Before GPS technology, we used to travel relying on maps and our knowledge of our cardinal directions. Today, many of us depend on our GPS to take us from point A to point B, but how exactly does our GPS get us to our destination? This blog will discuss how GPS works, as it’s honestly amazing that it works at all!
First, how do you find your current location without GPS? Using a map and something to tell distance (like a laser ranging device), you find three landmarks around you and measure distance to them. By drawing a circle equal to that distance around the landmarks, you are able to triangulate a position.
GPS and Satellites
GPS works off the same principle, but our landmarks are instead way out in space (about 12,550 miles away). This extra dimension over 2D maps means that a minimum of four satellites are required. Since these landmarks are so far away and so fast moving, the only way to reliably determine their location to find our location is with a map of the GPS constellation. This map is actually broadcast by the satellites themselves as a part of their standard signal, which tells you where each of the 24+ satellites will be at any given time in the next 30 days so we can more easily find their positions. A GPS satellite follows the same path around earth every day. If you have a satellite directly above you at noon, it will be in the same position at noon every day! However, the satellites are subject to gravitational forces from Earth, the Moon, the Sun, Jupiter, even Pluto and other bodies in (and outside) the solar system. These forces tug and pull the satellites ever so slightly, meaning the map the GPS satellites broadcast actually have to account for and predict movement based on all the known bodies in our Solar System. This is a huge undertaking, but thankfully there are very smart people figuring this out for us.
Satellite Delays and GPS Receiving
So imagine your device has just turned on and received its first signal and mapping data, but there’s a new challenge. The way we measure distance from each of these satellites to triangulate our position is by measuring the delay the signal takes to reach us. As the signal is broadcast at light speed, nanosecond delays could throw positions off drastically. It should only take about 0.134 seconds for each signal to reach us, so it is very important that both the satellite and our GPS receiver have the exact same clock times. But to get that precise, we would need an atomic clock. This poses a new challenge, as atomic clocks are very expensive. GPS would never have made it to the consumer world with that price barrier. So, what’s the answer? Let’s use the clocks on the satellites to sync our own. That’s right: we use the clock on a satellite to measure the delay from that satellite. It might not make that much sense, but with a constellation of satellites available, checking off the time from four satellites can give us an accurate measure of the real time with some reverse engineering.
Tackling Signal Problems
With that problem out of the way, the next issue is signal strength. These satellites have a broadcast area of roughly 1/3rd of the surface of Earth each. That’s a lot of area to cover. Such an incredibly weak signal from a satellite zooming through space is somehow expected to reliably cover so much surface area. It makes sense that something like a concrete wall, metal roof or coil of wires could degrade signal. In reality, GPS signal is heavily affected by those factors, as well as power lines, microwave ovens, even background cosmic radiation drowns it out. To combat this, the antennae in a GPS receiver are so incredibly fine-tuned to one specific frequency that they can block out all this other background noise.
But what if the issue isn’t external influence, but the GPS signal itself is wrong? There is still a unique phenomenon called GPS Reflection. In situations where your receiver cannot see many satellites, it may end up with a bounced or reflected signal coming off a wall, building or bridge. This extraordinarily small signal delay from a “bounced” signal is responsible for erratic GPS positions where the location moves wildly from several yards to several miles away. There is really no solution for this behavior other than addressing the GPS receiver’s sky visibility. If only one out of 6 visible satellites bounces, that bounce can be ignored. If one of the minimum 4 satellites bounces, the signal quality will suffer as a result. If this happens frequently, re-positioning the unit in the vehicle may help as it could be that the radio antenna just happens to block out some signals with it’s noise.
GPS has seen amazing leaps in quality since its introduction, both on the receivers and satellites. At its launch, GPS took around 26 minutes to provide a position using an antenna as tall as you or I. Now, a GPS signal can be acquired typically within 6-12 seconds from a receiver smaller than your fingertip. Regardless, the above considerations still have some influence on GPS reliability and could easily explain minor fluctuations in GPS signal.