four-range, triaxial fluxgate magnetometer mounted on a 1.52-m boom, was used to measure the ambient magnetic field. The three axes were sampled sequentially three times each 5.72 s. Sensitivities and accuracies of the four ranges were plus or minus 150 and 1.2, 450 and 3.5, 1,500 and 11.7, and 25,000 and 195.3 nT, respectively. The sensitivity was switched by ground command. Frequency response was dc to 1 Hz (flat); down 3 dB at 10 Hz; then falling at 6 dB per octave at higher frequencies. Satellite stray fields were constrained to be less than 0.1 nT, which was also the rms instrument noise level. Inflight calibration was performed once every 98 min.