A computer code has been developed for calculating ground magnetic perturbations due to a substorm current system in the magnetosphere. The relevant currents are determined using results of a numerical simulation of the substorm itself devised by R. A. Wolf and collaborators. The magnetic fields due to the currents are calculated numerically using Ampere's law. The substorm simulation and its relation to the current calculation are briefly described. The assumed paths and means of calculating the currents are discussed. The results of the first run are presented and compared with observational data from the AFGL magnetometer array.