A wifi or Bluetooth signal gets weaker the farther you carry it, in a known way — so the strength your phone measures is a crude tape measure. That’s how a store knows roughly where you are from its beacons, and how ‘find my’ tags estimate range. It’s noisy, but the trend is real. Slide away from the beacon and watch the bars drop.
Signal strength falls with distance by the log-distance path-loss law: RSSI = P₀ − 10·n·log₁₀(d), where P₀ is the strength at 1 m and n is the environment’s path-loss exponent (~2 in free space, 3–4 indoors). Invert it and distance d = 10^((P₀ − RSSI)/(10n)). It underlies indoor positioning from wifi/BLE beacons and proximity from AirTag-style tags — cheap but noisy (fading and obstacles scatter n). A fail-loud self-check throws unless a weaker RSSI maps to a larger distance. ◆ real RF positioning, node-verified.
The log-distance model with a single fixed n (the exact inversion); real RSSI ranging is noisy — multipath and body-blocking swing it metres, which is why it’s used with many beacons and filtering, not alone. The weaker-is-farther monotonicity is exact.