What Is A GPS Receiver And How Does It Work?
So about those trackers: If they don’t use a cellular signal to track the device, how do they work? Passive GPS trackers use GPS data loggers. That is, they keep track of where the device is at all times, provided it has power (whether battery-operated or plugged into the vehicle’s power), and log that data on the device itself.
This is done with a GPS receiver. But how does a GPS receiver work?
First, there are satellites that make up the Global Positioning System (GPS) network. There are currently 31 of these earth-orbiting satellites in operation. Developed by the American military, these satellites are in constant orbit, making two complete rotations around the earth at a height of 12,000 miles above the earth’s surface. Their orbits are designed so that no matter where you are on Earth and no matter what time it is, you should always have at least four satellites visible (if you could see that high, that is) in the sky above you.
A GPS receiver does the work of locating those satellites, determining the distance to each based on the time it takes to get a high-frequency, low-power radio signal from the satellite and the satellites’ known location, and then determines its own location based on the distance to each of the four (or more) satellites. This process works using the mathematical principle of trilateration.
Let’s start by thinking of two-dimensional trilateration, and then we can explore better how it works in three-dimensional space.
GPS Trilateration Method
Let’s say you’re lost somewhere with no idea where you are. A stranger walks by and tells you you are 690 miles from Minneapolis. Now, that’s great—but absent any other information, isn’t in and of itself very helpful information. You could be anywhere on the circle with a radius of 690 miles from Minneapolis.
Let’s say a second stranger walks by and tells you you are 615 miles from Tuscon. That’s a little more helpful because now you know you are in one of the two places where the Tuscon circle and Minneapolis circle might intersect. (Think of it as a Venn diagram of sorts.) If a third stranger tells you you are 625 miles from Boise, however, you can determine exactly where you are—because there’s only one place on earth that is at the intersection of those three circles. And sure enough, Denver is 690 miles from Minneapolis, 615 miles from Tuscon, and 625 miles from Boise.
It gets a little bit trickier with three-dimensional space because you’re looking at spheres rather than circles, but the principle remains the same. By the time you introduce three satellites, for instance, those spheres intersect at only two points—and only one of those points can be on Earth. GPS receivers use four satellites, however, instead of three to help ensure greater accuracy as well as to determine elevation.
As a result, if the GPS receiver knows
- The location of at least three (and ideally four) satellites in the sky above, and
- the distance between itself and each of the three (but ideally four) satellites, then
- it can determine its location using trilateration.
The best GPS tracker units have multiple receivers so that it can pick up signals from multiple satellites simultaneously.
And because the receivers know that radio waves travel at the speed of light, they can determine how far they are from each satellite based on the atomic clock time signature of when the radio wave was sent and the time it took for that signal to get to the receiver.
Actually, it’s a little more complicated than just having a time signature. Instead, both satellites and receivers run digital patterns of code; the receiver knows how long a signal from the satellite took to reach the receiver by noting the lag in the code. This means the receivers don’t need an expensive atomic clock (which can run $50,000 or more!). Instead, because each satellite has an atomic clock, the receivers can get away with having quartz clocks that don’t need to be quite as accurate. The receiver can simply look at the time signals from the satellites and gauge its own inaccuracies, correcting to that time value.
This works because using four satellites to determine a location builds in redundancy and allows the receiver to determine how the distances are off proportionally (since all of those distances will have been determined using its own clock). After determining that margin of error, the receiver can then automatically reset its clock based on the satellite’s atomic clocks. And because it does this constantly anytime it is getting radio signals from the satellites, the receivers are guaranteed to be nearly as accurate in keeping time as the atomic clocks on the satellites.
Another piece that helps the receivers stay accurate? An understanding of when and where each of the satellites should be. By storing an almanac of upcoming satellite locations, the receivers know where to look and another layer is built in that can help catch errors, especially as the Department of Defense is constantly monitoring each of the satellites exact positions so that they can update the almanac when variance results (such as courtesy the gravitational pull of the moon and sun, for instance).
There are still other areas where errors can pop up, however. For instance, the earth’s atmosphere slows radio signals at different rates depending on where you live and what the atmosphere is like in that location, making it difficult to account for that slowing. Similarly, objects such as skyscrapers, mountains, and anything else that might disrupt that “line of sight” to the satellites can create a bouncing signal, which also introduces error. Finally, satellites also sometimes send out bad data, meaning they misreport their own location.
In general, though, the most essential function of a GPS receiver is to lock in on at least four satellites, use the timestamp data to determine the distance to each of those satellites, and use trilateration to determine the receiver’s own location, which the GPS data logger then stores until someone next downloads the data files from the passive GPS tracker.
As a result, passive GPS trackers or no monthly fee GPS trackers often contain the following bits of information:
- How far the receiver has traveled, much like an odometer
- Where the receiver traveled, often as a map overlay
- How long the receiver traveled
- The speed(s) at which the receiver traveled
- The average speed at which the receiver traveled
How Accurate Is A GPS Receiver?
That varies. GPS-enabled phones, for instance, are usually accurate within 5 meters (16 feet). Better GPS receivers are obviously more accurate; signal transmission (that is, the signal transmitted by the GPS satellites) is 95% accurate within 1 meter (3 feet). Even if most receivers aren’t quite up to that standard, some receivers get closer than others.
Lots of things can affect that accuracy, however, some of which we’ve discussed previously. For instance, anything that might affect that line of sight to the GPS satellites (including underground use, buildings, bridges, trees, etc) will also affect accuracy. Similarly, atmospheric considerations that affect accuracy include solar storms, radio interference, and more. There can also be issues with the receiver itself, or with the GPS’ mapping software, which can, in turn, lead to accuracy issues or, at the very least, perceived accuracy issues.