• Lift-off: This is the time in Coordinated Universal Time (UTC) for the expected lift-off time of the launch vehicle. UTC is similar to Greenwich Mean Time but without adjustments for daylight savings time.
• Built-in Hold: This is the amount of scheduled hold time. Most launch providers add scheduled hold time into their countdown procedures. This scheduled hold time is used as contingency time to address issues encountered during the launch countdown procedure. This hold time (contingency time) gives the launch team time to address issues without having to change the scheduled lift-off time. Typically there will be 2 or 3 built in holds thought out the launch countdown. If there are no issues to be addressed the launch team simple waits for the built in hold to complete and then resumes launch countdown activities after the completion of the built in hold.
• L-TIME: This is the amount of time between the current time and Lift-off time. For example if L-Time is 02:00:00 then the lift-off time is 2 hours away.
• T-Time: This is the amount of time between the current time and Lift-off time minus any built in hold time. Therefore T-Time is equal to L-Time minus any remaining built in hold time. For example if L-Time is 02:00:00 (two hours from launch) and there is 15 minutes of built-in hold time then T-Time will be 01:45:00. During the countdown the T-Time will jump at the beginning of a built in hold. For example: if a built in hold starts at 30 minutes before launch and the built in hold time is 15 minutes T-Time will jump from 00:30:00 to 00:15:00 at the start of the built in hold. T-Time will remain at 00:15:00 during the built in hold and will not start counting again until the end of the built in hold.
• Window Remaining: The Window Remaining time is the amount of time left in the launch window. The launch window is the time period that the launch vehicle must launch to meet the mission’s requirements. The launch window typically begins at the scheduled lift-off time and continues until the window close time. Launch windows can be as short as 1 second to two or more hours. The larger the window the better since this gives the launch vehicle the most opportunity to launch. Typical reasons for not launching at the scheduled lift-off time are bad weather, technical issues with the launch vehicle or spacecraft, and technical issues with down range support equipment. At the start of the countdown procedure the window remaining will be the time between the scheduled lift-off time and the window close time. For example the lift-off time is 04:00:00 with an 1 hour launch window therefore the window remaining is 1 hour and the window close time is 05:00:00. If the lift-off time slides to 04:05:00 the window remaining will be 55 minutes and the Window close time remains unchanged at 05:00:00.