The Apple iPhone has four discrete radio frequency subsystems: 2G/3G mobile phone, Bluetooth, Wi-Fi, and GPS. Understanding how to enable or disable each will help extend your iPhone's battery charge throughout the day.* Also, understanding the GPS system setting is important when flying.

iPhone 3GS

Here's an overview of each radio frequency (RF) subsystems and how to activate or deactivate it.

Mobile phone service. This is a two-way radio system, designed for long range service. Frequencies for U.S. (and generally North, Central and South America) are:

GSM  850: downlink: 869-894 MHz, uplink at 824-849 MHz
GSM 1900: downlink: 1.930-1.990 GHz, uplink: 1.850-1.910 GHz

The iPhone 3G(S) is a "quad-band phone" which means it can also operate on the GSM 900 and GSM 1800 frequencies used in Europe, parts of South America, and elsewhere. (See the Wikipedia link.)

To enable both the 2G and 3G networks, go to Settings -> General -> Network and make sure 3G is on. Also in Settings, make sure Airplane mode is off. If you know you're in a rural area where 3G is not available, disabling 3G will save quite a bit of battery power.

Bluetooth. Bluetooth is a two-way, ultrashort range protocol that works in the 2.40-2.485 GHz band. It avoids interference with other systems in the same band by being very low power, about a milliwatt, and has a maximum range of about 10 meters.

You enable/disable Bluetooth with Settings -> General -> Bluetooth. Because this is an ultra short range protocol, it uses little battery power. It's used primarily for Bluetooth headsets. There's little advantage to leaving it off and not much impact if you do need it and leave it on.

Wi-Fi. Wi-Fi, the IEEE 802.11 standard, is a two-way, short range protocol and operates in two bands. Initially, all Wi-Fi used 2.412-2.472 GHz. The 802.11n protocol added the ability to use 5.15 to 5.25 GHz (indoor use), but note that the iPhone is restricted to 802.11b/g.

We know that the Apple Airport Extreme base station, for example, generates 100 milliwatts (20 dBm), but Apple doesn't publish the specs on the output power of the Wi-Fi unit in the iPhone. However, it does claim that the 3GS can last for nine hours of web browsing on Wi-Fi or five hours of talk on 3G or 12 hours of talk on 2G.  So that suggests that the Wi-Fi unit is generating roughly half the output power of the 3G radio in continuous service. And 3G is the biggest drain on the iPhone battery.  But Wi-Fi uses somewhat more power than 2G, according to Apple's relative numbers. So if you're out and about, not in range of a Wi-Fi signal, it might pay to turn off the Wi-Fi system. **

Disable the iPhone's Wi-Fi with Settings -> Wi-Fi ( or Settings -> General -> Network -> Wi-Fi).

Global Positioning System (GPS). GPS is a one-way system, via satellite, that operates at two frequencies: 1.57542 GHz and 1.22760 GHz. This is used for the iPhone's Location Services, for example, locating your current position and movement on a map.

Even though the radio unit is receive only, any circuit that oscillates at a given frequency, in order to receive, will also generate a small amount of residual emission. I haven't seen any numbers on that output power, but it's almost certainly less than any other RF system on the iPhone. That brings up the issue below.

GPS Considerations

Note that when you activate Airplane mode, the Wi-Fi, and Bluetooth settings are automatically set to off. However, Location Services shows that it's still on, even though the GPS system is disabled. Apple's iPhone manual says, on page 137, that Airplane mode stops all radio emissions and that map locations are unavailable. The fact that Location Services indicator remains on is a recognition that alternative location services such as IP address location (with Wi-Fi turned back on) could still be used. For example, turn on Airplane mode, turn Wi-Fi back on, and Location Services, if on, will use your IP address to generate a map location. Of course, it won't be as accurate as GPS would have been.

Note that it's the airline's call whether you can use a GPS device in flight. Some prohibit any radio device whether it's a radio transmitter or receiver.

Check with the airline you're flying on, and, to be safe, it's probably a good idea to, in addition to Airplane mode, manually turn off Location Services as well. That's for show, just in case a flight attendant asks to inspect your iPhone and mistakenly thinks your GPS unit is active. You could be in for a long session with a Federal Marshall when you land if a flight attendant even slightly suspects you've violating the airline's rules. Do all this before the aircraft cabin door closes so that when the iPhone is booted up in the air, it has the approved settings.

With Airplane mode OFF, the result of turning off location services on the GPS receiver in unclear. ***  It does, however, serve to keep apps from accessing available location service methods in order to preserve privacy.

Finally, note that in Airplane mode, the mobile phone radio is turned off, but Bluetooth and Wi-Fi can be individually, manually turned back on. When flying, do that only if allowed by the airline rules. For example, AirTran, American Airlines, Delta, United, and Virgin America offer approved Wi-Fi service on some flights. Knowing the specific rules for your airline is a good idea.

Power Summary

From the above discussion, the most power hungry systems, at least for continuous service are, from most to least:

1. 3G radio system
2. Wi-Fi
3. 2G radio system
4. Bluetooth
5. GPS

Right now, without any kind of "smart" Location Service that can identify your location, it's up to you to enable and disable these radio systems to get the most out of the battery charge. It would also be nice if just one of the 65,000 iPhone apps were able to show the relative power usage of each subsystem in real-time, but the fact that it doesn't exist suggests Apple hasn't supplied an API. Or no developer has thought of the idea.

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* There are other techniques to manage battery usage such as screen brightness and push notifications.  The focus of this HOWTO, however, is only the use of the RF systems.

** If the Wi-Fi system isn't very active when you're away from a signal, then, of course, it's not using much power. Then it's a matter of personal preference to disable it. 

*** I asked Apple for clarification, and the reply was a reference to location services. The comment there about turning off location services to conserve battery power suggests but does not prove that turning L.S. off shuts down the GPS receiver. A discussion with an iPhone developer revealed that, in fact, the GPS may be routinely off unless there is a request for location that can't be handled by the other methods. So turning L.S. off implies that the GPS radio won't be activated, as Apple said. The preferred method, however, (when flying) to keep the GPS system definitively off is Airplane mode.