When updating a running program in a system that uses a one-to-one backup program, fault-tolerance is lost while the backup program is itself being updated. The method includes receiving a graphical code image. The sensor head includes a transmitting and receiving means for emitting electromagnetic impulses to the ground within a range of detection of land mines. The bonding of the frame to the greensheet may be by lamination or by an adhesive, or by other means. The control element places a PWM current on the electromagnet, such that the length of the duty cycle controls the strength of the first magnetic field. A variable valve unit having the simple and same structure can be used in both banks.