Offering real-time location of devices, whether indoors or out, is in high demand. Having moved on from simply using a SatNav device in the car, users now expect precise location to be easy and seamless on all their devices. Whilst it’s usually possible to locate devices outdoors when a GPS chip is fitted, what if no GPS chip is installed, or what about locating items indoors, or other cases where GPS is impractical?
A number of techniques based around the method of triangulation help make accurate positioning possible with wireless devices. Triangulation is a method that uses direction from two or more points as a cross reference to pinpoint a location. Modern wireless positioning methods judge distance as well as direction and there are several tried and tested methods: signal strength, time-of-flight, beacons, angle-of-arrival and dead-reckoning techniques.
Technique 1: Signal Strength
Using signal strength is a straightforward way of providing location data; the closer the device, the stronger the signal. For wireless designers it is easy to utilise techniques that use existing system resources and can give a useful indication of location very quickly.
However, the chief disadvantage of this technique is accuracy. If people or objects get in the way the signal strength will drop and interfere with the location finding. Signals can also reflect and cancel each other out, sometimes causing confusion as the signal strength may even appear to drop the closer you get to the source. The use of omnidirectional antennas (such as those found in smartphones) can compound the problem by causing multipath fading with unhelpfully variable results.
Bluetooth signal strength measurement between smartphones formed the basis for the UK national COVID-19 contact tracing mobile application to determine if two people have been within 2m of each other for a period of time. The unreliability of the measurement makes this a very approximate measure.
It is possible to characterise the signal strength environment in, for example, a hospital room and use changes in signal strength to detect movement or a re-arrangement of equipment. To use this technique as a full asset-tracking and location solution is as complicated as it sounds
Technique 2: Time-of-flight
The time-of-flight technique involves measuring the round-trip time of a probing signal-request. It is possible then to measure the distance away and pinpoint the location within a defined radius. In some ways time-of-flight parallels the theory behind radar or sonar location. By recording the time a signal was sent and the time it was received back it is possible to calculate, using some simple maths and knowing the speed of light, the distance it has travelled.
This is a fairly tricky technique on a single path signal and on a multipath signal it is even more difficult, providing a large range error unless very large bandwidths are used. There are a few mobile devices that can do this successfully, but the complexity involved can easily create conflicting results – don’t always assume that headline accuracy is always achievable. The international standard IEEE 802.15.4a UWB (Ultra-Wide Band) is now in wide use, and some smartphones (e.g. iPhones) now have UWB that can be used to get an accurate distance measurement to tags for finding lost items and more accurate location awareness.
Technique 3: Time-difference-of-arrival
Only in terminology. Computing has been done at the edge for a very long time, really for as long as computing has become pervasive within industry to automate a task. How long? IBM released the 5531 Industrial Computer in 1984, arguably a good milestone to measure, which saw computers doing processing within industry exactly where the data was being processed locally. The internet, and the ‘cloud’ came a little later, giving the relative position of the edge a meaning.
Industrial and embedded computing manufacturers have therefore been producing computers for the edge, before it was even a ‘thing’!
Technique 4: Beacons, signal strength, and dead-reckoning
Using a combination of methods is often a more reliable way to calculate location. The combination of a beacon’s signal strength and dead-reckoning (using a previously calculated location and advancing it based upon speed, elapsed time and course) can give a surprisingly accurate fix compared to using these methods in isolation. Additionally, using the mapping of known WiFi points, MEMS accelerometers and compass applications can add further details for pinpointing location.
For indoor navigation around a large facility or venue for instance, companies like Google have shown how mapping wireless access points can be a good solution. When the location of a wireless access point is known within a space, receiving a high signal strength correspondingly indicates close proximity. Also, if the user is moving past the access point (the signal gets stronger, then weaker again) it can also offer a useable waypoint. By then using accelerometers inside a device, a dead reckoning procedure can determine if a turn has been made and calculate a new position.
Technique 5: Angle-of-arrival
Another approach to localisation is to estimate where a signal is coming from – its angle-of-arrival, rather than how loud it is (signal strength) or how far away it is (time-of-flight measurement). At least three angle-of-arrival receivers are needed to get a location fix in two dimensions. To obtain an angle-of-arrival measurement, at least two receiver antennas and suitable circuitry is needed. Notably, Bluetooth 5.1 supports angle-of-arrival measurement using special base station receivers.
There continues to be a large amount of commercial interest in the development of wireless location techniques, and combinations of signal strength, time-of-flight, beacons, angle-of-arrival and dead-reckoning techniques offer cost-effective usable solutions with results that are continuously improving. In many cases they are a viable alternative to ubiquitous, but more battery intensive, GNSS/GPS solutions.