Transponders are mandatory for flights in some types of airspace in many parts of Europe from March 2008 onwards. This includes hot air balloons and other light recreational aviation and aircraft. The following is a brief explanation of transponders from a hot air balloon pilots perspective. I hope you find it useful.
Mode A, C, & S Explained …
hot air balloons over the clouds above Ashton Court
A mode A balloon or light aircraft transponder will emit a signal in response to being ‘illuminated’ by the 3 degree wide primary radar beam from an air traffic control unit (ATC). The resulting 7ms response from the transponder contains a digital encoding of the 4 figure number assigned to the aircraft by the ATC unit and set by the pilot on the transponder unit. This is the ‘squawk’ code. The transponder response is received by the ATC radar and now it is possible to associate the squawk code with the aircraft and uniquely identify it on the radar screen.
A mode C transponder will in addition to the functionality of mode A transponder, transmit in its digital response an encoded hot air balloon or light aircraft altitude signal. The altitude signal is derived either from an attached or integral altimeter set to 1013.25 mb QNH. In addition to receiving the unique 4 figure aircraft identifier the ATC unit now also receives the flight level of the aircraft and these are displayed on the radar screen.
A mode S transponder also returns a digital encoding of an aircraft identifier and the balloon ? aircraft altitude to the ATC unit. However additional functionality allows the ATC unit to determine how often the transponder will transmit the encoded information.
This selective functionality has many advantages in crowded airspace. For instance only aircraft operating between certain altitudes can be asked to respond to the radar beam sweep. Or the unit can be asked only to send its response at certain times and hence Mode S transponders may be interrogated individually (Roll Call) or collective mode (All Call).
The limitation of a 4 digit air traffic controlled identifier has also been replaced with a 12 bit (2^24 combination) identifier, which is assigned by the relevant air worthiness authority. Therefore the ATC no longer needs to assign the Squawk code to the transponder although this is still possible.
An extension of mode S allows additional aircraft data (such as heading, speed etc) to be encode in the response message and this can be further used by the ATC unit. How ever this is unlikely to be relevant to hot air balloon transponders for obvious reasons.
ELS, EHS and LAST Explained
ELS stands for Elementary Mode S. According to this standard only the unique 24 aircraft identifier and the encoded flight level are returned in the transponder message from the hot air balloon or light aircraft.
EHS is the enhanced mode S standard, in which it is possible to encode additional information in the transponder message. This information would usually be derived from the flight management system or an approved GPS unit and would allow the ATC unit to retrieve additional aircraft parameters such as track, speed, roll angle etc.
LAST is a variation of ELS for Light Aviation (Light Aviation Secondary Surveillance Transponder). For aircraft operating at less than 15000 ft and at speeds less than 175 knot, a reduced power implementation of elementary mode S is allowed saving battery power.
Much of the above information was compiled from information from the the UK CAA website.
In 2008 I bought my first Mode S transponder for our hot air balloon syndicate, because we were now flying adjacent to a Transponder Mandatory Zone. I chose a Garrect VT01. I liked it so much that I now reccommend them.