A spacecraft timer that protects against faults in uplink commands (those transmitted from ground to space). It is part of the command and data-handling subsystem. Each time its spacecraft receives a command from the ground, the CLT is set to a selected period of time, such as a week. If it ever runs down to zero, this indicates a failure in the spacecraft's receiver or other command components. The CLT then issues commands for actions, such as switching to a back-up system, to restore the broken link.