GPS feels as if a satellite "sees" your vehicle. It does not: the satellites receive nothing, they only broadcast the exact time, over and over. All the work is done by a small device in the vehicle, and the simulation above shows how, with the real satellites and real numbers for Novi Sad. Here is the same explanation as text.
The satellites do not know you exist
GPS is about thirty satellites at a height of around 20,200 km, spread over six orbital planes so that at least four are always visible from anywhere on Earth. Each one circles the Earth in 11 hours 58 minutes, so twice a day.
The satellites only transmit: each one continuously sends the exact time from its atomic clock and data about its own orbit. The device in the vehicle sends nothing back, it only listens. That is why GPS can serve an unlimited number of users, and why the satellites cannot track anyone.
Four satellites, four distances
The signal travels at the speed of light and reaches the ground in 67 to 86 milliseconds, depending on how high the satellite is in the sky. The device compares the send time, written into the signal itself, with the moment it arrives. That difference multiplied by the speed of light is the distance to the satellite.
One distance puts the vehicle somewhere on a sphere around that satellite. Two spheres cross in a circle. Three spheres leave only two points, and one of them is thousands of kilometres out in space, so it drops out.
Why four: the device clock is not atomic
The device has an ordinary quartz clock. An error of just one millisecond makes every measured distance 300 km too long, so the position would be hundreds of kilometres off, often deep underground.
So the device does not trust its clock: it treats the exact time as a fourth unknown. Three coordinates plus time are four unknowns, which is why four satellites are needed. The solve takes a few steps, from a rough guess to the answer, and within a fraction of a second it yields both the position and the exact time. In practice a device sees eight to twelve GPS satellites at once and uses them all, which makes the result steadier.
Einstein in every GPS device
At 20,200 km gravity is weaker, so a clock on the satellite runs faster by about 45.7 millionths of a second a day. The satellite also moves at about 3.9 km per second, which slows the clock by about 7.2 microseconds a day. Net, the clock up there gains about 38 microseconds a day.
That sounds tiny, but light covers about 11 km in that time, so uncorrected the position error would grow day after day. That is why the clocks are tuned before launch: they run at 10.22999999543 MHz instead of 10.23 MHz, so in orbit they tick exactly right.
From satellite to your map
GPS gives the position to the device itself. For the fleet owner to see it, the device sends that position over its built-in SIM card to the PratiMe platform, together with speed, heading and vehicle data. It does not send on a fixed beat: in bends and when speed changes messages go out more often, events such as hard braking are sent at once, and construction machinery has its own operating modes.
When the mobile network drops, in the hills or at a border crossing for example, the device stores its positions and sends them as soon as it reconnects, so the route on the map stays complete. What the platform does with that data is described in how GPS vehicle tracking works, and the devices we use are on the GPS devices we install page.
How accurate GPS is
In the open, a modern vehicle tracker usually fixes its position to within a few metres. Accuracy drops between tall buildings, where signals bounce, and in tunnels and underground car parks there is no signal at all. Weather has little effect.
Besides GPS, the devices we install also use Europe's Galileo, Russia's GLONASS and China's BeiDou, so they see more satellites at once and stay accurate in cities too.
About the simulation
The satellite positions are calculated from the public GPS almanac published by the US Coast Guard Navigation Center (NAVCEN), using the algorithm from the official GPS specification, for the moment you are watching. The almanac refreshes automatically.
This is not a recording of a real radio signal: the travel times, sphere intersections and position solve are derived from those orbits for Novi Sad, with a few metres of noise like a real receiver has. That is why the numbers change from day to day, just like the real sky above us.
Frequently asked questions about GPS
Does GPS work without the internet?
Yes. The position is calculated from the satellite signals alone, without the internet or a mobile network. The network is only needed for a tracking device to send that position to the platform, and until it is available the positions are stored in memory.
Can the satellites track my vehicle or my phone?
No. GPS satellites only broadcast a signal and receive nothing. A vehicle can be tracked only when the device in it sends its own position, for example over a SIM card to a tracking platform.
How many satellites are needed for a GPS position?
At least four: three for the position and a fourth for the exact time, because the device clock is not precise enough. In practice a device sees eight to twelve GPS satellites at once, and more with Galileo, GLONASS and BeiDou.
How accurate is GPS?
In the open, usually within a few metres. Accuracy drops between tall buildings, and in tunnels and underground car parks there is no signal, so the device calculates its position again as soon as it picks up the satellites.
How often does a vehicle tracker send its position?
There is no single fixed rhythm. The device sends a new message when the vehicle turns or changes speed, so in bends it can be every few seconds, and it sends events such as hard braking at once. Construction machinery has its own operating modes. If there is no mobile network, messages are stored in memory and sent as soon as it reconnects.
Why does GPS have to account for Einstein's relativity?
Because the satellite clocks, due to weaker gravity and high speed, run about 38 microseconds a day faster than clocks on Earth. Uncorrected, the distance error would grow by about 11 km every day, so the clocks are tuned before launch.
Is the GPS signal free?
Yes. The GPS signal is free and available to everyone, with no subscription. A vehicle tracking subscription pays for data transfer, the platform, reports and support, not for the satellite signal itself. More on the GPS vehicle tracking price page.
Want to see your own vehicles on the map? Contact us by e-mail at podrska@pratime.rs or call +381 65 900 7380. We install the device at your location, and you test the system free for 15 days, with no obligation.
