Station Radial Waypoint Distance VOR flightpath RNAV flightpath Airport A VORTAC XYZ ZYX 108/15 ABC 348/19 ABC 015/30 XYZ 105/25 XYZ 167/16 VOR/DME ABC VOR/DME ZYX Airport B Phantom waypoints created by RNAV CLC computer Figure 11-123.  The pilot uses the aircraft’s course deviation indicator to fly to and from RNAV phantom waypoints created by computer.  This allows direct routes to be created and flown rather than flying from VOR to VOR.  in all directions. When the radio waves encounter an aircraft,  part of those waves reflect back to a ground antenna.  Calculations are made in a receiver to determine the direction  and distance of the aircraft from the transmitter. A blip or  target representing the aircraft is displayed on a radar screen  also known as a plan position indicator (PPI). The azimuth  direction and scaled distance from the tower are presented  giving controllers a two dimensional fix on the aircraft.  [Figure 11-126] A secondary surveillance radar (SSR) is used by ATC to  verify the aircraft’s position and to add the third dimension of  altitude to its location. SSD radar transmits coded pulse trains  that are received by the transponder on board the aircraft.  Mode 3/A pulses, as they are known, aid in confirming  the location of the aircraft. When verbal communication is  established with ATC, a pilot is instructed to select one of  4,096 discrete codes on the transponder. These are digital  octal codes. The ground station transmits a pulse of energy  at 1030 MHz and the transponder transmits a reply with  the assigned code attached at 1090 MHz. This confirms  the aircraft’s location typically by altering its target symbol  on the radar screen. As the screen may be filled with many  confirmed aircraft, ATC can also ask the pilot to ident. By  pressing the IDENT button on the transponder, it transmits  in such a way that the aircraft’s target symbol is highlighted  on the PPI to be distinguishable.  To gain altitude clarification, the transponder control must  be placed in the ALT or Mode C position. The signal  transmitted back to ATC in response to pulse interrogation  is then modified with a code that places the pressure altitude  of the aircraft next to the target symbol on the radar screen.  The transponder gets the pressure altitude of the aircraft  from an altitude encoder that is electrically connected to  the transponder. Typical aircraft transponder antennas are  illustrated in  Figure 11-127. VOR R.NAV HOLD USE DATA DSP PAR ENR NM KT MIN FRQ RAD DST APR VOR RNV HLD ILS USE DSP MODE DME WAYPOINT FREQ-RAD-DST NAV SYSTEM PULL ID OFF Figure 11-124.  RNAV unit from a general aviation aircraft.  11-55