GNSS fundamentals, GPS, GLONASS, Galileo, signals, pseudo-range measurements, and positioning.
Every global navigation satellite system has three segments:
| Segment | Role | Components |
|---|---|---|
| Space | Broadcast ranging signals and navigation data | Constellation of MEO satellites (~20,000 km altitude), atomic clocks |
| Control / Ground | Track satellites, compute orbits and clock corrections | Monitor stations, control stations, uplink stations |
| User | Receive signals, compute position/velocity/time | Antenna, receiver, ranging processor, navigation processor |
Satellites orbit in medium Earth orbit (MEO), inclined ~55–65° to the equator with roughly 12-hour periods. This provides better geometry and polar coverage than geostationary orbits.
Each satellite broadcasts signals with two key components: ranging codes (pseudo-random noise sequences for measuring transmission time) and navigation data messages (ephemeris parameters describing satellite orbits, clock corrections, ionosphere models).
GNSS positioning is passive ranging in three dimensions. The user measures the time each signal took to travel from satellite to antenna and multiplies by the speed of light to get range.
One range → sphere. Two ranges → circle (intersection of spheres). Three ranges → two points. A fourth resolves the ambiguity.
But there is a catch: the receiver clock is not perfectly synchronized to satellite system time. The receiver clock offset δtrc adds a common bias to all range measurements, turning true ranges into pseudo-ranges:
Satellite clock errors are measured by the ground segment and broadcast in the navigation message, so the user can correct for them. But the receiver clock offset must be solved as a fourth unknown. Hence: four satellites minimum for a 3D position fix (three spatial unknowns + one clock unknown).
GNSS signals are broadcast in the L-band (1–2 GHz). Each signal combines a carrier, a ranging code (PRN sequence), and optionally a navigation data message using BPSK modulation:
The ranging code is a pseudo-random noise (PRN) sequence known to the receiver. Multiplying by this code "spreads" the signal over a wide bandwidth, burying it below the noise floor. The receiver correlates the incoming signal with its local replica code. When the codes align, the signal pops out of the noise.
CDMA vs FDMA: GPS, Galileo, and BeiDou use code-division multiple access (each satellite has a unique code on the same frequency). GLONASS historically used frequency-division multiple access (each satellite on a slightly different frequency).
BOC modulation: Newer signals use binary offset carrier modulation, which adds a subcarrier that splits the spectrum into two sidebands. This reduces interference with BPSK signals sharing the same band and improves code-tracking accuracy.
Pilot signals: Data-free signals (no navigation message) enable narrower tracking bandwidth for better performance in weak-signal environments.
NAVSTAR GPS, developed by the U.S. Department of Defense, is the oldest and most mature GNSS. First prototype satellite launched 1978; full operational capability reached 1994.
Constellation: 24+ satellites in 6 orbital planes, 55° inclination, ~20,200 km altitude, ~12-hour period. Typically 5–14 satellites visible from any point on Earth.
Two services:
| Service | Access | Typical Accuracy (1σ) |
|---|---|---|
| SPS (Standard Positioning) | Open to all | ~3.8 m horizontal, ~6.2 m vertical |
| PPS (Precise Positioning) | Licensed (military) | ~1.2 m horizontal, ~1.9 m vertical |
Signals: 10 signals across three bands (L1: 1575.42 MHz, L2: 1227.60 MHz, L5: 1176.45 MHz). The legacy C/A code on L1 is the primary civil signal. Modernization adds L2C, L5, L1C, and military M code.
Selective Availability (SA): Intentional accuracy degradation (~100 m) was active until May 1, 2000. Circumventable via differential GPS. Deactivated but capability remains.
GLONASS (Global Navigation Satellite System) is Russia's GNSS. Developed during the Soviet era, it reached initial operational capability in the 1990s, declined after the Soviet collapse, and was rebuilt to full capability by 2011.
Constellation: 24 satellites in 3 orbital planes, 64.8° inclination, ~19,100 km altitude, ~11 hour 15 minute period. Higher inclination gives better polar coverage than GPS.
Key difference — FDMA: Legacy GLONASS uses frequency-division multiple access. Each satellite transmits on a slightly different frequency within the L1 and L2 bands. This simplifies the satellite but complicates the receiver (must handle multiple frequencies). Newer GLONASS-K satellites add CDMA signals.
Reference frame: GLONASS uses the PZ-90 datum (moving to ITRF-aligned). GPS uses WGS 84. The difference is centimeters, relevant only for high-precision users.
Time system: GLONASS time is linked to Russian UTC (UTC(SU)), which can differ from GPS time by up to several hundred nanoseconds. Combined GPS/GLONASS receivers must solve for an additional clock offset.
Galileo is Europe's GNSS, developed by the European Union and ESA. It is the first GNSS designed from the start as a civilian system under civilian control.
Constellation: 30 satellites (27 active + 3 spares) in 3 orbital planes, 56° inclination, ~23,222 km altitude, ~14-hour period.
Four services:
| Service | Access | Key Feature |
|---|---|---|
| OS (Open Service) | Free | Comparable to GPS SPS |
| CS (Commercial Service) | Paid | Higher accuracy, guaranteed performance |
| SoL (Safety of Life) | Free | Integrity alerts for aviation |
| PRS (Public Regulated) | Government only | Encrypted, robust against jamming |
Signals: Galileo broadcasts on four frequency bands: E1 (1575.42 MHz, shared with GPS L1), E5a (1176.45 MHz, shared with GPS L5), E5b (1207.14 MHz), and E6 (1278.75 MHz). The E5 band uses AltBOC(15,10) modulation — the widest bandwidth GNSS signal, offering the highest precision ranging code.
Integrity: Galileo was designed from the start with integrity monitoring, broadcasting alerts within 6 seconds if a satellite signal becomes unreliable. GPS does not yet have an equivalent built-in system.
In addition to the three global systems, several regional satellite navigation systems exist or are planned:
| System | Country | Coverage | Status (as of textbook) |
|---|---|---|---|
| BeiDou / Compass | China | Asia-Pacific (regional); later global | BeiDou-1 operational (GEO); Compass (MEO) in development |
| QZSS | Japan | Japan and East Asia | In development; highly elliptical orbits for high-elevation coverage |
| IRNSS | India | India and surrounding region | Planned; GEO + IGSO constellation |
BeiDou evolution: BeiDou-1 was a regional system using geostationary satellites with active ranging (user transmits back). Compass/BeiDou-2 moved to passive ranging with MEO satellites, like GPS/GLONASS/Galileo, with global ambitions.
QZSS: Japan's Quasi-Zenith Satellite System uses highly inclined, elliptical orbits that ensure at least one satellite is always near-zenith over Japan. This dramatically improves signal availability in urban canyons where high-elevation satellites are needed.
Using multiple GNSS constellations together brings significant benefits: more satellites visible, better geometry, higher accuracy, and insurance against single-system failure.
Frequency compatibility: GPS and GLONASS use completely separate bands — no interference concern. GPS and Galileo share L1 and L5/E5a bands but use CDMA with different codes, so they coexist. BOC modulation on newer signals further reduces cross-system interference.
Reference datums: GPS (WGS 84), Galileo (GTRF), and GLONASS (PZ-90) are all converging on ITRF. Differences are centimeters — negligible for most users, relevant for high-precision.
Time systems: Each GNSS maintains its own time. GPS time is based on US UTC; GLONASS on Russian UTC; Galileo on TAI. Broadcast conversion parameters exist for some pairs but not all, requiring multi-system receivers to solve for additional clock unknowns.
This simulation shows how pseudo-range measurements from multiple satellites constrain the user position. Observe how adding more satellites tightens the position estimate.