The ham particle tracing code is used to compute trajectories of charged particles in Earth's magnetosphere. It uses the T96 Tsyganenko model of the magnetic field and the Weimer model for the electrostatic potential in the ionosphere. The electric field is computed under the assumption that magnetic field lines are also equipotential lines with the electrostatic potential given by the Weimer model at the point where the magnetic field line hits the ionosphere. The electric and magnetic fields are defined on a grid, and when the test particle trajectories are computed, the fields at the particle positions are found by interpolation. The test particle trajectories are computed using Boris' method.