nRF51822 beacon kit: big notches in RSSI vs distance plot

[classicman]

Hello everybody,

I'm designing an app to track the position of the smartphone using six beacons and, in order to do that, I made some measures both indoors and outdoors and plotted a graph of RSSI vs distance using the data acquired from nRF Connect app (exported to .csv and fed to Matlab). The first set of measures was taken in a corridor about 3m wide and 35m long at different transmission power level, the second set was taken outdoors in an open field about 30x25 m at 0 dBm transmission power. Both the plots show serious notches, as you can see from the pictures, causing great troubles with the accuracy of the tracking.

I'd like to know if this behaviour is to be expected or there is something wrong in the setup of the measures.

Thanks.

[Roger Clark]

Very interesting data.

I suspect the dips and peaks in the RSSI may be caused by reflections.

When measuring outdoors, what height did you have the beacon?

Did you try varying the height of both the beacon and the receiver ( phone )

In general RSSI is not a reliable indication of distance, as there are loads of different factors that will effect RSSI, not just distance. Including orientation of the antenna in both the Tx and Rx devices, as well as reflections off various types of surfaces at various amplitudes, and absorption of walls and any other material between the Tx and Rx.