SPC  Statistical Process Control 
SPD  Speed 
SPE  Seller Purchased Equipment 
Speed of Light  Represented by the symbol c and has a value of 2.9979250 x 10 metres/second or 983,571,194 feet/second. 
SPI  Special Position Identification 
SPIP  Designation for a transponder ident pulse. 
SPKR  Speaker 
SPM  (1) Surface Position Monitor (2) Stabilizer Position Modules 
Spoking  Spoking refers to a display presentation which radi-ates outward from the display origin like the spokes on a wagon wheel. 
SPR  Sync Phase Reversal (term used in Mode S transpon-ders) 
SPS  Standard Positioning Service 
SQ or Sql  Squelch 
Squall Line  A squall line is a line of thunderstorms and develop-ing thunderstorms. 
Squawk  Reply to interrogation signal (XPD). 
Squelch  A control and/or circuit which reduces the gain in response of a receiver. The squelch is used to elimi-nate the output noise of the receiver when a signal is not being received. 
Squitter  (1) The random pulse pairs generated by the ground station as a filler signal. (2) The transmission of a specified reply format at a minimum rate without the need to be interrogated (filler pulses transmitted between interrogations) [XPD]. (3) Spontaneous Transmission generated once per sec-ond by transponders. 
SR  SwissAir 
SRADD  Software Requirements And Design Description