Building on this and other work, the FAA launched the TCAS program in 1981. TCAS is a relatively simple system to understand. Basically, the system identifies the location and tracks the progress of aircraft equipped with beacon transponders. Currently, there are three versions of the TCAS system in use or in some stage of development; TCAS I, II, and III. TCAS I, the simplest of the systems, is less expensive but also less capable than the others. It was designed primarily for general aviation use. The TCAS I transmitter sends signals and interrogates Mode-C transponders. The TCAS I receiver and display indicates approximate bearing and relative altitude of all aircraft within the selected range, usually about forty miles. Further, the system uses color coded dots to indicate which aircraft in the area pose a potential threat. This is referred to as a Traffic Advisory (TA). When a pilot receives a TA, it is up to him/her to visually identify the intruder and is allowed to deviate up to + 300 feet. Lateral deviation is not authorized. In instrument conditions, the pilot is required to notify air traffic control for assistance in resolving the conflict. TCAS II on the other hand is a more comprehensive system than TCAS I. This system was required to be installed on all commercial air carriers operating in the United States by December 31, 1993. It offers all of the same benefits but it will also issue a Resolution Advisory (RA) to the pilot. In other words, the intruder target is plotted and the system is able to tell whether the aircraft if climbing, diving, or in straight and level flight. Once this is determined, the system will advise the pilot to execute an evasive maneuver that will deconflict the aircraft from the intruder. There are two types of RAs, preventive and positive. Preventive RAs instruct the pilot not to change altitude or heading to avoid a potential conflict. Positive RAs instruct the pilot to climb or descend at a predetermined rate of 2500 feet per minute to avoid a conflict. TCAS II is capable of interrogating Mode-C and Mode-S. In the case of both aircraft having Mode-S interrogation capability, the TCAS II systems communicate with one another and issue deconflicted RAs. Since this system costs up to $200,000 per aircraft, manufacturers have built in an upgrade capability to the next generation TCAS III. This system will be virtually the same as TCAS II but will allow pilots who receive RAs to execute lateral deviations to evade intruders. This will be possible because the directional antenna on TCAS III will be more accurate and will have a smaller bearing error. There are also hopes that the new antenna will cut down on false alarms since it can more accurately determine an intruder’s location. Another upgrade that is proposed has to do with the Mode-S data link. Through this link, a system will be capable of transmitting the aircraft’s GPS position and velocity vector to other TCAS-equipped aircraft thus providing much more accurate information.