Editing Dead man's switch (section)

===Spacecraft===
Many spacecraft use a form of dead man's switch to guard against command system failures. A timer is established that is normally reset by the receipt of any valid command (including one whose sole function is to reset the timer). If the timer expires, the spacecraft enters a "command loss" algorithm that cycles through a predefined sequence of hardware or software modes (such as the selection of a backup command receiver) until a valid command is received. The spacecraft may also enter a [[Safe mode (spacecraft)|safe mode]] to protect itself while waiting for further commands.

While having some similarities to a dead man's switch, this type of device (a command-loss timer) is not actually a dead man's switch, because it aims to recover from a hardware failure rather than the absence of human operators. It is generally called a [[watchdog timer]], and is also used extensively in nuclear power control systems. System components on a spacecraft that put it into a safe mode or cause it to execute default behaviors when no command is received within a predefined time window can be considered a dead man's switch, but hardware or software that attempts to receive a command from human operators through an alternate channel is an auto-recovering or adaptive communications system, not a dead man's switch. [[Voyager 2]] recovered from a command receiver failure with a command-loss timer.<ref>{{cite web|url=http://tmo.jpl.nasa.gov/progress_report2/42-49/49E.PDF|title=Voyager Support. DSN Progress Report 42-49|publisher=[[Jet Propulsion Laboratory]]|date=1978|access-date=4 March 2009|first1=J.|last1=Allen|first2=H.|last2=Nance}}</ref>