GPS -> Moon

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rogers_buck

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I'm wondering if the terrestrial GPS constellation might not be useable for triangulating your position on the way to the moon. Suppose that a few times each day, four of the little fellars turned themselves towards the moon so that the CEV could get a fix. Granted, it wouldn't be your typical GPS receiver, but the principals should still apply after a few coordinate transformations - I think... Would that be more or less accurate than a stellar triangulation?
 
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najab

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Nope. 'Turning' a GPS satellite ain't gonna happen - there's too many critical applications on Earth to put the constellation at unecessary risk. Besides, once you get any appreciable distance to the Moon the Earth is so small that even if you used satellites on opposite sides of the globe, their separation isn't enough to do accurate triangulation.
 
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bobvanx

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We're discovering that GPS is so useful, that it is very likely that such a system will eventually be placed around Luna, as well as Mars.<br /><br />Interplanetary travel will likely continue to be guided by the stars, but for determining place and velocity in relationship with a hard surface, GPS is a tech that can't be beat.<br />
 
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rogers_buck

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The earth is ~ 13,000 km in diameter when you throw in the GPS orbits. That gives it an angular diameter of about 2 degrees at the moon, of course half way to the moon it is 4 degrees, and so on. Small angles compared to having the sats in your sky, but we can assume a "special" GPS receiver. Ignoring the antenna pointing problem for the moment, what would be the limit of range for a useable "fix"?<br /><br />If you had good coverage, wouldn't it make sense to consider asking the EU to add low power outbound omni antennae to their Gallaleo GPS? Could satellites in GSO use such a GPS for positioning?<br />
 
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flynn

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I've suggested a microsatelite constellation of GPS (or indeed LPS) around the moon before, someone pointed out they would interfer with any night side observatories but I'm I'm sure they could be programmed to anter a sleep mode on the darkside unless required for a incomming or outgoing craft. <div class="Discussion_UserSignature"> <font color="#800080">"All God does is watch us and kill us when we get boring. We must never, ever be boring" - <strong>Chuck Palahniuk</strong>.</font> </div>
 
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bobvanx

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<blockquote><font class="small">In reply to:</font><hr /><p>night side observatories<p><hr /></p></p></blockquote>Farside Radio observatories? I can't imagine that a little blip across the field of view on an optical observatory from a satellite compares at all with the light pollution terrestrial observers contend with.
 
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bobvanx

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I don't think antenna pointing is much of a concern. After all, if you're halfway to the moon, then all the GPS sats that are on the other side of Earth are pointing roughly in your direction. Those that aren't occulted are going to give you a pretty good signal, I should think.<br /><br />I'm sure it's more a question of the timing and triangulation errors adding up. Even here at the surface, if you want a real accurate location, you'll set up a base station and reference off of that. Back-of-the-envelope and referencing the trouble Scaled and Armadillo have in holding a lock on the vertical vector with GPS, I'd give a SWAG and say that at the moon the accuracy is going to be plus or minus 500m.
 
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