Magnetometer data recorded by the Cassini spacecraft during the spacecraft's Earth swing-by maneuver in August 1999 are discussed. The primary purpose of this paper is to show the instrument's performance in detection of magnetic phenomena in a planetary environment. Companion papers put the data in a broader geophysical context. The low noise and high time resolution of the instrument coupled with the unusual orbit did produce new information about old phenomena (e.g., a whistler-like signal inside the near-subsolar magnetopause, lower hybrid noise in the current sheets around a magnetosheath field null, and sharp changes in the ion cyclotron wave spectrum between the boundary layer and magnetosphere proper). The detection of irregular isolated 1--2 s duration small-amplitude field strength dips in the vicinity of the nightside plasma trough appear to constitute a new discovery, tentatively identified as being evidence of plasmasphere interchange instability during a lull in background convection. The absolute scalar element of the instrument worked to plan, operating within 4 RE of Earth in parallel with the fluxgate sensors. In all respects, instrument performance bodes well for the Saturn orbital phase of the mission. ¿ 2001 American Geophysical Union