The oldest conversation: somebody asks, you answer.
Secondary surveillance radar was built on a simple bargain. The ground station transmits a question on 1030 MHz; every transponder within reach answers on 1090 MHz. Nothing about the aircraft is measured — the reply is the data. In the original Mode A the reply was just the four-digit code the controller assigned you. Mode C added pressure altitude, always referenced to the standard 1013.25 hPa so that the controller and not the pilot decides how to convert it.
Mode S fixed the flaw that came with success. When the sky filled up, every interrogation was answered by everyone in the beam and the replies collided — the elegantly named problem of over-interrogation. Mode S gives each airframe a permanent, unique 24-bit address issued with its registration, so the radar can address one aircraft and get one reply. That address is also what makes an aircraft trackable, identifiable, and — the part that matters most in a glider — selectively interrogable by the collision-avoidance system of an airliner.
The newer conversation: nobody asks, you announce.
ADS-B inverts the whole idea. An ADS-B Out installation takes the position and velocity from a satellite navigation receiver and simply broadcasts them — in Europe on 1090 MHz Extended Squitter, roughly twice a second, whether or not anybody is listening. There is no interrogation, no rotating antenna, and no ground infrastructure required for one aircraft to hear another. Anyone with a receiver and an antenna gets your position, your altitude, your track and your climb rate.
That generosity is also its weak point. A radar measures range independently; ADS-B repeats whatever the aircraft says about itself. So the standard carries integrity alongside the position — figures that state how much the transmitting aircraft trusts its own solution. A receiver is entitled to ignore a target whose integrity is too low to be worth acting on, which is exactly why ADS-B Out is not something to switch on with any GPS you happen to have: the position source has to be approved for it, or you are broadcasting a number nobody may legally use.
Two supporting services fill the gaps where they exist: TIS-B rebroadcasts radar-derived traffic to ADS-B receivers so that transponder-only aircraft appear on ADS-B screens, and ADS-R relays targets between the two ADS-B data links used in the United States. Both depend on ground stations, both are regional, and neither should be assumed anywhere in Europe.
And the conversation built for us.
FLARM exists because neither of the above works for gliders. Transponders are heavy on power and money; ADS-B is a fine position report but a poor collision predictor for aircraft that spend the day turning. FLARM broadcasts a short packet on a low-power band containing not only where the aircraft is but where it will be — a predicted track — and compares that prediction against every neighbour. It costs a fraction of a transponder in current and produces a directional warning tuned to the way gliders actually collide: converging in a thermal, closing head-on along a ridge. The FLARM article in this series takes that algorithm apart properly.
The critical fact is the one pilots most often get wrong: FLARM and the transponder world cannot hear each other. They are different frequencies, different protocols, different eras. A glider with the finest FLARM installation in the country is entirely invisible to the TCAS of the airliner descending through its ridge. A glider squawking Mode S is invisible to every other glider in the thermal. The two systems protect against two different accidents, and carrying one does not excuse the other.
Switch on what you carry.
Start with FLARM alone — the standard European glider fit — and read the third row. Then add the transponder and watch the first row stay strong while the airliner finally appears. There is no single box that fills the table, and that is the honest state of the art: the coverage comes from carrying more than one conversation, or from a receiver that listens to all of them.
Four bands, one antenna farm.
Ground radar asks on 1030, every transponder answers on 1090. TCAS uses the same pair of frequencies to ask directly, aircraft to aircraft.
The same 1090 MHz, but nobody asked: the aircraft simply broadcasts its GNSS position, altitude and velocity about twice a second.
A low-power licence-free band, a short packet with a predicted flight path, and a collision algorithm built for aircraft that circle.
The paraglider and hang-glider mesh — different traffic, same neighbourhood of the spectrum, increasingly worth receiving.
Which is where the installation problem starts. A transponder transmits with hundreds of watts on 1090 MHz a few centimetres from a FLARM receiver trying to hear microwatts on 868 MHz. Separate the antennas as far as the airframe allows, keep the transponder aerial on a metal ground plane underneath and the FLARM aerials vertical and clear, and range-check the result from a real flight log — never from the ground, where everything looks fine.
One screen, or none.
If an aircraft ends up carrying two or three of these systems, the last engineering problem is human. Two traffic displays showing the same aircraft as two different targets is worse than one display showing it once — the pilot spends the encounter reconciling screens instead of looking outside. This is the whole argument for a receiver that takes FLARM, ADS-B, Mode-S replies and the rest, correlates the duplicates by address, and puts one symbol per aeroplane on one display.
And then the oldest rule survives all of it. Every system here has a blind spot: the aircraft with nothing fitted, the one whose antenna is behind a carbon spar, the one whose transponder was left on standby. The traffic display is there to tell you where to look. The looking is still yours.
FLARM, ADS-B, Mode-S, FANET, SSR, ADS-R and TIS-B received and merged into one traffic picture on every display.
Traffic view on the vario you already fly — the FLARM picture where your eyes already are.
Traffic, terrain and task on the biggest 80 mm display, fed from the FLARM port on the back.