PCM Pulse Code Modulation
PCU  (1) Passenger Control Unit (2) Power Control Unit 
PDC  Pre-Departure Clearance 
PDCU  Panel Data Concentrator Unit 
PDDI  Product Definition Data Interface. Standardizes digital descriptions of part configurations and properties needed for manufacturing. 
PDF  Primary Display Function 
P-DME  Precision Distance Measuring Equipment 
PDN  Public Data Network 
PDOP  Position Dilution Of Precision. A GPS term for error introduced into the GPS calculations. 
PDOS  Powered Door Opening System 
PDR  Preliminary Design Review 
PDS  Primary Display System 
PDU  (1) Protocol Data Unit. The N-PDU is a combination of the N-PCI and the N-UD or N-SDU. The N-PDU is the total information that is transferred between peer network members (OSI Model) as a unit. (2) Power Drive Unit 
Performance Index  A relative number used to compare the performance of different radar systems. It is calculated from trans mitter peak power, antenna gain,pulse width,prf, antenna beam width and the receiver noise figure. 
PED  Portable Electronic Devices 
PET  Pacific Engineering Trials 
PF  Power Factor 
PFC  Primary Flight Computer 
PFCS  Primary Flight Control System 
PFD  (1) Primary Flight Display. An EFIS presentation sub-stituting for the ADI. (2) Primary Flight Director. 

PFE  Path Following Error
PFR  Pulse Repetition Frequency. The rate at which pulses are transmitted.
PGA  Pin Grid Array
PHY  Physical Interface Device
Phase  A signal in which the phase varies (with respect to
Modulation  the original signal) with the amplitude of the modula tory signal,while the amplitude of the carrier wave remains constant. Similar to a modified frequency modulated signal.