What GNSS Is
GNSS stands for Global Navigation Satellite System. It is a generic term for satellite-based systems that provide positioning, navigation, and timing (PNT) services to users on or near the Earth. GNSS receivers compute latitude, longitude, and height by receiving signals from multiple satellites and solving for the user's position and clock offset.
Note: GPS is simply the U.S.-based constellation within the collective GNSS.
How GNSS Works - core concepts
- Satellites broadcast timed signals. Each GNSS satellite continuously transmits a precise timestamp and satellite ephemeris (its orbit data).
- Receiver measures time-of-flight. The receiver notes when it received a satellite's timestamp and uses the difference to compute the signal travel time.
- Range computation (pseudorange). Travel time × speed of light ≈ distance (pseudorange). Because receiver clocks are not synchronized to satellite clocks, a clock bias exists.
- Solve for position and time. With ranges to four or more satellites, a receiver solves a set of equations for three spatial coordinates and the receiver clock bias (4 unknowns → ≥4 satellites required).
- Corrections and refinement. Techniques such as differential GNSS (RTK, DGPS), SBAS, or PPP reduce errors and improve accuracy.
Note: The basic position from raw pseudoranges is called a single point solution and is subject to several error sources described later.
Types of GNSS Constellations
Several global and regional satellite constellations exist. Many modern receivers can use multiple constellations simultaneously (multi-GNSS) to increase availability and accuracy.
| System | Operator / Region | Notes |
|---|---|---|
| GPS | United States (US DoD) | Oldest global system; provides L1/L2/L5 signals. |
| GLONASS | Russia | Frequency-division RT; good global coverage and redundancy. |
| Galileo | European Union / ESA | High-precision civil services; modern signals for improved accuracy. |
| BeiDou (BDS) | China | Global service with multiple frequency bands and regional augmentation. |
| QZSS | Japan (regional) | Regional augmentation for the Asia-Pacific, compatible with GPS. |
| SBAS (WAAS, EGNOS, MSAS) | Regional | Satellite-Based Augmentation Systems that broadcast corrections for safety-of-life aviation use. |
Types of Fix Qualities (Common Classifications)
Receiver outputs typically indicate a fix type or quality level. Exact names vary by manufacturer, but the common hierarchy is:
| Solution Type | Correction Source | Typical Accuracy (H / V) | Notes |
|---|---|---|---|
| Autonomous (Single-Point) | No external corrections - uses satellite broadcast ephemerides & clocks | ~3–10 m / ~5–15 m | Basic navigation, handheld devices; accuracy degrades with poor geometry or multipath. |
| DGPS (Differential GPS) | Pseudorange corrections from a local base station (real-time) or post-processing | ~0.5–1.5 m (real-time) / ~0.1–0.5 m (post-processed) / ~1–3 m (V) | Common for marine, basic mapping and agriculture; reduces broadcast errors but not carrier-phase-level biases. |
| Float (RTK Float) | Carrier-phase corrections from base/NTRIP, but integer ambiguities not yet resolved | ~0.2–0.5 m / ~0.3–0.8 m | Transitional state during RTK initialization or under marginal conditions (long baselines, obstructions). |
| Fixed (RTK Fixed) | Carrier-phase corrections with integer ambiguities resolved (real-time) | ~0.01–0.02 m / ~0.02–0.05 m | Survey-grade precision within short baselines (commonly <10 km). Used for surveying, machine control, precision agriculture. |
Multipath - what it is and why it matters
Multipath occurs when GNSS signals reach the receiver antenna along multiple paths - one direct (line-of-sight) and others reflected (from buildings, water, ground, or vehicle surfaces). The reflected signals cause errors in the measured pseudorange and carrier phase.
Effects of multipath
- Bias in ranges - reflected signals arrive later and distort measured travel time, causing position bias.
- Increased noise - carrier-phase noise and jitter, which can degrade ambiguity resolution in RTK.
- Elevation dependency - low-elevation satellites are more prone to multipath.
Mitigation strategies
- Site selection: choose open-sky locations away from large reflectors (metal surfaces, glass, water).
- Antenna choice & mounting: use antennas with good multipath rejection (choke rings, ground planes) and mount them away from reflective structures.
- Signal processing: modern receivers apply multipath mitigation algorithms and use multi-frequency/multi-constellation observations to detect and ignore bad measurements.
- Post-processing filters: apply averaging, outlier detection, or use carrier-phase based processing to reduce multipath impact.
Common GNSS Error Sources (brief)
- Satellite clock & ephemeris errors: inaccuracies in satellite time or orbit broadcast.
- Ionospheric & tropospheric delay: atmospheric layers slow signals; multi-frequency corrections and models mitigate these.
- Receiver noise & multipath: instrument noise and reflections.
- Receiver clock bias: solved as part of the 4-unknown GNSS solution, but residuals cause small errors.
- Reference/coordinate frame mismatches: differences between ellipsoid/datum/geoid definitions.
Practical Tips for Better GNSS Results
- Use multi-GNSS & multi-frequency receivers for improved availability and robustness.
- Prefer open sky and elevated mounting with a stable rig.
- Record metadata: antenna type, antenna height, datum/EPSG, geoid model, and observation logs (RINEX) for post-processing.
- When you need centimeter accuracy use RTK with a nearby base or a trusted network (NTRIP), and verify integer ambiguity resolution status.
Glossary (short)
- Ephemeris
- Orbital parameters that describe satellite position as broadcast in the navigation message.
- Pseudorange
- Measured range derived from signal travel time; contaminated by clock errors and delays.
- Carrier phase
- Phase of the high-frequency carrier wave - extremely precise measurement used in RTK/PPP.
- RTK
- Real-Time Kinematic - a technique using a local base station and carrier-phase corrections for cm-level accuracy.