001 kn)[48] are classified as munitions (weapons) for which U.S. State Department export licenses are required."

So, unless you have a military standard GPS it wouldn't work on the moon.

Another similar question would be  what would happen if I built a really high ladder and took my GPS up it?
GPS is normally set up to give a latitude, longitude and altitude.
How far up the ladder could I climb before the GPS failed?
(in principle- so we ignore the impossibility of the ladder and the need for my air supply).

My guess is that it would get screwy once you were above the astellites, but how far could you get?.

[Geezer]

My guess is that it would get screwy once you were above the astellites,

I think so. The receiver has no idea about the direction of the satelites, only distance based on time. That means the receiver can be in one of two possible positions in three dimentional space; one on the Earth's surface and another far above the Earth's surface. Of course, it "knows" it is on or near the Earth's surface, so the other location is "impossible".

So, if you were to put the GPS in a position beyond the orbits of the satellites, I think it would try to report a position within the orbits.

[neilep]

Hang On, if you are on the moon...do you really need your GPS to tell you that......you're on the moon ?

[Geezer]

To obtain a position you make a comparison between the time signals transmitted by the satellites and the time you actually receive them, would not a problem arise because the time signals would be Earth time not Moon time.
What the signals you receive would tell you would be where you are relative to the surface of the Earth as though you were flying above the Earth in an aircraft.
I presume what you are interested in is where you are on the moon, no doubt this could be computed but it would be a complex problem.

I don't think local time would matter. The satellites send time and position signals and the receiver uses them to determine relative distances from the satellites. I don't think the receiver even needs to be aware of an absolute time reference (they would probably be a lot more expensive if that was the case). Assuming the signals could be received on the Moon, the receiver could (in theory) determine a spatial position that corresponded with a position on the surface of the Moon.

However, the accuracy would be lousy, and, as you point out, the receiver would have to perform some really fancy gymnastics to map that point in space to a meaningful location on the Moon's surface.

[Geezer]

Hang On, if you are on the moon...do you really need your GPS to tell you that......you're on the moon ?

How else would you find the nearest McDonalds?

[Bored chemist]

Hang On, if you are on the moon...do you really need your GPS to tell you that......you're on the moon ?

How else would you find the nearest McDonalds?

Easy.
Find the place with no atmosphere.