GPS 101

Ever wondered how your phone instantly knows your location, even if you’ve never been there before? Whether you’re booking a cab, finding a restaurant, or tracking a morning run, there’s an invisible network in the sky quietly making it possible — the Global Positioning System, or GPS.

GPS has become so ubiquitous that we rarely pause to consider the remarkable technology behind it. It's a satellite-based navigation system operated by the United States that determines your precise location anywhere on Earth. The system consists of a constellation of satellites orbiting about 20,000 km above us, constantly broadcasting signals containing their location and the exact time the signal was sent.

How Does GPS Work?

Your phone (or any GPS-enabled device) has a receiver that listens for these signals. By calculating how long the signals took to reach it, the receiver can work out the distance to each satellite. With distance data from at least four satellites, it can determine your exact position on Earth — a process called trilateration.

To visualise this, imagine you’re somewhere and know you’re 5 miles from school. This means you could be anywhere on a circle with the school at its centre. Now, say you know you’re 10 miles from the library. This narrows your location to the two points where the circles intersect. With a third data point—say, you’re 3 miles from a friend’s house—you can pinpoint your exact location at the single point where all three circles intersect. GPS trilateration works similarly, but in three-dimensional space with satellites instead of landmarks.

One of GPS's most valuable features is its complete independence from internet connectivity. GPS satellites continuously broadcast their signals, and your device simply receives them—it doesn't send anything back. This one-way communication means GPS works perfectly in remote wilderness areas, during natural disasters when cell towers fail, or anywhere internet access is unavailable. The maps on your phone might require an internet connection to download, but the core positioning functionality operates entirely offline.

The Satellite Constellation.

A fully operational GPS network requires at least 24 satellites for global coverage, but there are currently about 31 active GPS satellites in orbit. They are arranged in six orbital planes to ensure that anywhere on Earth, at any given time, a receiver can “see” at least four satellites.

While three satellites can theoretically determine your position, a fourth is essential for accuracy. Your GPS receiver's internal clock isn't as precise as the atomic clocks on satellites, so the fourth satellite helps eliminate timing errors and provides the exact time synchronisation needed for accurate positioning.

GPS vs. GNSS: A Global Perspective.

GPS is just one member of a larger family called Global Navigation Satellite Systems (GNSS). While GPS is the American system, other nations operate their constellations, such as Russia's GLONASS, Europe's Galileo, and China's BeiDou. Most modern devices can access multiple GNSS systems simultaneously, improving accuracy and reliability.

Signals from these systems are transmitted on agreed-upon frequency bands and use compatible formats so that a single receiver can decode multiple systems. The International Committee on GNSS (ICG), under the UN, facilitates this cooperation.

From guiding ships across oceans to enabling delivery apps to find your doorstep, GPS and other GNSS systems form an invisible but essential layer of our modern world.

Reader's Corner:

A curated set of additional references from previous issues, shared by the community.

• "The UPC - A bit of history of barcodes" by Marcos D. Alves