The time reference system consists of an electronic timer. a time correlation buffer, an event timer. and a clock.An interface exists between the time reference system and the digital command system, the digital computer. and the data transmission system.
The electronic timer records elapsed time in 1/8th-second increments from liftoff through impact; it counts time-to- retrograde from liftoff to zero in 1/8th-second increments; and it counts time to equipent reset on command in 1/8th second increments. The electronic timer exchanges signals with the digital command system, the digital computer, and the data transmission system.
The electronic timer has a crystal controlled time reference accurate to 35 parts in 1 million for 24-hour period. Stability over a 3-hour period is 10 parts in 1 million at 25 degrees C +/- 10 degrees C. The timer is mounted behind the center instrument panel.
Updated or revised time-to-go is forwarded to he electronic timer by the digital command system.
To prevent inadvertent or premature countdown to retrofire, the electronic timer is provided with a lockout set at 512 seconds. It will not accept any time to go quantity of less than 512 seconds.
The time correlation buffer accepts elapsed time and clock information from the time reference system pulse code modulation input. The time correlation buffer provide outputs to the voice tape recorder and the two biomedical tape recorders information to the recorders from the time correlation buffer is updated every 2.4 seconds. It provides serial data and clock data outputs to a buffer register every 2.4 seconds.
The event timer provides a cockpit decimal time display in minutes and seconds to a maximum 99 minutes and 59 seconds. This display permits countup and countdown timing by the astronauts. The display may be manually positioned or it may be started by face-mounted switching or independent electrical remote signal. The unit operates completely independent of the electronic timer.
The spacecraft clock displays Greenwich Mean Time (GMT) in hours and minutes. Launches are conducted at Cape Kennedy on GMT. The clock includes an additional minute hand and a second hand which may be stopped and reset to zero mechanically at any time. A calendar day display is also provided.
Copyright 1997, 1998, 1999 by John
Duncan |