how is RSSI measured?

[bluebeam]

I have nRF52 on both central and peripheral. I'm getting the RSSI value on the central by looking at the RSSI field in the ADV_REPORT event. I see the RSSI values has a wide range, even when the peripheral is just put still in front of the central. Looks like it generally have 6dbm fluctuation. Is that expected? Is that because it's measured on different adv channel? How reliable is it to be used as an indication of distance? Thanks!

rssi-sample2.PNG

[endnode]

As Matt wrote RSSI is notoriously known as unreliable source of precise location, it only works in combination with extensive knowledge of the actual topology (in 2.4GHz spectrum of course;) and combination of multiple sources/receivers typically with heavy backend processing. But you can see this in basically any Q&A on this topic on this forum (as well as all over the internet).