Mode S

Trig customers often ask, what’s significant about a Mode S transponder?

The transponder has been with us for many years, referred to by some pilots as the ‘set and forget’ device in the cockpit. However, its role in modern air traffic management is becoming increasingly important.

A transponder is a radio transmitter which replies to radar interrogations. Mode A/C transponders will reply with Squawk code and altitude information to assist air traffic controllers. The technology has its roots in wartime systems known as ‘identification friend or foe’ or IFF devices. Pilots relied upon IFF equipment to prevent incidents of friendly fire.

Transponders have evolved to become more efficient and provide better information, by using Mode S and ADS-B technologies. Many legacy Mode A/C transponders were based upon valve technology which generates large amounts of heat. Increasingly these transponders are beyond their service life and when they break repairs can be costly and parts difficult to obtain. In contrast Trig Mode S transponders use solid state technology, which allows them to run more efficiently and much cooler.

There are significant benefits in using a Mode S transponder. For the pilot a Mode S device will actively ‘speak’ to TCAS systems and can be seen by most military aircraft – enhancing flight safety. Air traffic controllers recognise the value of Mode S too; it provides them with a Squawk code, tail number and altitude. This in turn usually makes a transit through controlled airspace less challenging. The ‘S’ in Mode S stands for selective. This means that air traffic controllers can de-select the Squawk information. This is necessary in busy airspace when having control of numerous aircraft on a radar screen presents ATC with a high workload.

Finally, the other advantage of Trig Mode S equipment is that it comes ready for ADS-B Out. This enhanced surveillance technology is explained elsewhere on this site. You may notice that Mode S transponders are referred to as 1090ES capable. This is the transmission frequency used – 1090 MHz for ADS-B Out transmissions, this is an ICAO international standard used worldwide.