Also known as: GPS, Navstar, Navstar GPS, Global Positioning System
GPS (Global Positioning System, originally Navstar) is the United States’ satellite navigation system and the most widely used member of the GNSS family.1 Its civilian L1 C/A signal, centred at 1575.42 MHz, spreads a navigation message with a satellite-unique Gold code so that all satellites share one frequency by CDMA, and a receiver recovers position by multilaterating ranges to four or more of them.
Overview
GPS is a constellation of at least 24 satellites in medium Earth orbit, arranged so that four or more are visible from almost any point on Earth at any time. Each satellite carries atomic clocks and broadcasts its precise time and orbit. A receiver correlates the incoming signal against the known Gold code for each satellite, measures the code phase to derive a pseudorange, and solves for its own three coordinates plus its clock error — four unknowns needing four satellites.
Technical characteristics
| Property | Value |
|---|---|
| Access | CDMA via 1023-chip Gold codes |
| Carrier (civil) | L1 = 1575.42 MHz |
| Spreading rate | 1.023 Mcps (C/A code) |
| Nav data rate | 50 bit/s |
| Modulation | BPSK-modulated direct-sequence spread spectrum |
| Satellites | ≥24 in six MEO planes, ~20,200 km |
| Doppler range | ±5 kHz at L1 from satellite motion |
The 1023-chip C/A Gold code repeats every millisecond and has low cross-correlation between satellites, which is what lets many satellites share L1 without jamming each other. Because the received power is far below the thermal noise floor, the receiver’s correlation gain (about 43 dB) is what pulls the signal up to a usable level.
History
GPS was initiated by the US Department of Defense in the 1970s; the first Block I satellite launched in 1978 and full operational capability was declared in 1995. Selective Availability, which deliberately degraded civilian accuracy, was switched off in 2000, and modernized signals (L2C, L5, and L1C) have since been added for better civilian and safety-of-life performance.
Deployment
GPS underpins navigation, surveying, timing, and synchronization worldwide. Beyond the obvious phone and vehicle navigation, it disciplines telecom networks, power grids, and financial timestamps, and provides the reference in a GPSDO used to stabilize radio equipment. A commodity GPS receiver module performs the acquisition and tracking and outputs position plus a one-pulse-per-second tick.
Decoding it with GopherTrunk
GopherTrunk does not decode GPS. It is a land-mobile trunking scanner focused on VHF/UHF voice protocols, and satellite navigation is outside that scope. GPS is, however, a favourite software-defined-radio target for other tools: the signal sits below the noise floor, so a general-purpose SDR needs an active L-band antenna, a low-noise amplifier, and software that performs a two-dimensional search over code phase and Doppler shift before it can lock. Open-source receivers such as GNSS-SDR demonstrate the full chain. For GopherTrunk, GPS is relevant only as an external timing source, not as traffic on the air it decodes.
Sources
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Global Positioning System — Wikipedia, for GPS history, the L1 C/A signal structure, Gold-code CDMA, the 50 bit/s navigation message, and the constellation geometry. ↩