Rocketman - From the little reading I did, the cdma network would send a cue to the phone as to where it was, so that the gps software could do a faster fix on the best satellites. But it would still need to get that fix, and anyone using a gps receiver knows how easily the signal is blocked.
Actually there are two separate things that a GPS receiver needs to know from as many different satellites as possible:
1) The satellite's ephemeris (where it is, and how it is moving, for those without rocket in their name)
2) Timing information.
It turns out that the timing information is embedded in the chipping that the GPS CDMA does. This is very repetitive, like a metronome. Anyone ever noticed that it is often possible to sync to the beat of very softly playing music long before you can really distinguish the music itself?
In this analogy (which is actually reasonably accurate technially), the satellite position data is like the music. In order to hear it and decode it the signal has to be pretty strong. But, the cell basestation already knows this information. The cell phone itself doesn't need to 'hear' it. It just gets what it needs (doppler info, ...) from the cell network.
Voila - this system locks up with much less received signal strength and it does so much quicker (it knows where to look for the beat, and what kind of beat to expect) than a standalone GPS receiver. Very, very cool idea.
Clark
PS Note that multipath would, to some degree, throw off a system like this. But typically, the strongest multipath comes from nearby reflections, and thus the error is not likely to be huge.
PPS Somewhat oddly, it turns out that in a past life I knew one of the developers of this system, (although this isn't how I happened on the details of this system). It's a small world.
PPPS I haven't yet looked at the key patent in depth, but at first glance it looks pretty good. There is only one other patent I've seen which is anywhere near as crucial to the whole technique. (Note: In addition, I haven't looked to see how thorough the patent is in locking up the whole solution space. An amazing number of patents are just poorly worked from that perspective.) |
