A B C Range marks Rotating sweep Echoes or returns from aircraft Figure 11-126.  A plan position indicator (PPI) for ATC primary  radar locates target aircraft on a scaled field.  Figure 11-125.  A traditional transponder control head (A), a lightweight digital transponder (B), and a remote altitude encoder (C) that  connects to a transponder to provide ATC with an aircraft’s altitude displayed on a PPI radar screen next to the target that represents  the aircraft.  The ATC/aircraft transponder system described is known as  Air Traffic Control Radar Beacon System (ATCRBS). To  increase safety, Mode S altitude response has been developed.  With Mode S, each aircraft is pre-assigned a unique identity  code that displays along with its pressure altitude on ATC  radar when the transponder responds to SSR interrogation.  Since no other aircraft respond with this code, the chance  of two pilots selecting the same response code on the  transponder is eliminated. A modern flight data processor  computer (FDP) assigns the beacon code and searches flight  plan data for useful information to be displayed on screen next  to the target in a data block for each aircraft.  [Figure 11-128] Mode S is sometimes referred to as mode select. It is a  data packet protocol that is also used in onboard collision  avoidance systems. When used by ATC, Mode S interrogates  one aircraft at a time. Transponder workload is reduced by  not having to respond to all interrogations in an airspace.  Additionally, location information is more accurate with  Mode S. A single reply in which the phase of the transponder  reply is used to calculate position, called monopulse, is  sufficient to locate the aircraft. Mode S also contains capacity for a wider variety of information exchange that is untapped  potential for the future. At the same time, compatibility with  older radar and transponder technology has been maintained.  Title 14 of the Code of Federal Regulations (CFR) part 91,  section 91.413 states that all transponders on aircraft flown  into controlled airspace are required to be inspected and  tested in accordance with 14 CFR part 43, Appendix F, every  24 calendar months. Installation or maintenance that may  introduce a transponder error is also cause for inspection and  Transponder Tests and Inspections 11-56