Starting with GPS Basics
The Global Positioning System (GPS) is based on trilateration, the measurement of distances (not, as is often mistakenly stated in popular explanations, triangulation, which is the measurement of angles). To understand how this works, imagine that all you know about your position is your distance from two known points, such as a tower and a mountain peak. This means that you are at one of the two points were a circle centered on the tower and with a radius equal to your distance from it intersects a circle centered on the mountain peak with a radius equal to your distance from it. Since you can usually exclude one of the two points based on other information — for example, you are on land but one of the points is in the middle of a lake — by drawing those two circles on a map and choosing the correct intersect, you know your position.
You can expand this 2D operation into 3D space by using three spheres instead of two circles. Each GPS satellite is the center of one of those spheres. Therefore, if you know your distance from three satellites, you know that you are in one of two possible positions, where those three spheres intersect. Like in the 2D version of this exercise, it is usually easy to pick the correct one. The time it takes a satellite’s signal, which travels at the speed of light, to reach a receiver tells us the distance between the two. (How a GPS receiver synchronizes its time with the satellites to measure each signal’s time of travel, while a key feature of the system, is beyond the scope of this summary.)
If receivers, like the satellites, had atomic clocks, three satellites would provide them all the information they would need to calculate their 3D position. However, because receivers have much less accurate clocks, they need at least four satellites in view to do this. The more satellites in view and the more spread out they are, the quicker and more accurately a GPS receiver can calculate its position.
The GPS constellation consists of at least 24 satellites in six nearly circular orbits, about 20,200 km above Earth. However, the actual number varies and there are currently 31 GPS satellites in orbit. They cover the entire planet, making the system global. In order to calculate its exact position on or near Earth’s surface, a receiver needs to know not just its distance from at least four satellites but also the exact position of those satellites at that moment. Therefore, a network of ground stations constantly monitors the satellites’ positions and uploads that data to them. In turn, the satellites include this information, called ephemeris, in the navigation message that they broadcast to receivers. The satellites are known as the system’s space segment, the monitoring stations as the control segment, and the receivers as the user segment.
Inevitably, a combination of equipment errors and atmospheric errors corrupt GPS ranging, forming what is known as the “GPS error budget.” The principal way of mitigating these errors is by means of differential GPS (DGPS): GPS receivers in very well surveyed positions broadcast the difference between their actual position and their GPS position, so that other nearby receivers can apply that difference as a correction. There are several such corrections services (aka augmentation services). The U.S. Coast Guard operates the U.S. Nationwide DGPS network (NDGPS); the Federal Aviation Administration (FAA) operates the Wide Area Augmentation System (WAAS); the National Geodetic Survey (NGS) operates a network of Continuously Operating Reference Stations (CORS); other countries operate regional augmentation systems; and private companies also provide correction services, such as Trimble’s RTX, which delivers corrections via satellites and cell phones.
Because the system’s architecture requires extremely accurate atomic clocks on the satellites, GPS has also become the timing source for communications and broadcast networks, banking systems, and the Internet. For this reason, GPS is known as positioning, navigation, and timing (PNT) service.