GNSS (Global Navigation Satellite System) is the umbrella term for all satellite positioning systems. Many people refer to everything as “GPS,” but GPS is actually just one of them. The systems available worldwide today include GPS, GLONASS, Galileo, BeiDou, and QZSS. A receiver that tracks signals from several systems at once (known as multi-constellation) can greatly increase the number of visible satellites, speed up convergence, and maintain stable positioning in obstructed environments such as valleys and urban canyons. The following explains the differences between the major systems, how positioning works, and the signal and error sources.
Quick Comparison of the Five Systems
| System | Operator | Coverage | Orbit altitude |
|---|---|---|---|
| GPS | United States | Global | ~20,200 km (MEO) |
| GLONASS | Russia | Global | ~19,100 km (MEO) |
| Galileo | European Union | Global | ~23,200 km (MEO) |
| BeiDou | China | Global | Mixed MEO/IGSO/GEO |
| QZSS | Japan | Asia-Pacific augmentation | IGSO/GEO |
In addition, India’s NavIC (IRNSS) provides regional service over India and its surroundings. Located in the Asia-Pacific, Taiwan benefits simultaneously from GPS, BeiDou, and Japan’s QZSS coverage.
A Brief Look at Each System
GPS (United States)
The first fully operational and most widely used system, and the byword for satellite positioning before the term “GNSS” became common. It broadcasts multiple frequency bands (L1/L2/L5), offers global coverage and the best compatibility, and is supported by virtually every receiver.
GLONASS (Russia)
A global system complementary to GPS, using frequency division multiple access (FDMA; newer generations also adopt CDMA). Used alongside GPS at high latitudes, it increases the number of visible satellites and improves geometry.
Galileo (European Union)
The EU’s independent civilian navigation system, offering high-accuracy multi-frequency signals that are highly compatible with GPS and other systems. It has become an important component of high-precision positioning.
BeiDou (China)
Uses a mixed MEO/IGSO/GEO constellation, providing a large number of satellites and good geometry over the Asia-Pacific, making a significant contribution to visible-satellite counts for users in Taiwan and the wider region.
QZSS (Japan)
A regional augmentation system whose satellites mostly use inclined geosynchronous orbits (IGSO), dwelling at high elevation angles over the Asia-Pacific for extended periods. This is especially helpful for mitigating obstruction in urban canyons and mountainous areas, and it also provides augmentation signals.
How GNSS Determines Position
GNSS positioning is based on time of arrival and trilateration:
- Each satellite continuously broadcasts its own position and precise time.
- The receiver measures the signal travel time and multiplies it by the speed of light to obtain the “pseudorange” to each satellite.
- In theory three satellites can fix a 3D position, but because of receiver clock error, at least four satellites are needed to solve for position and time together.
- The more visible satellites and the better the spatial geometry (the lower the DOP), the more stable and accurate the fix — which is exactly the value of multi-constellation use.
Major Error Sources
- Ionospheric delay: delay as signals pass through the ionosphere, effectively removed using dual-/multi-frequency observations.
- Tropospheric delay: delay caused by water vapor in the lower atmosphere, corrected with models.
- Multipath: signals reaching the antenna after reflecting off buildings or the ground, biasing measurements.
- Satellite orbit and clock errors: reduced with precise ephemerides and differential/correction services.
Single-point positioning (a standalone receiver, no corrections) is accurate to the meter level. To reach the centimeter level, differential positioning techniques (RTK, PPK, PPP, etc.) are required — see What Is Differential Positioning? DGNSS, RTK, PPK and PPP Compared.
Application in Engineering Monitoring
In long-term deformation monitoring of structures and slopes, GNSS RTK monitoring combined with multi-constellation observations delivers 24-hour continuous, centimeter-level 3D displacement data that can be integrated and interpreted alongside InSAR, inclinometer, and other data.
FAQ
Q: What is the difference between GPS and GNSS? GPS is the United States’ system, while GNSS is the umbrella term for all satellite positioning systems. Modern receivers typically use GPS, GLONASS, Galileo, BeiDou, and QZSS together.
Q: Why use several satellite systems at once? Multi-constellation use increases the number of visible satellites and improves geometry, maintains positioning in obstructed environments such as valleys and urban canyons, and speeds up convergence while improving accuracy and reliability.
Q: How many satellites does GNSS positioning need at minimum? Three can fix a 3D position in theory, but because the receiver clock error must be solved simultaneously, at least four satellites are needed in practice.
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