In this blog post, we will introduce the principles and various techniques used by smartphones to accurately measure location indoors and outdoors using GPS and beacons.
Smartphones use various location measurement technologies to measure their location in various terrain environments. There are two types of location: absolute location and relative location. Absolute location is a location indicated by latitude and longitude, while relative location is a location relative to a specific location. This location information plays an important role in various applications, such as route finding, bicycle route tracking, and location-based notification services.
Outdoors, GPS (Global Positioning System) and IMU (Inertial Measurement Unit) built into smartphones are mainly used. GPS measures absolute position using signals from satellites. GPS does not accumulate position errors over time. However, due to radio wave delays, large errors occur for a short time at the beginning of the connection, and it is difficult to receive GPS signals indoors or in tunnels. IMU calculates position changes by measuring acceleration and speed with built-in sensors and finds relative positions based on the initial position. Although it has excellent measurement performance for short-term movements, position errors increase over time due to the accumulation of errors in the values measured by the sensors. Using these two methods together can compensate for each other’s shortcomings and reduce errors.
Meanwhile, a technology that utilises Bluetooth-based beacons is available for indoor positioning. Beacons are devices that are fixed indoors and periodically send signals containing identification numbers and location information assigned to each beacon. Beacons send signals of the same strength in all directions, but the further away from the beacon and the more obstacles such as walls there are, the weaker the signal becomes. When a device enters the range of the beacon signal, the receiver in the device recognises the signal. The following are some methods for measuring the position on a two-dimensional plane using this signal.
The proximity method determines the position of the terminal as the position of the beacon when the terminal receives a beacon signal. When multiple beacon signals are received, the position of the beacon with the strongest signal is determined as the position of the terminal. This method is simple and fast, but may lack accuracy.
The trilateration technique measures the signal strength received from three or more beacons and converts it into the distance between the terminal and the beacons. A circle with the distance as the radius is drawn centred on each beacon, and the intersection of the circles is determined as the current location of the terminal. If the intersection points do not converge at a single point, the centre point of the area common to the three circles is measured as the location of the terminal. This method is relatively accurate, but the calculations are complex and many beacons are required.
The location map method divides the measurement space into small areas, sets reference points for each area, and installs beacons around them. Then, the beacons transmit signals and the strength of the signals reaching each reference point is measured. The signal strength, beacon identification number, and reference point coordinates are recorded as a location map in a database on the server. This process is performed at all reference points. When a device reaches a specific location and receives a beacon signal, it measures the signal strength and transmits it to the server along with the beacon’s identification number. The server finds the reference point with the closest signal strength in the database and informs the device of the location of this reference point. This method enables highly accurate location measurement, but the initial setup and database maintenance are complex.
As such, smartphone location measurement technology provides a variety of methods that can be used appropriately in various indoor and outdoor environments and situations. Efficient use of these technologies can greatly improve the quality of location-based services.
