calculate distance from beacon when user is moving in a vehicle

Question

How to calculate distance using BLE beacons if the user is moving in a vehicle with 2-15kmph speed?Also,if the distance won't give me accurate results is there any other mechanism with the help of which I can calculate the nearest beacon.The default implementation does not give me proper results as there is a 20sec lag in distance estimates. Secondly,in which cases should ARMA filter be used.

Cargill

Answer 1

Beacon distance estimates are only estimates, so you must set accuracy expectations accordingly. In order to reduce noise on radio signal levels, many samples are averaged over a period of time.

Figuring out the appropriate averaging time depends on the use case. As you have noted, the Android Beacon Library uses a 20 second running average by default. This gives a distance estimate based on where the vehicle was on average over the past 20 seconds.

For a vehicle going up to 15km/hour (~5 meters / sec) with a beacon transmission radius of 50 meters, it will be in range of the beacon for a maximum of 10 seconds. How many distance measurements do you need to get? If one is enough, average over 10 seconds. If you need more, set the period even shorter.

Understand that reducing the sample interval will add more noise error to the distance estimates, because you will have fewer statistical samples to work with. A few things you can do to make the most of the samples you do get:

1. Set the beacon to advertise as frequently as possible. Some beacons advertise at 1Hz or less to save battery. This leads to poor distance estimates. Increase it to 10Hz or more if possible.

2. Turn off bluetooth configurability/connectability on your beacon. Some Android devices will only allow one signal level sample per BLE scan (1.1 secs by default in the library) in connectable mode. This can lose you 90% of your samples at 10 Hz.

3. Configure the beacon transmitter to be on its highest power setting and calibrate it properly. Stronger signals have a higher signal to noise ratio, giving better distance estimates.

The ARMA filter uses a nondeterministic sample interval based on % change in each sample. It is more appropriate for use cases with slow moving beacon receivers.

Upkeep

I decreased the sampling interval depending on your answer and got better results.Now,I want to support the distance estimates for stationary case,walking case as well as moving at 10-20kms.I thought of deciding device speed based on some sensors and depending on the speed,setting the sampling interval.What do you think of this solution? – Cargill

Changing the sampling interval based on speed from other sensors is a good optimization, yes. – Upkeep