A Simple Guide to How GPS works
How often do you try to get a location fix using GPS (Global Positioning System) and notice that it was not showing your location correctly on the map? Is it because the GPS, which was first launched some 40 years ago, is an old system and inherently unreliable? Or is it you who are using it without really understanding how it works?
Well, it will be the latter — because almost all navigational systems and military systems worldwide rely on GPS. So, do try to understand how it works! Without delving into complex technical details, we provided some answers below to explain all about GPS and its accuracy after some research — hopefully in a simple way that you can understand. If you are not interested in the details, at least read the section headers to gain some basic knowledge.
1. You need line-of-sight with 4 GPS satellites to get accurate location fix.
There are around 32 satellites in orbit around Earth and they are constantly broadcasting microwave signals to GPS receivers near the Earth's surface. These GPS receivers are usually found in mobile devices, GPS navigation systems and military equipment. Each receiver will compute its own locations from the satellite signals received.
To get a fairly good fix on your location, the GPS receiver in your device will need to be able to pick up the microwave signals from at least 3 satellites at sea level (ignoring altitude). To get more accurate fix and taking altitude into account, it will need line-of-sights with at least 4 satellites. This means that if you are not at sea-level, 3 satellites will still give a slight error in your location.
Having "line-of-sight" means there should be a clear straight path between your mobile device and each of the satellites. There should be no obstacles in the path.
2. The accuracy of GPS can be as high as within 5 metres.
The satellites are constantly orbiting around Earth, and so do Earth orbiting around the sun. It could happen that you are able to get an exact fix on your location in the morning but it goes off by some tens of metres in the afternoon at the same spot.
The accuracy of the GPS is specified as within 5 metres — provided your mobile device can pick up signals from 3 to 4 satellites in an open space and under a clear sky. Accuracy will worsen if there are tall obstacles nearby. In all diagrams in this post, the blue bubble underneath the man indicates the accuracy of the location (similar to the blue bubble in iOS Map). The smaller the bubble, the higher the accuracy.
The map on the left (above) has a large bubble around the user location, when inside Beauty World MRT Station, without GPS and WiFi. Notice that the user location is also off by few hundred metres from the station. The map on the right has a smaller bubble when walking along a road with low buildings on both sides. This indicates that the user location is accurate and within 5 metres error.
3. Obstacles in your surroundings can block signals from the satellites.
According to experts, you should be able to pick up microwave signals from at least 4 satellites at any one spot on Earth. However, that is provided you are in an open space with no obstacles to interfere with the signals. Standing next to a tall building or under the dense canopies of trees, dense rain clouds, atmospheric pressure, inside vehicles with metallic cage, inside buildings, etc, can affect the signals received by your device's GPS receiver.
The satellites are orbiting some 20 km from Earth's surface. By the time the signals from the satellites reached Earth, they are already very weak. Therefore, when any of the signals get "obstructed", the line-of-sight with a satellite is considered lost. Under such condition, the GPS receiver will have lesser data to compute its location, thus resulting in larger accuracy error.
In the diagram above, the red cross-hair indicates the erroneous location shown in map apps. The bigger blue bubble indicates that the accuracy error is larger in this case. When you are inside a building with no public WiFi hotspots available, don't try to get a location fix — it will be inaccurate anyway.
4. GPS does not require a network connection to send you your location.
A common misconception among many people, including engineers, is that a user's location is sent through a telecommunication network to the mobile device. This is apparently not correct. As explained above, a user's location is computed by the GPS receiver after receiving the microwave signals directly from the satellites. There is no data network in that process.
In other words, you do not need a data plan to get a location fix on your mobile device when travelling but you will need it to download the map. And if you can download and cache the map prior to setting off, no data plan will be required at all. You can switch off mobile data and still get your location shown on the map.
5. Turn on WiFi to improve location accuracy — in cities only.
So, why do I have to turn on WiFi to improve location accuracy if GPS does not need network connection?
First, when you turn on your device's WiFi, the device will scan for nearby public WiFi hotspots and take note of the signal strengths from the WiFi hotspots. It does not require you to log on to any of the WiFi hotspots, so data cannot be sent.
Second, apart from GPS signals, a GPS receiver can also calculate your locations by using signals from cell towers and WiFi hotspots. In a process called "triangulation", signal strengths from at least 3 known cell towers and WiFi hotspots are measured and the "best" guesses to your location is offered.
This usually takes place inside buildings where GPS signals are unable to penetrate steel and concrete structures. And in cities, a mixture of signals from one or two GPS satellites, cell towers and WiFi hotspots are used to compute your location.
We highlighted "in cities". If you are in areas that has no public WiFi hotspots, such as forests, mountains, out at sea, etc., do turn off WiFi to conserve your device's battery power.
6. Low battery level will affect your location accuracy.
When you want to get a GPS location fix on your mobile device, the GPS receiver will be turned on. When you want to make phone calls or send data, your network micro-controller will be turned on. When you want to detect a WiFi network, the WiFi micro-controller will be turned on. When you want to listen to music at the same time, your audio chip will be turned on. And also the accelerometer, gyroscope, altimeter, etc. All these requires power from the mobile device's battery.
When your mobile device's battery level goes low, what should the device do to conserve power? It can't really turn any of the hardware off because you requested to use them concurrently. So the mobile device will tune down the hardwares' performance. The screen will be dimmer, the CPU will run slower, the GPS accuracy level will be lowered, your music will be softer, and so forth, until the battery level drop below the minimum threshold to keep the device powered up.
Try not to "torture" your device with too many tasks at the same time — especially when you are mapping your hiking trail.
7. GPS is more accurate than cell towers and WiFi hotspots.
It is too technical to explain the engineering behind the three totally different systems and how they identify your location. Let's keep it simple:
GPS is above you and it can transmit signals to your device directly. So long as you are under an open sky, GPS will give you an accurate fix.
Signals from cell towers suffer from reflection from too many structures inside cities and also interference from other transmitters.
WiFi hotspots are network transmitters installed in shops and they can be moved easily within the shops or reconfigured, thus, changing the reference points that are known previously.
So, to get an accurate location fix from GPS, get under an open sky.