The Iridium¿ System satellite constellation consists of 66 satellites in circular, polar, 780 km altitude orbits in six equally spaced planes. Each satellite carries an engineering magnetometer which has sufficient resolution to sense the Birkeland currents. This paper presents a spherical harmonic fitting (SHF) technique for estimating field aligned currents (FACs) using the cross track component of the magnetic field measurements from the Iridium satellites. The SHF magnetic field perturbations along with Ampere's law are used to derive the global FACs from 40¿ MLAT to the pole in either hemisphere. Data for 10--11 UT, 23 August, 1999 were obtained from Iridium, Defense Meteorology Satellite Program (DMSP) F13 and the Ultraviolet Imager (UVI) onboard the POLAR satellite. The SHF data were evaluated along the DMSP F13 track. The range of east-west component, SHF magnetic perturbations from Iridium was (-530,465) nT compared with (-630,634) nT from DMSP F13. The derived upward FACs ranged from -0.8 to 0.9 &mgr;Am-2. Upward FACs were co-located with bright dayside UVI emissions. ¿ 2001 American Geophysical Union