Also known as: GNSS, satnav, satellite navigation, PNT
GNSS (Global Navigation Satellite System) is the collective name for the space-based systems that let a receiver fix its position, velocity, and time (PNT) anywhere on Earth by listening to timing signals from orbiting satellites.1 It is an umbrella term, not a single system: the four global constellations are the United States’ GPS, Russia’s GLONASS, the European Union’s Galileo, and China’s BeiDou, joined by regional systems such as India’s NavIC and Japan’s QZSS.
How it works
Every GNSS shares the same principle: one-way ranging. Each satellite carries an atomic clock and continuously broadcasts a signal stamped with the exact time of transmission and the satellite’s own orbit (the ephemeris). A receiver measures how long each signal took to arrive by comparing the incoming timestamp with its own clock, and multiplies that delay by the speed of light to get a pseudorange — the distance to that satellite, biased by the receiver’s own clock error.
Because the receiver clock is cheap and imperfect, that clock bias is treated as a fourth unknown alongside the three spatial coordinates. Solving for four unknowns needs four equations, so a receiver must hear at least four satellites simultaneously; more satellites over-determine the solution and improve accuracy. Geometrically this is multilateration: each pseudorange defines a sphere around a satellite, and the position is where the spheres intersect once the common clock error is absorbed. The satellite orbits at roughly 20,000 km altitude in medium Earth orbit, so signals arrive extremely weak — below the receiver’s thermal noise floor — and are recovered by correlating against a known spreading code (see GPS).
The constellations differ in the details that ride on top of this shared idea. GPS, Galileo, and BeiDou separate satellites by unique spreading codes on a shared frequency (CDMA); classic GLONASS instead gives each satellite its own frequency (FDMA). All of them place signals in the L-band (roughly 1.1–1.6 GHz), and modern receivers combine several constellations and frequencies at once for faster fixes and better resilience.
Two effects complicate the measurement. Satellites move fast relative to the ground, so their carriers arrive with a substantial Doppler shift of several kilohertz that the receiver must search over and track. And the signal passes through the ionosphere, which delays it by a frequency-dependent amount; dual-frequency receivers cancel most of this error by comparing two bands.
Relevance to SDR
GNSS is everywhere in radio, but mostly as infrastructure rather than as a decode target. Any system needing precise time or frequency — cellular base stations, the timestamps in trunked-radio simulcast, a GPSDO disciplining an SDR’s reference oscillator — leans on a GNSS receiver in the background. A dedicated GPS receiver chip does the heavy correlation and simply outputs position and a one-pulse-per-second tick.
Decoding GNSS from raw IQ with a software-defined radio is a well-known but demanding exercise: the signals are below the noise floor, so recovery requires long coherent integration and a code/Doppler search, and a typical SDR needs an active L-band antenna with a low-noise amplifier to see them at all. Open projects (GNSS-SDR and others) do exactly this. GopherTrunk is a land-mobile trunking scanner and does not decode GNSS — satellite navigation is out of scope for its VHF/UHF trunking focus. GNSS matters to GopherTrunk only indirectly, as the timing source that can discipline the receiver hardware it runs on.
Sources
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Satellite navigation — Wikipedia, for the definition of GNSS, the four global constellations, the one-way ranging principle, and the need for four satellites. ↩