Development of a superconducting ELF receiving antenna

The development of an extremely low frequency (ELF) submarine receiving antenna that uses a triaxial array of superconducting quantum interference devices (SQUIDs) has addressed the following six problem areas: 1) achievement of a SQUID sensor with a sensitivity of
10^{-14}
T-Hz
^-1/2
; 2) acheivement of a SQUID linear dynamic range of 140 dB; 3) achievement of sensor orthogonality of
10^{-4}
rad;4) stabilization of receiver platform motion to
10^{3}
rad; 5) processing of SQUID outputs in order to remove residual motion noise;6) provision of a suitable cryogenic environment. The required sensitivity, approximate linear dynamic range, and sensor orthogonality were successfully obtained in a prototype point-contract type triaxial SQUID magnetometer. This magnetometer was used to detect the ELF signal broadcast from the U.S. Navy transmitter in Wisconsin, both in the atmosphere above the water surface and at a depth of 100 m below the ocean surface in a stationary configuration. No measurements have been made as yet with the magnetometer in a towed buoy. Achievement of the platform motion requirement was demonstrated by motion spectrum measurements on a hydrodynamically stabilized buoy designed to be towed by a submarine. Motion excursions within the ELF pass band of 30- 130 Hz were found to be about
10^{-4}
rad. This amount of motion requires that motion-generated noise be further reduced by more than 80 dB. The required noise rejection can be achieved by adaptive determination of a vector that is approximately equal to the earth's magnetic field vector. When the adaptive vector is evaluated and combined vectorially with the SQUID outputs, a quantity proportional to the ELF signal vector results in which motion noise is suppressed. This adaptive processing has been studied using a computer simulation of the SQUIDs' motion noise derived from towed-buoy motion data. The feasibility of cooling the SQUID sensors with liquid helium was demonstrated by a prototype long-hold-time dewar of a size capable of fitting a towed communications buoy. The dewar successfully maintained the temperature of a volume sufficiently large to hold a triaxial SQUID sensor package at the temperature of boiling liquid helium for a duration of 102 days.