A set of clocks in the sky
A navigation constellation is a group of satellites arranged so that, from nearly anywhere on Earth, several are above the horizon at once. Each carries very accurate atomic clocks and continuously broadcasts a signal that says, in effect, who it is, where it is, and what time it sent the message.
The main global systems are GPS (operated by the United States), GLONASS (Russia), Galileo (the European Union) and BeiDou (China). Some countries run regional systems too. These satellites typically orbit around 20,000 kilometers above Earth in what is known as medium Earth orbit, and each circles the planet a couple of times per day.
How a receiver finds your position
The core idea is measuring distance with time.
- A receiver picks up a signal and notes when it arrived.
- The signal carries the time it was sent. The difference, multiplied by the speed of light, gives the distance to that satellite.
- One distance places you somewhere on a sphere around the satellite. A second distance narrows that to a circle, and a third to two points, one of which is usually absurd, such as out in space.
- A fourth satellite is needed because the receiver’s own clock is not nearly as accurate as an atomic clock. The extra measurement lets the receiver solve for the clock error along with its three position coordinates.
This is why people often say a receiver needs at least four satellites. In practice, receivers use many more when they are visible, and many combine different constellations to improve results.
The numbers show why precision matters. Radio waves travel about 300,000 kilometers per second, so a timing error of one millionth of a second equals about 300 meters. Receivers therefore must work at nanosecond-scale timing, and the satellites’ clocks are corrected by ground stations. The phone version of this story appears in the explainer on how GPS works in your phone.
What the constellations give people
Position is the famous feature, but there are three outputs:
- Position in three dimensions.
- Velocity, derived from how position changes.
- Time, usually very precise and shared widely.
Everyday uses
- Maps and turn-by-turn directions on phones and in cars.
- Aircraft and ship navigation.
- Surveying, construction and farming equipment that follows precise paths.
- Fitness and delivery tracking.
- Time stamps for networks, including parts of wireless networks and the internet infrastructure behind them, as well as financial systems.
- Scientific uses, such as measuring slow ground movement or studying the atmosphere.
What affects accuracy
A basic receiver is often accurate to a few meters in open sky, though results vary. Factors include:
- Atmosphere. The ionosphere and troposphere slow signals slightly.
- Reflections. In cities, signals bounce off buildings and can cause errors.
- Satellite geometry. Satellites spread across the sky give better results than those bunched together.
- Augmentation. Correction services can improve accuracy considerably for those who need it.
One-way service
Navigation satellites broadcast only. They do not receive your location, which is why a receiver needs no transmitter and why the number of users does not strain the system. Tracking of a device happens in the apps and networks built on top of it.
Understanding this basic design also clarifies its weaknesses: weak signals can be blocked or interfered with, and a dependency on one service is risky. Later articles in this group address safety and security.