GPS DemystifiedPosted: March 16, 2012
Not long ago, a friend of mine told me she was planning on taking a long road trip to visit some national parks. I asked her if she had all her maps, and she laughed and said, “Oh I have my trusty GPS.”
“What if it doesn’t always work?” I responded.
She looked at me questioningly. “Why would it not work?”
I realized then how blindly we rely on technology today, and just how little many of us actually know about it. “Do you know how it works?” I asked.
“Um, I think it has something to do with a satellite, right?”
This is the conversation that inspired this blog post. Ancient travelers used to navigate based on the positions of the stars and constellations. Nowadays, many people use a different kind of constellation to find their ways, and most that I have spoken to don’t know that it even exists or how it really works. They view their GPS devices as pseudo-magical objects, and although I am by no means a total expert on the science, I do know the basics of how they work and hope to help explain it so that it is a little more understandable. This is the super-simplified answer.
Let’s start with a few definitions. “GPS” stands for the Global Positioning System, which is a combination of satellites and their ground stations that enable users to navigate if they have a receiver (for most people, it’s that piece you carry or put in your car). There is a network of 24 satellites with an additional 4-6 in reserve that act as ‘spares’ and form what is considered a ‘constellation’ that orbits the earth. These satellites are pretty far out—about 12,500 miles up. To put this in perspective, the International Space Station is only about 185 miles up, and the Hubble Space Telescope is about 365 miles up. Despite this distance, they move very quickly, and it takes each GPS satellite roughly 12 hours to circle the earth one time.
The GPS’s ground-based stations monitor where each satellite is at all times, how well each is working, as well as make corrections to the signals being transmitted from them and coordinate which ones are running at a given time. The master control station is at Schriever Air Force Base in Colorado, but the other stations are located in Hawaii and Kwajalein in the Pacific Ocean, Ascension Island in the Atlantic Ocean, Diego Garcia in the Indian Ocean, as well as Cape Canaveral and Colorado Springs.
This constellation of satellites also has a name: NAVSTAR. This stands for “Navigation System using Timing and Ranging.” This is a very important concept. Each NAVSTAR satellite is equipped with an atomic clock, and each emits its own unique radio signal that can be picked up and identified by a receiver on earth. The receivers have their own clocks too.
Your receiver’s clock is synched with the ones on the satellites as closely as possible. However, since atomic clocks are extremely expensive and super accurate, the average consumer receivers typically have to have the next best kinds. These clocks may not be quite as accurate, so there is usually a miniscule difference between the ones in the receivers and the satellites. Usually, the more expensive your device, the better the clock can keep time compared to the satellite and/or correct for any inconsistencies.
The receiver makes note of the time it was when a signal left a satellite compared to the time it receives it. Based on this information and the speed of the radio signal transmission itself, it can then calculate how far away each satellite is that it gets a signal from. If your receiver’s clock is not too accurate, the reading can sometimes be a little off. This typically doesn’t affect someone who is trying to get from one place to another, but it may if you are trying to get exact coordinates of a specific location.
Then, the receiver does some figuring. For example, if it is known that you are 10,000 miles away from a particular satellite, you can be anywhere on earth within a circle of that distance from it. But if you are also 11,000 miles from another nearby satellite, you would thus have to be at one of the two points where the two circles overlap on the ground, like a Venn Diagram. This narrows your location down significantly, but it is not precise. This is why if your GPS receiver says there are “only two satellites in range,” it won’t work well if it works at all. You need a third that would narrow it down even further and intersect one of the two points to confirm where you are, and the more satellites you have sending signals, the more accurate your receiver will determine your location. There usually is a minimum of 3 satellites needed at all times, which is why this is called trilateration.
In addition, the reading on the receiver may not be as accurate if the satellites happen to be clumped in the same general area of the sky, as opposed to being more spread out. Think of a bunch of circles being drawn on top of each other rather than being spread out enough to see where they actually overlap. This is called the Positional Dilution of Precision (PDOP), which can be another reason why your GPS receiver may not always work well.
But ultimately, once your receiver has figured out your location, it compares that to pre-loaded maps of roadways, etc. and displays them together. That is why you are supposed to periodically update your maps, though I don’t know anyone who actually does.
Finally, you may sometimes lose a radio signal in the event that a satellite moves behind a tall building or mountain that blocks it. If you have more than three satellites in range, this isn’t a problem, but I know my receiver gets confused when I drive in a city or through a tunnel. Some newer models can actually plan ahead—if they know based on the pre-loaded maps that a tunnel is coming up for instance, they can calculate your future trajectory and take that into account, so it never appears that the signal is lost in the first place.
Civilian GPS units weren’t always as accurate as they are today, but it was on purpose. Selective Availability was a term coined by the US military for their intentional degradation of the satellite signals accessible to the public for national security reasons, which would ensure a receiver’s reading would be at least 300 feet off. In the year 2000, President Bill Clinton ordered Selective Availability to be turned off because he and the government felt that it would be for the greater good. What do you think?