What makes a cell phone a GPS? The first requirement is a GPS receiver. GPS signals originate simultaneously from three to twelve satellites orbiting about 11,000 nautical miles above the earth. The GPS receiver and software sort out the signals from the different satellites and pinpoint the receiver’s location on the surface of the earth to within a few yards, sometimes closer.More and more cellular phones are manufactured with GPS receivers built into the handset. Cell phones without built-in GPS receivers can still become a GPS unit by connecting with a separate GPS receiver through a USB or serial port or by a wireless Bluetooth connection. Bluetooth is a specification for wireless, short-range communication of voice and data between two devices.Both methods can provide turn-by-turn driving instructions, often with voice and illustrated maps that show your location on the cell phone screen. The built-in GPS receiver method is more convenient. There are no wires or devices to connect and there is nothing to download from your desktop. Attach your GPS enabled cell phone to the windshield holder or pull it out of your pocket if you are on foot, push a few buttons and off you go.You will, however, need a special cellular plan with your mobile phone service company, and you may or may not also need to subscribe to their data service. The usual cost is between $10 and $25 per month, but that can vary up or down with usage. Sprint/Nextel and Verizon provide GPS cell phone service with different features.In addition to convenience, another advantage to a cell phone GPS plan is greater accuracy and fewer lost signals from overhead obstructions. The phone uses a signal from the cell phone tower to correct or enhance the satellite signal. This wireless assisted GPS, or enhanced GPS, can sometimes work inside buildings or under heavy forest canopy, both of which are terminal to line of sight satellite signals. A disadvantage with the cellular plan is that your GPS will work only in places that you can get cellular telephone service.If you use a separate GPS receiver that connects to your mobile phone then you will continue to receive GPS signals regardless of the availability of cellular service so long as your receiver and the satellites can see each other.Separate GPS receivers usually come bundled with software on a SD or similar card that you insert into a slot on the phone. The software has the maps that display on your phone screen. You can continue to receive cell phone calls or connect to the internet with your cell phone while the GPS is working, assuming of course that you have the proper subscriptions for doing so.The separate GPS receiver method avoids monthly plan fees but you will have to balance that against the cost of the GPS receiver and software, which is from $100 and up depending on the bells and whistles you want to add.If you live in an area with good cell phone service, and you are a non-technical person, or your are buying GPS for somebody who would never read a manual, then the Sprint/Nextel and Verizon cellular GPS plans are a good choice. When purchased as part of cellular plan the cost of a GPS enabled smartphone is not much different than a regular handset, and $10 – $25 per month is not unreasonable for the service.