[Formendacil]

If we want to go with an overly simplistic GPS analogy, there are a number of possible reasons for Sauron not noticing it until Mt. Doom.

1.) Frodo was simply a stronger power source than Gollum (as already mentioned by Galadriel55).

2.) Sauron's ability to pick up a transmission was limited in range to something transmitting in Mordor (not unlike rabbit ears on a television...)

3.) Mt. Doom's innate connection to the Ring gave it a signal boost that was otherwise unknown.

Also, I think it's worth noting that while Sauron first rose again after his defeat at Elendil and Gil-galad's hands in 1050 when the "shadow fell on Greenwood," he was probably still growing in power over the course of the Millennium. Actually, we know he was, at least in terms of military might, but I think it clearly goes beyond military might. Sauron's identity as the Necromancer was a secret until the very end of his time in Dol Guldur, and I think that Sauron only returned to Mordor and declared himself openly when he did because he finally felt strong enough to do so. Prior to that point, while I imagine he would have known right away if someone had destroyed the Ring (the "pop" as his existence was reduced to a shadow that would never have the time to grow back to a solid form would have almost certainly given it away), it's possible he wouldn't have had the ability sense more than the fact that the Ring was still out there.

Also, the connection to Mt. Doom should not be underemphasized either. This was the place of the Ring's forging and the very fact that it could only be destroyed there indicates the strong connection between the two. Frodo's claim of the Ring there is different in kind from Gollum's claim on the Ring and from Sam's use of the Ring walking into Mordor simply because of the location. What is more, Frodo is clearly acting in a ritualistic way: somewhere, deep down in the Hobbit who no longer has the will to fight the Ring, he knows the significance of the location. Whereas Gollum and Sam (and Bilbo and Frodo-priorly) certainly used the Ring and even claimed it as their own, Frodo-at-Mt.Doom is the first to be thinking of the true Master of the Ring as they do so. The other claims on the Ring were theft, perhaps, but Frodo's Mt. Doom claim was a challenge. A challenge, admittedly, that a Hobbit could possibly hope to win, but a challenge nonetheless.

In many respects, Frodo's claim of the Ring at Mt. Doom is probably what Sauron was expecting from Aragorn, after he wrenched control of the palantír away from him--a challenge that Aragorn won. Even if he was feeling "twinges" from the Ring, Sauron had no reason to be concerned about them, because he was confident he knew where the Ring was--well away from Mordor. In other words, until Frodo challenged him for real at Mt. Doom, there was nothing about his "Ring GPS" that would actually have given away the Ring's location.

Thus suggesting, perhaps, that "Ring GPS" is less accurate an analogy than I would have said at the beginning of this post.