GPS & e911?

It uses Assisted GPS, which is both. You need this because GPS doesn't work indoors, and not all emergencies happen out of door.

All phones in the US are required to support E911, but NO phone can provide exact location if it can't receive a GPS signal, even if it has a GPS.

It's far more dependent on the carrier. Using GPS for E911 requires the ability for a carrier to initiate GPS operations over a control channel, and that's mostly done only by CDMA based carriers who have GPS in all their phones.

GSM carriers use tower triangulation for E911, which requires less stringent accuracy by the FCC, and no phone cooperation. In real life, GSM tower methods can often find you within 300-600 feet, which is much better than the required 1,000 - 3,000 feet.

CDMA phones with A-GPS can find you within a few yards in good conditions. When having to fall back on CDMA tower trilateration, it's within 150 - 600 feet.

Since most E911 calls come from inside buildings where GPS doesn't work as well, most E911 calls therefore use cell tower methods. Newer A-GPS chips can change this ratio somewhat, as they can receive usable signals even deep in buildings. Tighter FCC rules are also trending even GSM carriers towards eventually using A-GPS. But for now, they don't except for location based services (not E911).

It uses Assisted GPS, which is both.

A-GPS does NOT mean both GPS and cell tower triangulation. They are separate methods.

You need this because GPS doesn't work indoors, and not all emergencies happen out of door.

True. That's why Verizon, which uses GPS as its primary E911 location method, has to fall back on tower based if GPS isn't available.

So if the iPhone is receiving a GPS signal, does it relay this to 911?

ATT doesn't use GPS for E911... just tower based. No, I don't think they can ask the iPhone's (or any other ATT phone's) GPS for location. From the ATT developer docs:

+"TDOA (Time Difference of Arrival) is the location determination method that AT&T uses to locate a caller when they dial 911 from their mobile phone. TDOA calculates the location of a mobile phone by using the difference in the time of arrival of signals at different cell sites."+