Mode S is a datalink technology that uses discretely addressed interrogations. Mode S equipment includes ground stations with sensors, and transponders aboard aircraft. The interrogations that ground sensors send planes include the identification information of the target plane. This method ensures that when other aircraft receive the interrogation, they will not respond.
Replies contain information such as altitude reports.
In studying the development of Mode S, we found that interoperability was the driving pressure behind many design decisions. The primary reason for this requirement was the needs of the aviation community. Being diverse in interests and large in size, this group is alternately safety-conscious, economical, skeptical, and concerned about the bottom line. The overall effect is extreme conservatism, which has two major consequences for designers of air traffic control systems. Adoption of a new air traffic control technology requires upgrading hundreds of ground stations, installing new equipment in thousands of aircraft, and training tens of thousands of people.