Why Satellite tracking software predicts where a satellite will be by propagating its orbit mathematically, and that requires the orbital parameters known as Keplerian elements (often distributed as two-line element sets, or TLEs). These describe the orbit's size, shape, tilt, orientation and where the satellite sits along it at a given epoch time. Combined with your grid square or latitude/longitude and the current time, the program computes azimuth, elevation, pass times and Doppler shift.