Are you wondering how GPS (Global Positioning System) functions? Invisible and all-reaching, this mysterious system is present worldwide and users take it for granted. Their devices such as smartphones, laptops, smartwatches, and compasses have a tiny bit of electronic, called GPS receiver (and some use multiple systems) that tells them what their location is. But who provides the information, how accurate is it, what affects its functionality, and does it have any drawbacks? We provide a lot of answers as we explain how GPS works. Let’s start.
What is GPS?
GPS or Global Positioning System is a three-component network of navigation satellites in orbit (orbiting Earth fully twice per day, once every 12 hours), ground stations, and GPS receivers of several types. The system works non-stop (24/7/365) by sending and receiving signals, independent of weather conditions, and is free to use. It is used to determine the accurate location and time thanks to the process called trilateration and super precise atomic clocks built into satellites, respectively. The navigation system was devised by Ivan A., Roger L. Easton, and Bradford Parkinson and based on Decca Navigation System and LORAN, both utilized for plane and ship navigation during World War II.
The GPS project was started in 1973 by the United States Department of Defense and had its first satellite launch in 1978. Further, it had a full network of 24 satellites active by 1993, but the GPS or NAVSTAR project remained strictly for governmental and military use. It was only around 1989 and 1990 that the first commercially available GPS receivers became available. However, they were bulky, expensive, and didn’t last long. It wasn’t until the 2000s that the technology became widespread. After 2000, the price of a GPS receiver and a processing chip dropped from thousands of dollars to roughly only $1.5, making it a commodity.
Components of GPS
To help you understand how the Global Positioning System works, we must study the three main GPS components:
1. Satellites (Space component)
The space segment of GPS is an American navigation system consisting of over 30 (31 at the time of writing) satellites in orbit roughly 12,500 miles (ca. 20,117 km) above Earth. They complete a full revolution every 12 hours or twice per day and tell us exactly where we are. As mentioned, the first iteration began with 24 satellites, and Russia’s version, GLONASS (Global Navigation Satellite System or Globalnaya Navigazionnaya Sputnikovaya Sistema), still has that many. However, their satellites are in a faster orbit that completes a full revolution every 11 hours at a height of 11,900 miles (ca. 19,151 km) above Earth. The application remains the same, however.
2. Ground station (Control component)
Ground stations or control segments are receiving stations on Earth that accept radio waves from satellites and make sure they are the satellites are in the assumed correct position, i.e., properly distanced and at the right altitude. In short, ground stations use radars to monitor and control satellites and ensure they are in a proper location.
3. Receiver (User component)
GPS receivers are a user segment of GPS that listen to radio waves coming from satellites at all times and identify the location on Earth at any given time. Common GPS receivers can determine a location down to 7 meters 95% of the time. However, some expensive ones can increase the accuracy of GPS on Earth to a few inches up to roughly 0.4 inches (ca. 1 cm). Receivers can also tell accurate times thanks to atomic clocks in each satellite.
How does GPS work?
GPS works through the combined functionality of its three members: satellites high in orbit and ground stations and GPS receivers on Earth. You can think of satellites as star constellations high in the sky—we know exactly where they are. To confirm their position, we need the control component or ground station.
The GPS is based on the Doppler Effect, where radio waves are transmitted at the speed of light. Throwing a ball is the simplest example. If someone throws a ball while running toward you, we can calculate the speed of the ball by adding the speed of running and the speed of the ball when it left their hand. Because the speed of radio waves is constant, the GPS receivers on the ground can determine the time it took for the radio wave to travel from each satellite to the receiver. Adding that time measurement lets receivers calculate the distance.
The process of using three or more satellites to find our location is called trilateration. The example of a flashlight is usually utilized to demonstrate this. With one circle of light (one satellite) the location can be anywhere in it. Adding another circle makes them intersect but is still not precise enough. When we shine the third flashlight (third satellite), the beams of light intersect or cross in one place only. Therefore, receivers require three satellites to perform a 2-D (two-dimensional) trilateration or determine two measurements, the longitude, and the latitude. For a three-dimensional (3-D or 3D) satellite reading (longitude, altitude, latitude) of an accurate location, we require data from at least 4 satellites.
GPS satellite and receiver functionality and transmission
GPS satellites send an encoded, unique signal to GPS receivers on the ground, which decode it and calculate the precise location based on the speed of light and consequent distance. Satellites send two or more low-power radio signals that contain three types of data:
- Pseudorandom code or PRN code, consisting of a complex sequence of 0s and 1s, identifies each satellite. Because of its intricacy, the sound it creates during transmission sounds like uncontrolled electrical noise, hence the name.
- Ephemeris data contains data such as the current location, time, and health of the satellite. It also lets the GPS receiver determine the position of the transmitting satellite to other satellites.
- Almanac data represents individual information each satellite transmits throughout the day and that of the entire constellation of GPS satellites. It helps receivers find viable satellites drastically faster.
Although GPS receivers require at least 3 satellites (2D trilateration) or 4 (3D position fix), in reality, they typically use 8 or more satellites for increased accuracy. This depends on the position on Earth, i.e., how many satellites a receiver can detect, but also weather conditions and time of day, which can obstruct the signal. To sum up, the calculation of the distance to each satellite based on which GPS works consists of these values:
- Earth longitude
- Earth latitude
- Elevation, altitude, or height (only for 3-D triangulation)
For more details, read the advantages and disadvantages of GPS.