What Is GNSS? GPS, Galileo, GLONASS and BeiDou

GNSS stands for Global Navigation Satellite System. It is the general name for any satellite system that provides positioning anywhere on Earth. GPS is the best-known example, but it is only one of several. When people say their phone uses GPS, the device is often actually using GNSS signals from more than one constellation.

The main global systems

• GPS is operated by the United States.
• GLONASS is operated by Russia.
• Galileo is operated by the European Union.
• BeiDou is operated by China.

Each has its own satellites, ground control and signal design, but they all follow the same basic principle: broadcast the satellite position and time, so that a receiver can work out its position by timing signals.

Some regions also have regional systems and augmentation services that improve the basic signals, but the four above are the global constellations most receivers use.

Why multiple systems help

A receiver that listens to several constellations sees more satellites at the same moment. This brings several benefits:

• Faster first fix: more satellites are available to lock onto quickly.
• Better coverage in difficult places: with more satellites overhead, some will be visible even where the sky is partly blocked, such as streets lined by tall buildings. See urban canyons.
• Better geometry: satellites spread widely across the sky give a more reliable position than ones clustered together.
• Redundancy: if one system has a problem, the receiver can still work with others.

Do they all use the same coordinates?

Each system defines its own reference frame, but they are all aligned closely with each other and with the global standard that most software expects. In everyday use, a receiver converts the result into latitude and longitude based on WGS 84, so you can treat the output as one consistent set of coordinates.

What GNSS cannot fix

More satellites do not remove all error. Signals still bounce off buildings, weaken indoors and are affected by the atmosphere. For the typical reasons a position is off, read why phone GPS is inaccurate.

Where UPCS fits in

UPCS does not provide positioning. You select a point on the map, and UPCS turns that latitude and longitude into a short code for a 3 m by 3 m cell. Your device's GNSS receiver may help you find where you are standing, but the code is derived from the coordinates you choose.