Regarding nRF51822 RSSI to Distance Calculation

[Nazmus Sakib]

Hi All.

I made my PCB with nRF51822 and achieved around 150 Feet range (LoS). My target is to calculate a approximate distance (in Feet) from RSSI values.

I did a field test and measured some data and observed its quite non-linear to determine a distance value. My test data is attached herewith.

Any anyone please advise the best way to formula to calculate distance from RSSI under open field?

Thanks in advance.

Regards.

[Dan Danknick]

I've experimented with this quite a bit. If you want to believe RSSI-to-distance then more/good data is your friend.

More = increased advertising or connected measurements per second Good = throwing away outliers and smoothing what remains

To start, I suggest tossing the bottom and top 10% of readings. And then applying an IIR filter with a ratio like:

signal = (15*signal + new_signal) >> 4;

giving you a good confidence after say, 30 iterations.

RSSI is messy and noisy due to a myriad of physical phenomenae. Welcome to the world of converting noisy sensors into meaningful data!

Dan

[Nazmus Sakib]

Hi Dan.

Thanks for your reply.

I know its quite challenging task plus its value will be approximate.

Could you please explain more about your given formula?

Regards.

[Dan Danknick]

It's just a simple infinite impulse response filter using fast integer math.

The equivalent with floating point, which you more typically see, is:

strength = 0.0625f; signal = (1.0f - strength)signal + strengthnew_signal;

Dan