We report on the design of an absolute field magnetometer being develo
ped to measure the ultra-low magnetic fields (<10-7 Gauss) produced fo
r the Gravity Probe B Relativity Mission. The hysteretic behavior of f
luxon motion in thin-film Niobium is used to devise a superconducting
thin-film fluxgate magnetometer. Comparisons with conventional fluxgat
e and SQUID systems will be discussed. (Research supported by NASA, co
ntract number NAS8-39225)