Understanding GPS

gps-satellite-constellationPassive location, position, navigation and mapping.

News and entertainment media as well as now common devices such as cell phones have obscured the true function of GPS (Global Positioning System). Its only function is to provide accurate position information to a receiver that can “hear” three or more orbiting satellites that transmit signals toward earth. Sometimes a transmitter at a known place on earth is combined with information from the satellites to increase accuracy.

The device on earth that computes the location is a multi-channel receiver with powerful computer logic that decodes the location of the receiver. Through programming it can also provide speed and direction and many other details of a moving receiver. Many units can create and store a path map in memory.

The important note is that GPS is a passive, receive only system. A pure GPS receiver does not transmit any signals to the satellites or any other system. All is does is compute a received position code that can be used by other systems. Early civilian GPS simply displayed the location on a self-contained viewing screen. Accuracy was deliberately reduced. I have a Garmin brand unit on my shelf. Hunters, boaters, and aircraft pilots used them only to view their location on a screen. Data is saved in memory but is not actively transmitted real time. It is downloadable to a PC which was useful.

8f51Active location broadcasting or reporting. (Tracking)

So inherently the GPS “system” is strictly passive. At least as far as civilian use. What makes it useful for remotely tracking something or someone is an entirely separate component of a system. The data created by the GPS receiver can be TRANSMITTED to another system by any data transmission method available. It usually involves at least one radio link such a Bluetooth or Wi-Fi. Actually any data capable transmission method can be used. Cell phones work great!

So GPS is a component of a remote tracking system. The passive receiver/decoder (the GPS receiver) and some kind of active but separate transmission system.

Common usage of the term GPS now creates the mental image that any and every GPS is a remote tracking system. This is technically incorrect but very commonly assumed. I have not researched this, but I can comfortably imagine that the vast majority of GPS equipped devices today, because of cars and cellphones, do have remote tracking as their most common function.

My point is that GPS is a location system. The information it creates can be used by any other system (like tracking) that needs to know location.

In the “old days” the tracking device was a small radio transmitter (no data) that was tracked with receivers with directional antennas. At least two receivers far apart were needed.

So the TV Movie statement, “I put a GPS on his car so we can track him.” Is technically incorrect. The GPS receiver (alone) can never be used that way. A “GPS equipped tracking device” is the accurate statement. But so what? Even I can admit it is a fairly insignificant error.

Entertainment media and the folks enjoying it seldom care or know if something technical being shown is accurate or even possible. Its’s not considered a “learning experience” even when that is exactly what happens. Almost no one cares about total factual truth except maybe Myth-Busters

A few of us techno-nerds do know the truth and will carry the torch… 🙂