There are not one, not two, not even three, but four types of GPS signals your cell phone can use. Allow me to demystify them for you.

On the Droid X and Droid 2, there are four GPS signal options. They’re found in the Settings app, Locations & Security category:

• E911 Only
• Use Wireless Networks
• Use GPS Satellites
• Enable Assisted GPS

Here’s the rundown:

E911 Only. If you’re used to seeing cryptic techy acronyms, you may not see the sense in the E911 location service. That’s okay: I didn’t get it at first, either. But when you think “This is a 911 emergency,” then you can see that E911 is the location service used in North America so that 911 operators can use the GPS signal on your cell phone to locate you. E911 equals Emergency 911.

You cannot disable the E911 signal on your phone, which is why the setting is called . Well, you probably could if you hack the phone, but I assume that most people don’t hack their phones, nor would I see why you would legitimately want to disregard this vital feature.

Use Wireless Networks. Turning on this setting directs the phone to monitor Wi-Fi networks that can help pinpoint your location — with the assistance of GPS satellites as well. That’s because the physical location of some Wi-Fi networks is known. In fact, Google gathered Wi-Fi networking locations when it does its Street View imaging. (And that was controversial, but I see their point.)

An issue with using Wireless Networks is that Wi-Fi networks can move. If you use this option and suddenly notice that the Maps app has you placed several hundred miles away, then it’s because the Wi-Fi network either moved or is being misinterpreted.

Obviously this item can’t be used when you’ve turned off the phone’s Wi-Fi radio. (Though the option doesn’t become disabled should you turn off the Wi-Fi radio.)

Use GPS Satellites. This setting is the standard GPS setting, the one that uses the signal from the Global Positioning Satellites. That signal helps determine your location anywhere on the globe. So in theory, the standard GPS option works even when your phone isn’t getting a cell signal. (Though the Maps app may not update in that instance.)

Even if you turn off the GPS Satellite option, the authorities can still use your phone to determine your location. That’s because the E911 option allows emergency operators to turn on your phone GPS signal, as well as the AGPS (covered next).

Enable Assisted GPS. Also known as AGPS, the Assisted GPS option supplements the standard GPS system with information from the cell towers near you. Basically it uses triangulation based on the signal strength of the towers to help pinpoint your location.

The problem with AGPS, and the reason some people don’t use it, is that it can incur more data charges and it can drain the battery faster. I believe both situations are rather insignificant, however: The data sent between your phone and the cell towers for AGPS isn’t that much. And, I suppose, if battery juice is getting low, then you can turn off AGPS. Even so, you’ll probably be using AGPS for navigation, and if you’re in a car I’d recommend using the car adapter anyway.

In addition to these options, various apps warn you whether they use or need GPS information. In all cases, you’ll be alerted to apps that use the GPS when you first install them or when they’re upgraded. That information appears on the alert screen before you install the app.

Further, some apps query you before they access the GPS. They may say something like, “Okay to use location information?” You then have the option whether or not to share you and your phone’s location with the app and, generally, with the rest of the Internet.