How GPS Works

How GPS Actually Works: Trilateration Explained Simply

What's actually happening inside your receiver when it shows a little blue dot on the map? It isn't magic, and it isn't triangulation either, despite that word getting thrown around constantly. GPS uses trilateration, and the difference matters.

Each GPS satellite constantly broadcasts a signal containing the exact time it was sent, down to the nanosecond, along with its own position in orbit. Your receiver picks up that signal and works out how long it took to arrive. Since radio waves travel at a known, fixed speed, timing delay converts directly into distance. Catch a signal from one satellite and you know you're somewhere on a sphere a certain distance from it. Catch a second, and you're somewhere on the circle where those two spheres intersect. Catch a third, and you're narrowed down to one of two points. A fourth satellite resolves the last bit of ambiguity and corrects for tiny errors in your receiver's own clock, which is nowhere near as precise as the atomic clocks on board the satellites.

Think of it like being lost in the bush and asking three separate trampers how far away you are from each of them. Line up all three answers and there's only one spot on the map that fits all three distances at once. That's trilateration, and it's the entire foundation of how GPS works.

• Distance to each satellite is calculated from signal travel time, not signal strength
• A minimum of four satellites in view gives a full 3D position fix, including altitude
• More satellites in view generally means a faster, more reliable fix, particularly in tricky terrain

What's Really Up There: Today's GPS Satellite Constellation

How many satellites does GPS actually need, and how many are up there right now? The system is built around a minimum of 24 operational satellites, spread across six orbital planes roughly 20,200 kilometres above the earth, positioned so at least four are visible from virtually anywhere on the planet at any time. Today there are 31 operational satellites in the constellation, giving a healthy buffer above the minimum.

GPS was originally built and is still operated by the United States Department of Defense, going live for military use in the early 1980s. Civilian access improved through the 1990s, and a big jump happened in May 2000, when the US government switched off "Selective Availability", a deliberate degrading of the civilian signal that had capped everyday accuracy at around 100 metres. Overnight, ordinary GPS receivers everywhere became roughly ten times more accurate, and that single decision is arguably what kicked off consumer GPS as an industry.

Why the Satellite Count Isn't Really the Interesting Number Anymore

For most of GPS's history, more satellites in view meant a better fix, full stop. That's still broadly true, but it's no longer the whole story. The bigger shift in the last decade has been what else is up there working alongside GPS, and what's now correcting its signal from the ground.

GPS vs GNSS: Why Modern Trackers Use More Than One System

Is "GPS" even the right word for what's in your tracker or your phone anymore? Strictly, GPS refers only to the American system. Most modern devices actually use GNSS, Global Navigation Satellite System, which is the umbrella term for GPS plus several other independent satellite constellations run by other countries.

• GPS (United States): the original, still the most widely supported globally
• GLONASS (Russia): fully operational, commonly paired with GPS in dual-system receivers
• Galileo (European Union): operational since December 2016, offering high-precision civilian signals
• BeiDou (China): a large modern constellation, increasingly supported in newer chipsets

A receiver that only talks to GPS satellites is limited to whichever of the 31 GPS satellites happen to be overhead. A multi-constellation GNSS receiver can pull in signals from GPS, GLONASS, Galileo and BeiDou simultaneously, sometimes seeing well over 100 satellites combined. More satellites in view means a faster initial fix, better accuracy under partial obstruction like light bush cover, and more resilience if one constellation has a temporary outage.

SouthPAN: The Upgrade Making GPS Sharper in NZ

Here's the part most GPS explainers never mention, and it's directly relevant if you operate in New Zealand. Standard, unaugmented GPS typically gets you accuracy of somewhere between 5 and 10 metres. Good enough for most navigation, not always good enough for precision agriculture, asset tracking on a tight site, or knowing exactly which side of the fence a worker's beacon has gone off on.

SouthPAN, short for the Southern Positioning Augmentation Network, is a joint New Zealand and Australian government initiative that fixes this. It's a Satellite-Based Augmentation System, the first of its kind in the Southern Hemisphere, using a network of ground reference stations to calculate the exact errors in GPS and Galileo signals, then broadcasting corrections back down via satellite.

The early open services of SouthPAN have been live since September 2022 and are free to use on a compatible device, no subscription required. Depending on which SouthPAN service tier a device supports, accuracy improves to as good as 3 metres, 1 metre, or in the most precise mode, under half a metre. Full safety-of-life certification, mainly relevant to aviation, is expected by 2028, but the accuracy benefits for everyday commercial use are available right now.

What This Actually Means On the Ground

• Compatible fleet tracking and precision agriculture hardware gets meaningfully sharper positioning without any extra cost
• Corrections come directly from the satellite, so they work even without mobile coverage or an internet connection
• Not every existing device is SouthPAN-compatible. Some older receivers need a firmware update, and some hardware simply doesn't support it

If you're buying new tracking or navigation hardware in 2026, it's worth asking specifically whether the device supports SBAS or SouthPAN corrections. It's a genuinely free accuracy upgrade, and plenty of buyers don't yet know to ask for it.

Where GPS Struggles, and What Actually Fixes It

Does GPS work everywhere, all the time? No, and anyone who tells you otherwise hasn't used it in the real world. GPS signals are weak by the time they reach the ground, having travelled over 20,000 kilometres, and they don't cope well with anything solid in the way.

• Dense forest canopy: Signals get scattered and absorbed by heavy foliage, common across NZ forestry blocks
• Deep valleys and gorges: Steep terrain physically blocks line of sight to enough satellites for a reliable fix
• Urban canyons: Tall buildings reflect signals, causing multipath errors that can throw a position off by tens of metres
• Indoors and underground: GPS signals don't penetrate structures well, which is why indoor positioning uses entirely different technology

The genuine fix isn't a better antenna alone, though that helps. It's a device built around a modern multi-constellation GNSS chipset with SouthPAN support, paired with sensible expectations about where it will and won't get a clean fix. For lone workers or vehicles regularly operating well outside GPS-friendly terrain entirely, that's exactly where a satellite communicator or PLB, which doesn't depend on line of sight to as many satellites and can message via satellite networks like Iridium, earns its place alongside a standard tracker.

Who This Actually Matters To

You don't need to understand orbital mechanics to run a business. But knowing roughly how GPS works helps you ask the right questions before you buy tracking or safety gear, rather than after something doesn't perform the way you expected.

This Matters Most If You're:

• Running a fleet where accurate location, route history and geofencing genuinely affect fuel costs and driver safety
• Tracking non-powered assets like trailers, plant or containers that move between sites without warning
• Operating in forestry, farming or backcountry tourism, where canopy cover and terrain regularly challenge weaker devices
• Responsible for lone or remote workers who need a reliable way to raise the alarm well outside mobile coverage

It Matters Less If You're:

Just navigating around town on a phone. Standard smartphone GPS, already GNSS-based and often SouthPAN-compatible on newer devices, is more than accurate enough for everyday driving directions. The detail in this guide starts to earn its keep once accuracy, reliability in poor terrain, or safety in an emergency actually has consequences.

Turning GPS Into a Working Tracking System

Understanding how GPS works is one thing. Choosing the right hardware and setting it up so it actually delivers useful, reliable data for your business is another. Mobile Systems Limited has been supplying and supporting communications and tracking equipment from Mount Maunganui for over 25 years, and GPS fleet tracking is a category we work with daily across the Bay of Plenty and nationwide.

We stock a genuine range of GNSS-based tracking and safety hardware, from vehicle and asset trackers through to satellite messengers and personal locator beacons for teams working well outside GPS-friendly terrain. Our team can talk you through which technology actually suits your operation, rather than selling you the most expensive option on the shelf.