How Does Your Phone Know Its Location?
Today’s show explains how your mobile device determines your location, commonly displayed on a map using the popular ‘blue dot’. Our guest is Ed Parsons, Google’s Geospatial Technologist. He has been at Google for over 15 years; but before that he came from academia, and even helped to set up one of the first GIS courses taught at Kingston University. Prior to that, he worked for the Ordnance Survey and the National Mapping Agency in the UK.
The Blue Dot and GPS
The blue dot on a map shows your location as determined by your mobile device. With the blue dot, you no longer have to manually figure out where you are, as you would have to if you were using a paper map.
The very first mobile devices with the capability to determine location were using GPS (Global Positioning System). To this day, many people still think that GPS is the only satellite constellation that all mobile devices use to determine location. GPS is actually only one of the several GNSS (Global Navigation Satellite System) constellations used to determine location. GPS is an American system; other GNSS include GLONASS of Russia, Galileo of the European Union, and Baidu of China, among others.
Why Does Your Phone Need To Know Its Location?
Despite helping you in navigation, your phone also needs to know its location for its basic functions. Knowing a phone’s location, service providers are able to route its calls through the closest cell towers. Messaging and calling is not possible if a phone’s location is not known. Therefore, all mobile networks have to maintain a rough location of where a device is in relation to their cell towers. This is a key way that a device is able to know its location.
Setbacks in GNSS
When a device is using GNSS to determine its location, it needs a line of sight to the satellite sending the timing signals so as to calculate an accurate location. If there’s an obstruction between the device and the satellite, (such as a tree, a building, or a mountain) the device may not get a signal at all, or may suffer multipath problems, which is when the signal bounces off the objects during its journey, resulting in inaccurate timing.
For a device to use GNSS, its radio receivers have to be turned on so as to receive the satellite signals. The power needed to keep these radios on for prolonged periods, as well as the power needed to do the actual computation of location from the satellite signals, can drain the phone’s battery rather quickly. Power management is a major concern in using GNSS, and several alternatives such as using Wi-Fi hotspots to determine location have been developed over the years to try and minimize power consumption.
Location Determination with Wi-Fi Hotspots
Determining location based on Wi-Fi hotspot proximity is widely used today due to the popularity and presence of Wi-Fi. From a home broadband to public Wi-Fi in railway stations, airports, or coffee shops – there are Wi-Fi hotspots all over. Each Wi Fi hotspot has a unique identifier – an address that is part of the internet’s network infrastructure. The Wi-Fi hotspots and their locations can be built into a database, and cached into a mobile device for use in location determination. There are a number of providers of the databases that match Wi -Fi hotspots to locations. A device just needs to keep its database updated to capture the changes when Wi-Fi hotspots are moved around.
Using Wi-Fi hotspot to determine location is more efficient in terms of power consumption than using GNSS technology. In most cases, people already have their Wi-Fi on as that is how most people access the internet today. This means the device does not use a large deal of extra battery power to find its location. Since the phone switches between the location determination technologies in the background, where you may be thinking that your device is giving you a location coming from GNSS technology, it is most likely the Wi-Fi infrastructure and technology identifying where you are.
Sometimes a device displays an incorrect location before correcting itself in the background and updating to the correct location. A common cause for this sudden shift is when your device initially picks up a stale Wi-Fi location, especially on startup. By checking against the nearest cell tower, it disregards the location from the stale Wi-Fi and updates to your correct location.
For multipath problems when using GNSS, using a model of a city’s morphology can help in error correction. The model gives insight on how the satellite signals bounce off objects in the area. The multipath errors caused by these bounces can then be canceled out and corrected for, increasing the location accuracy.
Fused Location Service
Cell towers, Wi-Fi, and GNSS constellations are the three main technologies used to determine a device’s location. A fused location service means that the choice of the technology being used at any particular point in time is largely invisible to the user, and in many cases from the application developer as well.
The device’s choice of which technology to use is a capability at the operating system level. All that is needed at the application level is making a single call to the operating system with a request for the location level that is required. Depending on whether the application needs a precise location or a relative location, the operating system will decide on the best technology for providing the location at the required accuracy. Hence, it is very likely that a device will only use GNSS to get location when precise location is needed, for instance, if an application is giving turn by turn directions. If proximal location is needed, Wi-Fi will most likely be used due to its power efficiency.
Visual positioning uses the phone’s camera to sense the environment around it. It makes a comparison between what the camera sees against a database containing images of that street. The device can then orient itself to the correct position and orientation during navigation. Google in particular has a rich resource of street view imagery of many cities and locations around the world. Google Maps users can use visual positioning to orient themselves correctly and know which side of the street they are on, and which direction they are facing. The visual positioning system supplements other location services to not only identify your location, but also the orientation (i.e. the compass direction to which you are facing or heading).
Do You Always Need A Precise Location All the Time?
Precise location is not always what we need for most of our day to day location needs. Now, if you are doing turn by turn navigation as a pedestrian, then you definitely need precise location. If your goal is to get a local weather forecast, or find your local Starbucks, then a proximity location that puts you on the right street would suffice.