## Monday, August 10, 2009

### How Do Satellites Track Mobile Phones?

Have you ever wondered how satellites can track mobile phones? It seems like they’re so far away, yet they can be accurate within about 15 meters. Just how is that done?

The key to tracking any signal, whether it be cellular or radiowave signals, is something called trilateration. In order for trilateration to be used effectively, your phone needs to be picking up on at least 3 satellites – 4, or more, is better.

Your GPS-enabled phone receives a constantly streaming signal, from the satellites, containing information such as the time the signal was sent and the orbital information of the satellite.  Based on that, your phone’s GPS receiver calculates your location in latitude and longitude. It can also calculate your current speed, based on the time between readings and distance covered.

So how exactly does this trilateration thing work?

##### How Do Satellites Track Mobile Phones?

Imagine a cone extending down from each of the three satellites covering your location. These cones make ellipses, close to circles, when the hit they Earth. Now, you have three intersecting circles. The centers of those circles are then used in a trilateration equation to determine roughly where you are. The point where all three circles intersect is your position. Let’s see how that might look in a simplified view.

The actual equation is more complex than the scope of this article will allow. However, I think this will give you a good high-level understanding of how it all works.

Since the GPS system employs from 24 to 32  global positioning satellites, your position can be determined almost anywhere on Earth. Take a look at the animation below and you’ll see how multiple satellites can transmit to your phone simultaneously. When there are 4 or more satellites transmitting their signal to your phone, the calculation is far more accurate.  Originally, the GPS receivers could only use data from 4 or 5 satellites at once. Now they can use as many as 20.

What can stop it from being tracked is anything that is going to block the line of sight. For instance, if you are in a building, away from a window, the satellites cannot communicate with your cellphone. So it isn’t a foolproof system.

Not all phones have the hardware to be tracked by satellite, but many do. BlackBerry and Treo are popular brands of cellphones that have satellite-trackable models. If your phone has GPS but doesn’t use satellites to trilaterate, then it is relying on the trilateration of cell towers, or even WiFi, to determine your location. Same principle, slightly different technologies.

Once your phone has calculated your position, how does it let anyone else know what that position is? I wasn’t able to find a definitive answer, but I’m led to believe it does it via your cell signal when a call is made,  or SMS. I bet one of our readers has an answer to this question.

Interestingly, the entire Global Positioning System of satellites is run by the United States Air Force’s 50th Space Wing, located in Colorado. The system has been around for quite some time and was made accessible to the public during President Reagan’s administration.

Of course, there are those people that fear that having their location tracked by their phone will lead to some sort of uber-surveillance. Personally, I don’t see it. How many cellphones are there in the world? What kind of massive server farm would you need to keep track of all that?

Very generally speaking, if law enforcement wants, or needs, to track your phone, then a warrant must be obtained and provided to the service carrier. There are situations of emergency where this process is sped up and may not include the full warrant procedure. Surely you’ve seen at least one story of how a stranded or kidnapped person was found alive, thanks to cellphone GPS services. Due to all those safeguards, the system is reasonably free from abuse and a technology that truly does add to our lives.