A method and apparatus for determining transmit and receive gain of a wireless transceiver as well as the path loss between multiple (e.g., two or more) transceivers. The technique described herein allows the transmitter gain, receiver gain, and path loss to be determined in real-time in a closed-loop control scheme with each transmitter in the wireless system adjusting its output power level to a predetermined power level. In one embodiment, the real-time gain and path loss calibration scheme can determine the gain and path loss of a wireless system during network establishment and at periodic intervals during regular operation.
This is from http://www.freepatentsonline.com/6272322.html
If you register at this site, you will be able to see more specific data about periodic calibration.
Obviously, periodic calibration is a feature of Atheros chipset (and patent owned by Atheros) and Mikrotik probably just use this feature. Same applies to madwifi
Atheros driver for Linux. Maybe we should see ast_per_cal function of madwifi driver to figure out what is going on, but as I said, it seems this is hidden inside Atheros hardware and firmware. We should just use it and don't worry