2.1 NM 3.3 NM 25 seconds 40 seconds 20 NM RA issued  for 300 KT TA issed  Closure Surveillance Range Targets displayed on-screen (TCAS I & II) Traffic advisory (TA)  region (TCAS I & II) TA Region RA Region (TCAS II only) Resolution advisory (RA) region (TCAS II only) Pilot commanded to  take evasive action Pilot alerted to traffic in range Range criterion Intruder Intruder Altitude criterion 1200' 850' 1200' 850' 11-59 Figure 11-131.  Traffic collision and avoidance system (TCAS) uses an aircraft’s transponder to interrogate and receive replies from  other aircraft in close proximity. The TCAS computer alerts the pilot as to the presence of an intruder aircraft and displays the aircraft on  a screen in the cockpit. Additionally, TCAS II equipped aircraft receive evasive maneuver commands from the computer that calculates  trajectories of the aircraft to predict potential collisions or near misses before they become unavoidable.  There are two classes of TCAS. TCAS I was developed to  accommodate the general aviation community and regional  airlines. This system identifies traffic in a 35–40 mile range  of the aircraft and issues Traffic Advisories (TA) to assist  pilots in visual acquisition of intruder aircraft. TCAS I is  mandated on aircraft with 10 to 30 seats.  TCAS II is a more sophisticated system. It is required  internationally in aircraft with more than 30 seats or weighing  more than 15,000 kg. TCAS II provides the information  of TCAS I, but also analyzes the projected flightpath of  approaching aircraft. If a collision or near miss is imminent, the  TCAS II computer issues a Resolution Advisory (RA). This is  an aural command to the pilot to take a specific evasive action  (i.e., DESCEND). The computer is programmed such that the  pilot in the encroaching aircraft receives an RA for evasive  action in the opposite direction (if it is TCAS II equipped).  [Figure 11-131]  The transponder of an aircraft with TCAS is able to  interrogate the transponders of other aircraft nearby using  SSR technology (Mode C and Mode S). This is done with  a 1030 MHz signal. Interrogated aircraft transponders  reply with an encoded 1090 MHz signal that allows the  TCAS computer to display the position and altitude of each  aircraft. Should the aircraft come within the horizontal or 