During the past years a prototype of a LTS SQUID gradiometer system for mobile applications measuring the full tensor of the Earth’s magnetic field gradient with extremely high sensitivity was built at IPHT Jena.

The system contains six large low temperature SQUID gradiometers with a base length of 3.5 cm and an intrinsic noise of better than 50 fT/(Hz^1/2) and three orthogonal reference magnetometers with a field sensitivity of about 1 pT/Hz^1/2. The magnetometer triple is used to enhance the intrinsic balance of 10-4 of the gradiometers further by up to three orders of magnitude. Both type of sensors are robust against magnetic field of up to 65 µT.

A highly sophisticated data acquisition unit was built which is close to the sensor system and does not introduce magnetic disturbances. Twelve auxiliary sensor signals (temperatures, currents, and pressure inside the cryostat) are digitized with a 12 bit ADC, additionally to the extremely low-noise differential inputs for 21 analogue channels which are digitized with 24 bit ADC. A new completely nonmagnetic active pressure regulator was implemented in the data acquisition unit, which produces a constant pressure within 0.5 mbar inside of the cryostat even under strong barometric (altitude) changes of the outside pressure. As well a new low power inertial system to calculate the Euler angles in space was implemented.

The whole system is mounted in a bird for towed operation Together with the new tow cable construction the whole system is now much more rugged for field operation.

During the past three years successful tests of the system were conducted by towing the system from a helicopter as well as for a stinger mounted configuration in a Cessna 208 aircraft are reported.