Turns out, there is a way to use GPS while having the cellular radios disabled.
If you lock the SIM card with a PIN, it apparently disables the cellular radios. Locking is explained here (Apple technote HT1316), but there is a critical piece of info missing. The default PIN (at least on my AT&T iPhone) is 1111. Entering the wrong code three many times will require a call to AT&T support, so beware. Entering the wrong code ten times kills your SIM card.
You can enable, disable, or change your SIM PIN in the following locations:
- iPhone: Settings > Phone > SIM PIN
- iPad Wi-Fi + 3G: Settings > Cellular Data > SIM PIN
Set the slider to “on” and enter 1111 (for AT&T)
After the SIM is PIN locked, power the phone off and back on and DO NOT ‘unlock’ the SIM. The carrier will now read “locked SIM” and there will be no cellular data. Turn off WiFi and Bluetooth also, they use a lot of power as well. I haven’t tested this yet, but a poster the forum I found the hint from posted running the GPS in tracking mode for 15 hours with the battery still showing 89%
This is also a great tip for traveling abroad without getting hit with huge roaming charges. No SIM, no billing.