Development of a Superconducting ELF Receiving Antenna

The development of an ELF submarine receiving antenna that uses a triaxial array of superconducting quantum interference devices SQUIDs has addressed the achievement of a SQUID sensor with a sensitivity of 0.01 picotesla per hertz to the 12 power, achievement of a SQUID linear dynamic range of 140 dB, achievement of sensor orthogonality of 0.1 milliradian, stabilization of receiver platform motion to 1 milliradian, processing of SQUID outputs to remove residual motion noise, and provision of a suitable cryogenic environment. The required sensitivity, linear dynamic range, and sensor orthogonality were obtained in a prototype point-contact-type triaxial SQUID magnetometer used to detect the ELF signal from the Navy test transmitter in Wisconsin. The required platform stabilization was demonstrated in a towing basin by motion spectrum measurements on a hydrodynamically stabilized buoy designed to be towed by a submarine. Motion excursions within the ELF passband of 30 to 130 Hz were about a microradian hence motion-generated noise must be further reduced by about 80 dB. This noise in the ELF signal can be reduced by adaptively determining a vector approximately equal to the earths magnetic field vector and combining it vectorially with the SQUID outputs. This adaptive processing has been studied using a computer simulation of the SQUIDs and motion noise. A prototype dewar capable of fitting a towed communication buoy maintained a volume large enough for a triaxial SQUID sensor package at the temperature of boiling liquid helium for 102 days.