How does GPS work?

The Global Positioning System is a US-owned service that provides positioning, navigation and timing. GPS.gov describes it as three segments:

• The space segment. A constellation of satellites in medium Earth orbit, at about 20,200 km, each circling the Earth twice a day. The satellites fly in six orbital planes with four baseline slots in each, an arrangement GPS.gov says lets users see at least four satellites from virtually any point on the planet. The US commits to keeping at least 24 operational satellites available 95% of the time.
• The control segment. A global network of ground facilities that track the satellites, monitor their signals, adjust their clocks, keep them in their orbits, and upload fresh navigation data. A master control station turns the monitor station data into precise satellite positions and builds the navigation messages the satellites broadcast.
• The user segment. The receivers, in phones, cars, aircraft and survey kit, that pick up the signals and compute position and time.

Each satellite transmits a one-way signal that carries its current position and the time it sent the signal. The receiver notes when the signal arrives. The difference between the two times, multiplied by the speed of light, gives a distance to that satellite.

There is a catch. The satellites carry atomic clocks; a receiver’s clock is far less precise. Any error in the receiver’s clock turns directly into an error in every distance it computes. ESA’s Navipedia calls the measured value a pseudorange, an apparent range that differs from the true geometric distance because of the offset between the receiver and satellite clocks, plus delays from the ionosphere and troposphere, signals bouncing off nearby surfaces, and receiver noise.

If the clock were perfect, three distances would pin down a point in space. Because it is not, the receiver has four unknowns: its three coordinates and its own clock offset. Navipedia sets out the standard approach: take pseudoranges from at least four satellites in view, use the satellite positions computed from the broadcast navigation message, and solve the equations together for position and clock offset. When more than four satellites are in view, the receiver can use them all as extra measurements.

A side effect is that every GPS receiver is also a precise clock. Once it has solved for its clock offset, it knows the time as well as its position. GPS.gov notes that GPS time transfer is a common method for synchronising clocks and networks to Coordinated Universal Time.

GPS satellites broadcast on several radio frequencies. GPS.gov lists the civil signals by satellite generation: the original coarse acquisition (C/A) code on L1, a second civil signal on L2 (L2C), a third on L5, and a fourth on L1 (L1C) on the newest GPS III satellites. A separate military signal is encrypted for authorised users, while the civil service is open to anyone.

The exact structure of these signals is public. IS-GPS-200, published on GPS.gov, defines the interface between the space and user segments for the L1 and L2 links, which is what lets anyone build a receiver. The same openness has a cost: the civil signal carries no proof of where it came from, which is what makes GPS spoofing possible.

GPS.gov’s answer is that it depends. The government commits to the accuracy of the signal in space, not of any device. Its commitment is a global average user range error of 2.0 m or less, 95% of the time, across healthy satellites; GPS.gov notes that this is not the same as user accuracy.

What a receiver achieves depends on satellite geometry, signal blockage, the atmosphere and the quality of the receiver. GPS.gov states that GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft) radius under open sky, and that accuracy worsens near buildings, bridges and trees. High-end users with dual-frequency receivers or augmentation systems can reach real-time positioning within a few centimetres.

GPS.gov lists the common causes of a wrong position as signals blocked by buildings, bridges or trees, indoor or underground use, and multipath, where signals reflect off walls. Less common causes include radio interference or jamming, major solar storms, and satellite maintenance. Selective Availability, which deliberately degraded civil accuracy in the 1990s, ended in May 2000.

GPS is one of several satellite systems. Galileo, GLONASS and BeiDou work on the same principle of timing signals from satellites; what GNSS means covers how they fit together.

GPS also has limits worth designing around. Its signals are easily blocked indoors and underground, so phones combine satellite fixes with other sources, as explained in how phones find their location without GPS. And because the receiver computes its own position, a GPS coordinate is something a device reports, not something anyone else has observed. Checking where a device really is takes separate evidence.