A method and apparatus involve a plurality of first devices, and a second device movable relative thereto. According to one aspect, the first and second devices respectively transmit first information and second information containing unique identification codes, the first information also containing position information. The devices collectively include circuitry that estimates distances therebetween, one of the first and second information including the estimate. According to a different aspect, one of the second device and each first device transmits its identification code, and the devices collectively include circuitry that determines two successive sets of estimates representing respective distances between the second device and each first device. One of the second device and each first device transmits information that includes both estimates for that first device, and identification codes for the second device and that first device.