Short for “Mission Elapsed Time”. As the name suggests, a clock on your spacecraft starts counting from the moment it’s turned on. For most missions, this happens when the vehicle is released from the rocket and the computer starts for the first. Other missions launch with their computers turned on and time starts at liftoff. The parameters that define it are stored in non-volatile memory so that computer restarts don’t reset the clock. The ground team also keeps their own record of it.

Mission Elapsed Time is used often by mission operations teams to anchor events and plan the next steps. For instance, the solar panels may be scheduled to trigger after a Mission Elapsed Time of eighteen minutes and three seconds so that, if ground controllers lose contact, the spacecraft will keep going.

Note that the computer on the spacecraft is usually tracking both Mission Elapsed Time and UTC/GPS time, at a minimum.

In cFS, it’s short for “Message Identification”. Core Flight Software works by passing messages around between systems. The Message Identification number is unique to a particular Message Type. It’s like ordering a “#5” meal at the Drive Thru. Instead of saying everything that comes in the meal, you just order #5 and everyone knows what you’re talking about. Message ID’s are listed in a table that you update when you define a new message type.

Short for “Memory Management Unit”. This is (usually) a hardware-based system that helps you manage memory resources. It can provide a relatively simple interface to you while working complex memory allocation behind the scenes.