In 2022, GPS is all around us – maybe even more so than some people are comfortable with. Short for “Global Positioning System,” GPS is the hallmark technology used for locating and tracking people and things, and for that reason, surrounds our everyday lives.
We can not only access detailed route guidance from San Diego to San Francisco, but we can also search for generic term like “food” and see only relevant, nearby restaurants recommended to us. Even mobile applications can utilize GPS to offer us a streamlined experience.
But how exactly does GPS work? And what does the technology really do? Without any knowledge on the subject one might assume that it utilizes the Internet to find its location. I know I did at first. I mean, every innovation nowadays comes from the Internet, right?
Let’s consider the GPS navigation systems in our car sand on our phones. Cars – at least for the moment – generally do not connect to the Internet. However, in-car GPS systems have existed for over 20 years. And in phones/mobile devices, our Maps route does not crash or quit once we enter an area with no service. By those two examples alone, we can see that “Internet” does not hold up as the answer to our question. In a practical sense too, if GPS relied on the Internet to function, it would be a huge limitation to the technology. Imagine being in a remote location and losing your route because of weak connection or having your route buffer right before a turn in a busy city. The limitations could be dangerous.
But if not thanks to the Internet, then how does GPS work?
At its core, GPS is a system that includes 31 satellites that orbit the Earth and send out information that devices receive and use to determine their (the device’s) location. There are 31 satellites to ensure that GPS devices can be anywhere on Earth and still pick up signals from at least 4 satellites (more on this below). This process breaks down into five major steps:
STEP 1: Satellites send a signal via radio wave that includes its location and time sent
STEP 2: Radio signal travels from space to Earth for devices to receive
STEP 3: GPS device (smartphone, tracking unit, etc.) receives signal and notes the
exact time the signal was received
STEP 4: GPS device calculates distance from the satellite
STEP 5: GPS device repeats steps 1-4 for four satellites, and uses geometry to find
GPS: A Closer Look
Let’s take a closer look at each step to better understand this process.
Step 1: Satellites send out a radio wave signal
Starting at step 1, each of the 31 satellites is constantly sending out radio signals. These signals are sent out regardless of any action on the receiving end. In other words, GPS devices do not request data from a satellite; satellites send out signals and GPS devices receive when needed. So, if all GPS devices were shut down, these satellites would still be transmitting the signal, they would just not be received by anything. This allows for maximum coverage from the GPS system because it does not rely on the device being able to reach the satellite. If every GPS device had to communicate with each of the 31 satellites, these satellites would be overloaded, and the system would either crash or be extremely slow. Instead, satellites send out their signals into open space and devices that pick up the signal determine whether or not they need the information. This is just like our FM & AM radio stations. Each radio station will send out its signal regardless of who is listening, and it will be up to us to “tune in” to that signal and receive the data. But even if we do not tune in, that signal is still being sent out.
Looking at the image below, we can see that a satellite can send its data to many different GPS devices at once, because it sends out one wave into the open space, and GPS devices choose whether or not to receive the signal.
This radio wave signal transmits four pieces of information: the x, y, and z axis position of the satellite (one value for each dimension) and the exact time the signal is being transmitted.
Step 2: Radio Signal Travels from Space to Earth
As mentioned above, these radio signals are not directed towards any particular device. So, when the satellite sends out a radio wave, it travels to Earth and is picked up by whatever devices can find the signal and want to pick it up. The key thing to note in this step is the rate at which the signal travels. These radio waves travel at the speed of light (c), which is 3x10^8 meters per second.
Steps 3 & 4: GPS Device Receives Signal and Calculates Distance
In these steps, the device detects the satellite signal and notes the time the signal was received using its own internal clock. Now the device has five pieces of information: the location of the satellite in all 3 axes, the time the signal was sent, and the time the signal was received. From here, the GPS device does some computing to determine the distance between it and the satellite using the equation:
Distance = rate * time
Here, we have the rate (speed of radio wave) as a constant, the speed of light (c), and the time is just the difference between the transmission time and the arrival time.
Step 5: Use Distances from 4 Satellites to Find Location
While it seems like we solved our equation after step 4, we are still not quite there. Knowing the distance from our GPS device to the satellite is great information, but in the end the distance does not tell us our location. However, we can utilize distances from multiple satellites to calculate the device location.
If we have data from four different satellites, we can use geometry to figure out the location of the receiver. Each satellite creates a radius of its respective distance, and the intersection of them will be where the GPS tracker is located.
The reason that four satellites are needed is because there are four unknown parameters. The diagram above shows a 2D space, but in reality, there are three dimensions to factor for. Next, the fourth unknown parameter is the “clock bias,” which is the difference between the GPS device clock and the satellite clock. While we can assume the satellites are synchronized, the GPS device will register a slightly different internal time than the satellites (think about how your microwave clock is slightly different than your phone clock). This creates the fourth unknown parameter. With four unknowns, we need four different distance equations to find the exact location of the GPS device. Hence, we utilize 4 satellites, and by doing so, we can find the exact longitude, latitude, and elevation of the GPS device.
In reality, there are some more computations and factors at play in the process of calculating location using GPS, but this description provides the foundation. Overall, GPS is incredibly accurate at telling time and location, and is utilized in many different ways across many different industries. GPS is a critical factor at play in telematics, and allows solutions like GCF’s EZ750 to provide reliable, accurate data to fleet managers, no matter the surrounding conditions. And for fleets that traverse many thousands of miles through all sorts of terrains, this means safety and security for their drivers.