A multilateration method basically includes receiving a signal strength indicator for at least one client wireless device from a plurality of access point devices, and calculating a position of the at least one client wireless device based on the signal strength indicator for the at least one client wireless device, prestored positions of the access point devices and a radio transmission parameter. The radio transmission parameter is obtained as a value that minimizes an error between a calculated position of at least one reference wireless device and a prestored position of the at least one reference wireless device.