GPS receivers and satellites work together to calculate your exact location

            Today, many people cannot even read a map because we are so dependent on our GPS system.  Everyone who has a smartphone has their GPS with them at all times because they are conveniently built into our phones.  But does anyone actually know how a GPS works? I was unaware of what GPS stood for before doing a little research.  The GPS, or Global Positioning System, was originally created by the United States military to use as a navigation system.  The Global Positioning System is composed of 29 satellites orbiting the Earth at an altitude of 20,000 kilometers.  These 3,000 to 4,000 pound solar-powered satellites make two complete rotations around the Earth per day.   It only takes 24 satellites to cover the Earth, but there are five spare satellites to improve accuracy in calculating one’s location. No matter where you are on earth at any given time there are four satellites 'visible' to you and your location. 

            A GPS receiver uses a process called trilateration to locate itself.  

Trying to understand this process is simpler by first examining it from a two dimensional perspective.  If someone were to say, "You are 216 miles from Boston, Massachusetts do you know exactly where you are?" No, because you could be 216 miles from Boston in any direction, north, south, east, or west.  But, if someone says, "You are 216 miles away from Boston, 81 miles from New Haven, Connecticut, and 229 miles from Washington, DC," you can figure out where you are by calculating where all the radiuses overlap.  This is exactly how GPS satellites locate a GPS receiver.  Each satellite creates a sphere indicating where the GPS receiver could possibly be.  The four satellites that are “visible” from your location create four spheres.  Where these spheres overlap is where your exact location is.  For the best results, including altitude as well as latitude and longitude, more than four satellites are necessary. Technically, the Earth can act as the fourth sphere but usually receivers search for four satellites. But even though we have 29 satellites, sometimes they cannot locate the receivers.  Have you ever turned on your navigation system and the message “Searching For Satellites” appears?  This is because the receiver cannot locate enough satellites to determine your location. Usually trees or buildings block the transmissions from the receiver to the satellites making them unable to send signals to each other. 

            In order for your navigation system to locate exactly where you are we have already established receivers need at least three satellites that are ‘visible’.  But, your navigation system must also know the distance between you and each of those satellites. 

Your GPS receiver does this by analyzing high-frequency, low-power radio signals from each of the GPS satellites.  These radio waves are classified as electromagnetic energy because they travel at 186,000 miles per second, or the speed of light.  The GPS receiver can derive how far away the signal has travelled by timing how long it took for the signal to reach itself.

            The process in which your GPS receiver calculates the distance to the GPS satellite is extremely complex.  

At a certain time the GPS satellite will begin to transmit a pseudo-random code, or a long, digital pattern. The GPS receiver also begins transmitting the same pseudo-random code at the time.  When the satellite’s signal reaches the receiver on earth, the code created by the satellite will lag behind the receiver’s code.  The length of the delay between the two codes is believed to be the signal’s travel time.  The receiver on earth multiplies the travel time by the speed of light to calculate how far the signal has travelled.  With this, the receiver has now determined its distance to the satellite in orbit.

            In order to do these calculations both the receiver and satellites need clocks that are exact to the nanosecond.  

Both the satellites and receiver need to have atomic clocks to ensure they are in sync.  But, atomic clocks cost about $50,000 to $100,000, which is too expensive to place in consumer products.  To solve this problem, the Global Positioning System planted the expensive atomic clocks in each of the GPS satellites.  In each receiver is a typical quartz clock, which is consistently resetting.  The receiver compensates for this by determining it’s own inaccuracies as it is analyzing incoming signals from the satellites.

            Technology today not only allows you to find your stationary location but you can now track your movement by constant communication with the satellites. 

Today GPS receivers can calculate how far you’ve traveled, how long you’ve traveled, your current speed, your average speed, a map of exactly where you have traveled, and the estimated time of arrival at your destination.  Who knows what else GPS receivers will be able to do in another five years.