GPS—the Global Positioning System—has become a household term, synonymous with any satellite navigation device in North America. But, particularly for technical users like scientists and surveyors, references to GNSS—the Global Navigation Satellite System—are on the rise. So what’s the difference?
At a basic level, the terms GPS and GNSS refer to the same thing—a system that uses signals from satellites to accurately determine your location on Earth. Similar to the way “Kleenex” is one brand of tissues that is often used as a generic term for all tissues, GPS is one example of GNSS. GPS was the first such system to become available, and as a result it has great name recognition. While the GPS satellite constellation operated by the United States became accessible to the public in 1994, there are now also constellations operated by Russia (GLONASS), the EU (Galileo), and China (BeiDou). All four constellations fall under the umbrella of GNSS.
Each constellation has enough satellites to ensure that at least four are available from any spot on the globe—making a position calculation possible. But because precision increases with the inclusion of more satellites, there is an advantage to utilizing more than one constellation. This has given rise to multi-constellation “GNSS” devices.
One reason that permanent GPS stations are sensitive enough to measure the slow motion of tectonic plates is simply that they are constantly collecting data. Averaging a day’s worth of positions cancels out some of the sources of error that cause variation from datapoint to datapoint. But because a permanent GNSS station receives signals from many more satellites, it can average over many datapoints in a much shorter period of time. That makes it much easier to detect earthquakes in real-time, for example, so stations can contribute to earthquake early warning systems.
The benefits of multi-constellation GNSS stations are also particularly clear for reflectometry—the use of signal reflections around a station to measure things like water level or soil moisture. As satellites pass in an arc overhead, it’s their time near the horizon that is useful for reflectometry. The measurement comes from the signal-to-noise-ratio while a significant portion of the signal bounces off the surface around the station before reaching it, and this is greatest when the signal comes in at a low angle. The more satellites that pass over a station, the more opportunities there are to make a reflectometry measurement.
In short, GPS and GNSS refer to the same technology. There is a technical difference between GPS and GNSS devices, though—the satellite constellations they utilize.