A system comprises an outer shell having an inner spherical cavity and an inner sphere located in the spherical cavity. The inner sphere comprises a sensor; at least one transmit antenna; and at least one transmitter coupled to the sensor and to a respective one of the at least one transmit antenna. The system also comprises a first receive antenna located in the spherical cavity; a second receive antenna located in the spherical cavity; and a receiver located outside of the outer shell. The receiver is configured to determine the signal to noise ratio of a first signal received at the first receive antenna and the signal to noise ratio of a second signal received at the second receive antenna; and to combine the first and second signals based on the respective signal to noise ratios such that interference due to multi-path signals in the spherical cavity is reduced.