AGC

Apollo guidance computer, main digital computer guiding theCSMs

AGS

Abort guidance computer, backup digital computer for lunar landings

AOS

Acquisition of signal, the moment the LM reappears from behind the moon during an orbit, allowing earth-based tracking to receive radio signals

apogee

High point of an orbit

apolune

High point of a lunar orbit

attitude

Orientation of a spacecraft or aircraft

CAPCOM

Capsule communicator; astronaut on the ground who speaks with crew during a mission

CSM

Command and Service Module

delta v budget

Means of keeping track of velocity changes (delta v), roughly equivalent to fuel, during spacecraft maneuvers

DELTAH

Quantity describing the difference between the on-board inertial estimate of height above the moon and that measured by the radar altimeter

DPS

Descent propulsion system, rocket engine on the bottom of the Lunar Module

DSKY

Display/Keyboard interface to Apollo computer, pronounced “dis-key.”

gimbal lock

Condition where 3-gyro inertial system in CSM and LM loses track of attitude due to unusual orientation of spacecraft

LGC

Lunar Module Guidance Computer, identical to AGC but installed in LM

LLRV

Lunar landing research vehicle – “flying simulator” developed to research lunar landing trajectories and control techniques

LLTV

Training version of LLRV

LM

Lunar module, pronounced “lem”

MIT IL

Massachusetts Institute of Technology Instrumentation Laboratory, today known as the Charles Stark Draper Laboratory Inc. (spun out of MIT in the early 1970s)

MPAD

Mission Planning and Analysis Division – group at NASA Houston that developed landing trajectories

PDI

Powered Descent Initiation – moment at 10 miles above the moon when descent engine (DPS) began to fire to begin descent to lunar surface

perigee

Low point of an orbit

perilune

Low point of lunar orbit

PNGS

Primary Navigation and Guidance System (“pings”) – the digital computer plus the inertial and optical systems

PRO

Proceed button on DSKY display; astronauts had to press this button before the computer would initiate critical events

RCS

Reaction control system – collection of small thrusters that controlled the attitude of the CSM or the LM

SETP

Society of Experimental Test Pilots – professional organization of test pilots

IMU

Inertial measurement unit – collection of gyroscopes and accelerometers that measured velocity changes in CSM and LM

DAP

Digital autopilot – computer programs that controlled steering of Apollo spacecraft

State vector

List of numbers that described position and velocity of spacecraft at a given time