There was a hiker lost in the Rincon Mountains a few weeks ago. She was alone, with little water and had thought that she was hiking the Miller Creek Trail when, in fact, she was on the Turkey Creek Trail. About halfway to Heartbreak Ridge she realized her mistake and decided to go cross-country to the Miller Creek Trail; a distance of about 6 miles through cliffy, dense chaparral. She ran out of water and energy and was benighted and thus overdue home. The Pima County Sheriff got a call and then Saguaro National Park was notified. She was located by pinging her cell phone using it's GPS feature and was helicoptered out. The following quote about pinging is from the website at:

http://pursuitmag.com/locating-mobile-phones-through-pinging-and-triangulation/
"A cell phone “ping” is quite simply the process of determining the location, with reasonable accuracy, of a cell phone at any given point in time by utilizing the phone GPS location aware capabilities, it is very similar to GPS vehicle tracking systems. To “ping” in this context means to send a signal to a particular cell phone and have it respond with the requested data. The term is derived from SONAR and echolocation when a technician would send out a sound wave, or ping, and wait for its return to locate another object. New generation cell phones and mobile service providers are required by federal mandate, via the “E-911” program, to be or become GPS capable so that 911 operators will be able to determine the location of a caller who is making an emergency phone call. When a new digital cell phone is pinged, it determines its latitude and longitude via GPS and sends these coordinates back via the SMS system (the same system used to send text messages). This means that in instances where a fugitive or other missing person has a GPS enabled cell phone (and that the phone has power when being polled, or pinged) that the cell phone can be located within a reasonable geographic area- some say within several feet of the cell phone.

With the older style analog cellular phones and digital mobile phones that are not GPS capable the cellular network provider can determine where the phone is to within a hundred feet or so using “triangulation” because at any one time, the phone is usually able to communicate with more than one of the aerial arrays provided by the phone network. The cell towers are typically 6 to 12 miles apart (less in cities) and a phone is usually within range of at least three of them. By comparing the signal strength and time lag for the phone’s carrier signal to reach at each tower, the network provider can triangulate the phone’s approximate position."

I am sure that the system has a lot of blind spots but it worked for this woman. Sure beats just yelling.