Intro to GNSS: Orbiting Satellites

Have you ever stopped for a moment to think about the satellites orbiting us? There are thousands of GPS satellites orbiting Earth that play a crucial part in GNSS and assured positioning for our technology. From autonomous shuttles to objects in the sky, satellites help us navigate our world.

What is the difference between GPS and GNSS?

GPS, or Global Positioning System, was launched in the 1970s by the United States Department of Defense. However, the term “GNSS” or Global Navigation Satellite System is not as commonly known, it is used to describe the collection of satellite positioning systems. GNSS includes GPS, GLONASS, Galileo, BeiDou, IRNSS and QZSS systems. These GNSS satellites orbit Earth, approximately 20,000 km above the surface groups of 20 to 30 satellites called constellations.

Orbiting Earth at High Speeds

Satellites are not as small as one may think. The latest generation weighed over 1,400 kg, slightly more than a Volkswagen Beetle. (There are small nanosatellites developed and used for specific missions). Although they may weigh close to the same, these satellites can cover ground a bit faster than a Beetle, flying above us at several kilometers per second.

These satellites are also incredibly accurate when it comes to their time and orbit ephemerides. For example, if you ask a GPS satellite for the time, it won’t tell you “8:30.” Instead, it will tell you “8:30.3924136.” The latest generation of GNSS satellites uses rubidium clocks that are synchronized by even more accurate ground-based cesium clocks. These clocks are so accurate that it will take one over 100,000 years to gain or lose a second. By comparison, a quartz watch will lose about a second every two days.

Why is time an important factor in GNSS systems?

The time used for a GNSS signal to travel from satellites to receivers determines distances/ranges. Timing accuracy is crucial. The radio waves GNSS signals travel upon move at the speed of light. In one microsecond, light travels 300 meters. So if the timing is off by even a few seconds, the satellite’s position has already changed. Small errors in time can result in large errors in position.

Learn more about satellites and other basic GNSS concepts in the “Introduction to GNSS: GPS, GLONASS, Galileo, and other Global Navigation Satellite Systems.”

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