GPS vs. GNSS: Understanding PNT Satellite Systems

To most people, GNSS satellites are something vague that fly around in the upper atmosphere. From there, the technical details tend to get a bit hairy. As long as they function correctly, who cares which ones we use for tasks like positioning and timing, right?

Not so fast. Not all systems are the same and when it comes to business use, it's vital to understand the nuances. Here's a quick rundown that should cut through the noise to help you get a clear signal.

Positioning, Navigation and Timing Systems

A GNSS, or Global Navigation Satellite System, is a generic name for a group of artificial satellites that send position and timing data from their high orbits. The GPS, or Global Positioning System, is just one of the many different sets of satellites that can provide such data.

Most satellite navigation systems operate on similar principles. The satellites are arranged in geosynchronous orbits – Each one's speed and altitude is carefully controlled so that it stays in the same place in the sky.

GNSS System Orbits.png

Geosynchronous satellite orbits have a few interesting properties. Since the individual satellites maintain their relative positions, they form stable artificial constellations. By equipping each satellite with a radio transmitter, it's possible to calculate position data from the ground by listening to the signals coming from recognized devices and estimating the timing delays. Alternatively, a receiver that knows its position can calculate the local time.

These satellite constellations vary in terms of their orbital heights and the speeds at which they whiz around the sky. Other technical distinctions include the fact that they employ mostly unique radio frequency bands for their data transmissions.

No satellite navigation system is perfect. For instance, it takes time to prepare and launch reliable devices, and keeping them running is no minor feat. A ground-based receiver needs signals from multiple transmitters to get an accurate timing or positional fix, and the most-populated systems only include a few dozen satellites. In other words, loss of a single signal may have a more significant impact in a constellation with fewer members to spare in the first place.

The U.S. spent years dominating the scene with the Global Positioning System. Even with the later addition of Russia's GLONASS, the options remained limited.

More recently, however, the playing field has become host to the European Union's global Galileo and China's regional BeiDou, once known as COMPASS. These systems are scheduled to be fully operational by 2020.

Other options, like Japan's Quasi-Zenith Satellite System, or QZSS, are currently restricted to limited regions. If things go according to plan, however, they should eventually expand to provide broader coverage.

As nations compete to create functional, accurate and secure satellite positioning and timing networks, it is only natural to differentiate between the diverse groups of artificial constellations. Here are the major players:

GPS

The Global Positioning System is owned and operated by the United States. The official US Department of Defense name for GPS is NAVSTAR.

BeiDou

BeiDou (or BDS – formally known as Compass) is a regional GNSS owned and operated by the People's Republic of China. China is in the process of expanding the system to 35 satellites by 2020 to provide global coverage.

Galileo

Galileo is the European Union’s global GNSS. The EU plans to complete the system by 2020.

GLONASS

GLONASS (Global Navigation Satellite System) is owned and operated by the Russian Federation. The fully operational global system consists of 24+ satellites.

QZSS

QZSS, or Quasi-Zenith Satellite System, is a regional GNSS operated by QZS System Service Inc. (QSS) and owned by the Government of Japan. QZSS works with GPS to improve coverage in East Asia. The constellation is operational with 4 satellites. By 2023, Japan plans to have 7 satellites for autonomous capability.

Which GNSS System Should You Use?

What does all this mean for companies that depend on GPS or GNSS for tasks like high-fidelity network timekeeping? In the end, users may be better off not playing favorites. Multi-GNSS can solve problems like outages by using a variety of different systems. For instance, if part of the QZSS satellite constellation fails, the receiver might fall back on GLONASS, BeiDou or Galileo. This capability means that users don't have to stick to a particular option to enjoy reliable timing and positioning feedback.

In the end, your choice of satellite constellations should reflect your use case. For instance, if you were deploying a network in a particular country, you'd probably go with whichever positioning system provided the most accurate information. National security embargoes and other political issues can also impact the viability of specific alternatives. GNSS threats like jamming and spoofing can also impact performance.

Masterclock offers both GPS and multi-GNSS receiver options with our configurable master clocks. If you’d like to learn more about GNSS-synced timing, contact us today.