Older military aircraft often had INS/IRSs that could be aligned inflight. An example is the USAF C-130Hs with self-contained navigation system (SCNS). Inflight INS alignment requires valid GPS or Doppler velocity sensor data. The alignment process requires that the airplane fly straight and level for a minimum of 1 minute after commanding the alignment, although 5 minutes is recommended. After 5 minutes in alignment, S turns are recommended. The KC-135 fleet was equipped with Carousel IV INSes that also be aligned inflight using similar procedures.