GPS devices don’t actually contact satellites and transmit information to them. They only receive data from satellites – data that’s being always-transmitted. However, GPS isn’t the only way devices can determine your location.
From Satellites to the Palm of Your Hand
The global positioning system was originally created by the United State for military use, but was eventually opened up to civilian use. At least 24 GPS satellites are always in orbit around the Earth, and they’re constantly broadcasting data.
The satellites are arranged in orbit such that four satellites are visible in the sky from any point on Earth. (You can’t actually see them, but there’s a direct path for the radio transmissions.) This means that GPS won’t work if the signals are being blocked – you will want a fairly direct path between you and the sky. In an underground bunker or in a cave under a mountain, it won’t work.
GPS satellites are constantly transmitting radio signals towards the Earth. Each transmission includes the location of the GPS satellite and the time the signal was sent. Each satellite has an atomic clock onboard, so the time is very precise.
How GPS Determines Your Location
A device with built-in GPS – whether it’s a dedicated in-car GPS navigation unit or a smartphone – only acts as a GPS receiver. A device with GPS isn’t actually “contacting” satellites to determine its location. Instead, it’s just listening for the radio signals that are being broadcast from these satellites all the time.
A GPS receiver “listens” for signals from four or more satellites. Signals from the closer satellites will arrive sooner, while signals from the farther satellites will arrive later. (The actual time difference is very small, but can be detected by the GPS receiver.) By comparing the time the signal was broadcast and the time the signal arrived, the receiver can estimate its relative distance from all four satellites. Using trilateration, the receiver can then determine its location.
Trilateration may sound a bit complicated, but it’s actually fairly simple. Imagine if someone told you you were 500 miles from New York, 800 miles from Miami, and 700 miles from Kansas City. With this information, you could determine a region that is the correct distance from all of these cities and estimate your current location. If we told you your distance from a fourth city, you could estimate your location even more precisely. That’s trilateration in a nutshell, and it’s what GPS receivers are doing whenever you use them.
Alternatives to GPS
GPS isn’t the only way devices can estimate your current location. The 911 service uses cell tower strength information to triangulate the position of mobile phones. This works in a similar way – by measuring the signal strength differences between multiple cell towers, your device can estimate your current location.
Some devices can also use a Wi-Fi based positioning system (WPS) to determine their current location. Google’s street view trucks drive around, capturing the names of nearby access points and their relative strengths at certain locations. Your smartphone scans for nearby wireless networks, then sends a list of their names and signal strengths to Google’s servers. Google uses their database and estimates where you are. (Google isn’t the only provider of Wi-Fi-based positioning system data, but it’s the one most people will be familiar with.) This can be particularly convenient in indoor locations GPS signals can’t reach.
The GPS system isn’t the only network of satellites that can be used for positioning, either. Russia has its own GLONASS system and China has BDS. Europe is also working on its own alternative to GPS, known as Galileo. GPS could be shut down or restricted in times of war or conflict, so nations want their own satellites to be self-sufficient.
GPS on its own isn’t a privacy concern – for example, if you have an old GPS unit for your car, it likely isn’t capable of transmitting your location. However, GPS can be a privacy concern when combined with transmitting technology. GPS tracking devices don’t just use GPS receivers – they store the GPS data for later retrieval or transmit the GPS data.
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