BY ODD ERIK GUNDERSEN
In a map application, such as Google Maps or Apple Maps, the blue dot tells you where you are. How can your mobile phone know where you are?
This is the first blog post on a series of posts that examine and discuss problems and solutions related to mobile positioning. The series will entail the following related posts:
Part 1: Outdoor positioning
Part 2: Indoor positioning
Part 3: Fusing different position sources
How does your phone know where you are?
Your mobile phone cannot find your location on its own. It needs help, and this aid comes from asking other devices that has a known position. By collaborating, your phone and these other devices are able to find your location so that your phone can illustrate the blue dot in a map service.
There are four different methods a mobile phone can use to find its position:
- It can ask the global positioning system (GPS),
- It can ask the GSM phone network,
- It can ask GSM base stations equipped with GPS receivers (aGPS),
- It can scan which Wi-Fi networks are within its range and ask a web service that keeps track of Wi-Fi networks.
By combining some or all of these four positioning methods, the phone is able to interpret its position fairly accurately in most situations.
The global positioning system (GPS) consists of a network of satellites. These satellites know where they are and what the time is – with high precision. They broadcast this information continuously to everyone. GPS receivers, such as your average smart phone, need the signal from at least three satellites to determine its position and at least four satellites to estimate the altitude as well. The GPS receiver fetch information on each satellite’s clock time and location, and computes its position based on these data.
GPS technology has clear advantages in that the position is quite accurate and it works in any weather condition. However, it takes between 30 and 60 seconds for a GPS receiver to assemble and understand all the information required to calculate a position. When you are in a hurry, 60 seconds is just too slow. Further, as a lot of us have experienced, calculating GPS positions is quite resource demanding and the phone battery is quickly drained when using GPS for navigation over time.
“When you are in a hurry, waiting 60 seconds for a GPS position is just too slow.” tweet
The emergency services often already know your position when you call them, which is a good thing if you want their help. And you probably do, since you called them in the first place. They do this by requesting the caller’s position from the phone company. When your phone communicates with a telecommunications mast (a GSM base station), the phone sends its round-trip time and signal strength to the base station. This information can be utilized to find how far away from the base station your cell phone is. Based on this estimation, your location can be decided. The position reported by one GSM base station has an uncertainty equal to your distance from it. To increase the accuracy, multilateration between several base stations is possible. The more base stations the phone can communicate with, the better the accuracy gets.
Using GSM base stations for positioning seems handy, as the position of any phone connected to the network can be found. However, GSM positioning has clear disadvantages when it comes to accuracy. Since the accuracy is directly related to the density of GSM base stations, the positioning service is most accurate in urban areas. Still, the accuracy in urban areas is reported to have a median error between 94m and 196m, which is to say the least not impressive. In rural areas where there is a lower density of base stations, the accuracy can be as poor as several kilometers. Albeit, the latency of the GSM-based positioning service is very low.
In order to improve the GPS’ positioning latency and battery drainage, most smartphones today use an augmented version of GPS called aGPS or assisted GPS. The assisted version of GPS relies on GSM base stations that are equipped with GPS receivers. aGPS has several advantages, as it provides a location much faster than GPS and the accuracy is much better than GSM positioning. The accuracy range is normally between 5m to 50m. In worst case, it is no better than GSM positioning. Additionally, less processing power is required by the phone, which saves battery life, and a position is provided even in non-optimal environments, such as indoor locations. In conclusion, aGPS is less accurate than GPS, but it is much faster and treats your battery nicely.
Outdoor positioning can also be done using the position of known Wi-Fi networks. If Wi-Fi is enabled on your phone, the phone scans for nearby Wi-Fi networks that broadcast their name. The name and signal strength of this Wi-Fi is sent to a database that keeps track of Wi-Fi networks, which then replies with a position. The accuracy of this position is slightly better than for GSM-based positioning, but Wi-Fi coverage can be spotty even in urban areas. In rural areas without wireless networks, Wi-Fi positioning does not work at all, which clearly represents a disadvantage.
The great mutual thing about these four positioning methods is that they are all integrated into your phone, which means that your phone is able to find your position under most conditions. Do you have network connection on your phone? You can get a position! Do you have an unobstructed line of sight to three GPS satellites? You can get a position! Do you see any public Wi-Fi networks around and Wi-Fi turned on on your phone? You can get a position!
Outdoor positioning can obviously get more accurate and work faster. Still, it works really well, and I personally use it all the time. However, all outdoor positioning services proves very unsatisfactory when doing indoor positioning, a problem for which we will explore solutions to in the next blog post of this series.
“All outdoor positioning services proves very unsatisfactory when doing indoor positioning.” tweet
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