Magsat data provide an unequalled opportunity to study the magnetic field produced by currents external to the earth. Analyses of Magsat's measurements are facilitated by an appropriately chosen magnetic coordinate system but existing coordinate systems are inappropriate. An eccentric dipole system based on Magsat measurements is presented which is useful both for positional information and data display. These two aspects must be carefully matched or unacceptable biases will be introduced. The proposed system facilitates intercomparison of Magsat measurements and numerical modelling results. Additionally the system permits a partial separation of magnetic field aligned currents, further facilitating external current analyses.