A high-Tc second-order gradiometer for use in an unshielded environment

We have developed a highly balanced five-channel planar high-T_c second-order SQUID gradiometer. The gradiometer incorporates two long-baseline, first-order single-layer SQUID gradiometers and three orthogonal flux-gate magnetometers used as reference sensors. The outputs of the five channels are combined synthetically on a computer to form a fully balanced second-order gradiometer. We have investigated the use of both time-domain and frequency-domain balancing. We report on the performance of the system when used to measure weak magnetic signals in an unshielded environment.