GPS/GNSS for real-time location: how it works, range and accuracy

GPS (Global Positioning System) is one specific example of a GNSS (Global Navigation Satellite System), alongside others like GLONASS, Galileo and BeiDou. All work on the same basic principle: satellites in known orbits transmit timed signals, and a receiver calculates its own position based on how long those signals took to arrive. GNSS is the standard technology for outdoor location, but it's included here because it defines the boundary many RTLS deployments run into: it generally doesn't work reliably indoors.

Key specifications

How it works

Each GNSS satellite continuously broadcasts its own precise position and a timestamp. A receiver picks up signals from multiple satellites and measures how long each one took to arrive, the same Time of Flight principle described in our measurement basics guide, just applied at a much larger scale, with satellites instead of local fixed receivers. Each measured travel time defines a distance, called a pseudorange, from that satellite. With signals from four or more satellites, a receiver can solve for its own position and correct for its own clock error, since receiver clocks are far less precise than the atomic clocks carried on the satellites themselves.

Because GNSS depends on receiving clear signals from multiple satellites simultaneously, it needs a relatively unobstructed view of the sky. As explained in our radio frequencies guide, building materials attenuate and reflect RF signals, and GNSS signals arriving from satellites are already comparatively weak by the time they reach ground level. Roofs, walls and floors typically block or badly distort them, which is why standalone GNSS is generally unreliable indoors and is treated as an outdoor technology for real-time location purposes.

Standalone GNSS accuracy is also limited by atmospheric effects, satellite geometry, and clock and orbital errors. Augmentation systems, such as satellite-based augmentation systems (SBAS) or ground-based correction services, can reduce these errors and improve accuracy beyond what a basic receiver achieves alone. Real-time kinematic (RTK) positioning, covered on its own page, takes this correction approach much further to achieve centimetre-level accuracy.

Measurement methods available

• Time of Flight / pseudorange – distance to each satellite calculated from signal travel time, combined across multiple satellites through trilateration

Advantages

• Genuinely global coverage outdoors, with no local infrastructure required beyond the receiver itself
• Well-established, standardised technology with receivers built into most smartphones and a huge range of outdoor devices
• No ongoing signal transmission required from the tracked object beyond receiving satellite signals
• Multiple constellations (GPS, GLONASS, Galileo, BeiDou) can be combined for improved availability and accuracy

Limitations

• Requires a reasonably clear view of the sky; performance degrades badly or fails entirely indoors, in tunnels, and in dense urban canyons
• Standalone accuracy of a few metres is not sufficient for many precision indoor or close-proximity use cases
• Multipath from nearby buildings can distort accuracy even outdoors, particularly near tall structures
• Receiver power consumption is higher than passive or low-power RF tags, a consideration for long-life battery-powered trackers

Typical applications

GNSS is the default choice for outdoor asset and vehicle tracking: yard and fleet management, outdoor logistics, construction site equipment tracking, and any application where the tracked object spends most of its time outdoors with a reasonably clear sky view.

Frequently asked questions

Does GPS work indoors? Generally not reliably. Building materials block and distort satellite signals enough that standalone GPS accuracy indoors is typically poor or unavailable, which is why indoor deployments use other RF technologies covered in this series.

What's the difference between GPS and GNSS? GPS is one specific satellite navigation system, operated by the United States. GNSS is the general term for all such systems, including GPS, GLONASS, Galileo and BeiDou. Many modern receivers use signals from several of these systems together.

How accurate is standard GPS? Typically around 3–5 metres for a standalone consumer receiver with a clear sky view. Correction techniques like SBAS or RTK can improve this substantially.

Related reading

• RTK GPS for real-time location
• Measurement basics of location systems: distance and direction
• What are radio frequencies and how are they used for indoor location