A phone can tell you how far you have walked, run or driven with genuinely impressive accuracy under good conditions, using nothing but signals received from satellites orbiting roughly 20,000 kilometres above the Earth — a remarkable feat of engineering that most people use daily without ever thinking about the underlying mechanism. Understanding how GPS actually calculates position and distance clarifies both why it works as well as it does under open sky, and why it becomes noticeably less reliable in specific, predictable situations like dense cities or indoors.
The basic principle: triangulation from multiple satellites
GPS (Global Positioning System) works by having a receiver — your phone — listen to precisely timed radio signals broadcast continuously from multiple satellites orbiting Earth, each signal encoding both the exact time it was sent and the satellite's own precise position at that moment. By measuring how long each signal took to travel from a satellite to the receiver (using the tiny, precisely measurable time delay, since radio waves travel at the known, constant speed of light), the receiver calculates its distance from that specific satellite. Repeating this calculation using signals from multiple satellites simultaneously — typically at least four are needed for a full three-dimensional position fix — lets the receiver mathematically triangulate its exact position on Earth through the intersection of these multiple distance measurements.
From position to distance travelled
Once a GPS receiver can determine its current position with reasonable accuracy, calculating distance travelled becomes a matter of repeatedly determining position at frequent intervals and summing the straight-line distance between each consecutive pair of position points along the path taken. A smoother, more frequently sampled path (more position readings per unit of time) produces a more accurate total distance than a path calculated from only a few widely spaced points, since a route with many turns or curves sampled too infrequently will have its true distance underestimated — the straight lines connecting sparse points cut across the actual curved path rather than following it closely.
Why GPS accuracy varies so much by situation
GPS accuracy depends heavily on having a clear, unobstructed view of the sky, since the radio signals from satellites can be blocked, reflected, or delayed by physical obstructions between the satellite and the receiver. Under open sky with a clear view in multiple directions, modern GPS typically achieves accuracy within a few metres. In dense urban environments, tall buildings can block direct line-of-sight to some satellites while also reflecting signals off building surfaces (called "multipath" interference), causing the receiver to calculate a subtly or sometimes significantly incorrect position, an effect often visible as a GPS track that shows implausible zigzagging along what was actually a straight city street. Indoors, GPS signals are typically blocked almost entirely by roofing and walls, which is why indoor GPS positioning is generally unreliable or unavailable altogether, and why indoor navigation systems rely on entirely different technology (WiFi signal mapping, Bluetooth beacons) instead.
Why altitude readings are often less accurate than horizontal position
GPS vertical (altitude) accuracy is typically noticeably worse than horizontal accuracy, for a geometric reason inherent to how satellite triangulation works — since satellites are all positioned somewhere above the receiver (never below), the geometry available for calculating vertical position is inherently less favourable than the geometry for horizontal position, where satellites can be usefully spread across the full horizon in every direction. This is why a phone's Altitude Meter reading, when based purely on GPS, tends to show more variation and less precision than the same device's horizontal position accuracy, and why dedicated altitude measurement sometimes instead relies on a barometric pressure sensor (measuring air pressure, which changes predictably and quite precisely with elevation) either as a supplement to or instead of pure GPS altitude data.
GPS-based speed calculation
A phone's GPS-based Speedometer function calculates speed indirectly, from the rate of change in position over successive, closely spaced time intervals, rather than measuring speed as a direct, independent quantity the way a car's mechanical speedometer (connected directly to wheel rotation) does. This means GPS speed accuracy inherits the same underlying position-accuracy limitations described above — brief signal loss or degraded accuracy (driving through a tunnel, a dense urban canyon between tall buildings) produces a corresponding brief inaccuracy or gap in the calculated speed reading, until clear satellite signal is reacquired.
How GPS accuracy has improved over time
Modern GPS accuracy is considerably better than it was in GPS's earlier years of civilian availability, thanks to a combination of technical improvements — more capable receiver chips in phones, the availability of additional satellite constellations beyond the original US GPS system (some devices also use signals from other countries' equivalent satellite navigation systems, effectively increasing the total number of usable satellites visible at any moment), and improved algorithms for filtering out or correcting the kind of multipath interference described above. This ongoing improvement is why GPS-based distance and position tracking on a modern phone tends to feel noticeably more reliable and precise than older devices or older navigation systems from just a decade or so earlier.
Practical implications for distance-based tracking
Because GPS accuracy genuinely varies by environment, distance measurements from GPS-based tracking (a run, a hike, a drive) tend to be most reliable in open outdoor areas with a clear sky view, and least reliable in dense cities, forests with heavy tree canopy (which can partially obstruct signals similar to buildings), or anywhere with significant physical obstruction between the device and open sky. For planning purposes where accurate distance genuinely matters — estimating fuel needs for a long drive, for instance, where the Fuel Cost Calculator depends on an accurate distance input — cross-checking a GPS-derived distance against a known reference (a mapped route distance, an odometer reading) is worth doing when precision genuinely matters, rather than assuming a single GPS reading is necessarily exact.
Key takeaways
- GPS calculates position by triangulating signal travel time from multiple satellites, typically at least four for a full position fix.
- Distance travelled is the sum of straight-line segments between frequently sampled position points along a path.
- Accuracy depends on a clear sky view — dense cities (signal blocking and reflection) and indoors (near-total signal loss) reduce reliability significantly.
- Vertical (altitude) GPS accuracy is inherently worse than horizontal accuracy due to satellite geometry — barometric sensors often supplement it.