Apollo Inertial Measurement Unit, where Inertial Reference Integrating Gyros (IRIGs,Xg,Yg,Zg), sense altitude changes, and Pulse Integrating Pendulous Accelerometers (PIPAs, Xa,Ya,Za) sense velocity changes
NASA
2009-01-01
Apollo Guidance, Navigation, and Control (GNC) Hardware Overview, <a rel="nofollow" class="external free" href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090016290.pdf">https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090016290.pdf</a>