A GPS unit helps you determine your exact position on Earth, expressed as latitude and longitude coordinates, along with your altitude and the precise time. But that’s just the starting point. From those core measurements, a GPS receiver calculates a surprising range of useful information, from your speed and direction of travel to your estimated arrival time, distance covered, and even the area enclosed by a route you’ve walked or driven.
Your Exact Position
The most fundamental thing a GPS unit determines is where you are. It does this by receiving signals from multiple satellites orbiting Earth and calculating the distance to each one. The result is a set of coordinates, typically displayed as latitude and longitude in either degrees-minutes-seconds or decimal degrees. A standard civilian GPS receiver is accurate to about 8 meters horizontally under open sky conditions, based on the global average. Even at the worst-performing locations on Earth, accuracy stays within about 15 meters.
Your GPS unit also determines your altitude above sea level, though vertical accuracy is generally less precise than horizontal. The quality of any GPS position depends heavily on satellite geometry. When the satellites your receiver can see are spread widely across the sky, the position fix is more reliable. When they’re clustered together in one part of the sky, uncertainty increases. GPS receivers express this as a value called dilution of precision: a low number means strong geometry and a trustworthy fix, while a high number is a warning that your reported position may be less accurate than usual.
Direction, Speed, and Distance
Once you start moving, a GPS unit determines several navigation metrics in real time. Your heading is the direction you’re currently traveling, and it updates continuously whether or not you’ve set a destination. If you have selected a destination, the unit also calculates your bearing, which is the direction from your current location to that point.
Speed is derived from how quickly your position changes over time. From speed, the unit calculates your estimated time of arrival at a chosen destination. Most GPS receivers also log a track, which is essentially a breadcrumb trail of everywhere you’ve been. This track lets you see total distance traveled, retrace your steps, and in many models, calculate the perimeter distance and enclosed area of a route. That last feature is particularly useful for anyone measuring a property boundary, a farm field, or a search area.
Precise Time
Every GPS satellite carries atomic clocks, and your receiver synchronizes with them to determine time with extraordinary precision. For everyday users, this means your GPS-connected devices always display accurate time. But precise GPS timing has far broader applications. Financial markets use it to timestamp stock transactions. Telecommunications networks rely on it to coordinate data transmission. Power grids use GPS-derived time to synchronize electricity generation and distribution across vast distances. The time displayed on your phone traces back to these same satellite clocks.
This makes GPS one of the most important timekeeping tools in modern infrastructure, well beyond its role in navigation.
Professional and High-Precision Measurements
Standard GPS accuracy of a few meters is fine for driving directions or hiking, but specialized equipment pushes precision dramatically further. Real-Time Kinematic (RTK) systems compare satellite signals between a fixed base station and a mobile receiver to achieve horizontal accuracy down to about 0.6 centimeters. That’s roughly 100 times more precise than a standard GPS unit. Surveyors, construction crews, and autonomous vehicle systems depend on this level of detail.
At the scientific end of the spectrum, networks of permanently installed GPS stations measure the slow movement of Earth’s tectonic plates. These stations record daily position data over decades, detecting surface displacements of just a few millimeters per year. Researchers at institutions like the Scripps Orbit and Permanent Array Center have used over 25 years of continuous GPS data to track crustal deformation, identify earthquake-related shifts, and even detect subtle ground movement caused by groundwater changes or human activity. In this context, GPS helps determine not just where something is, but how the ground itself is moving.
Fleet and Vehicle Monitoring
GPS units installed in commercial vehicles determine far more than location. Fleet tracking systems combine GPS position data with vehicle sensors to monitor operational details that directly affect costs and efficiency. One of the most valuable is idle time, the period when an engine is running but the vehicle isn’t moving. An idling vehicle typically burns between 0.25 and 1 gallon of fuel per hour, so tracking this adds up fast across a fleet.
These systems distinguish between working idle, where the engine powers job-related equipment, and true idle, where fuel is being wasted with no productive purpose. GPS data ties each idle event to a specific location, time of day, and duration. Fleet managers can see whether vehicles idle most at the warehouse, at customer sites, or during lunch breaks. The system flags events that exceed set thresholds, such as idling for more than a defined number of consecutive minutes or idling after business hours. Combined with geofencing, which creates virtual boundaries around specific locations, GPS tracking can also determine when a vehicle enters or leaves a designated area, how long it stays, and whether it deviates from planned routes.
What GPS Cannot Determine on Its Own
A GPS unit determines position, time, and movement, but it has real limitations. It cannot tell you what is at a location, only where that location is. Map data, points of interest, and street names come from separate databases loaded onto the device. GPS signals weaken or disappear indoors, underground, and in dense urban canyons where buildings block satellite lines of sight. Vertical accuracy is consistently weaker than horizontal accuracy because of how satellite geometry works. Altitude readings from a handheld GPS can be off by 15 meters or more, which is why dedicated altimeters still matter for mountaineering and aviation.
Weather does not significantly affect GPS signals, but heavy tree canopy, mountains, and even your own body can partially block reception and reduce accuracy. The number of satellites visible at any given moment varies, and fewer visible satellites means a less reliable fix.