Airborne Traffic Detection Systems in European Airspace

As I get so many questions about this subject, I finally wrote this summary. If you have any comments or questions please just write me.

This document is about systems which allow pilots to detect other traffic in their vicinity. The goal is to avoid collisions and close encounters. This text focuses on the current situation in Europe.

All the systems mentioned only work if the target aircraft operates some sort of transponder or other transmitter. There are no civilian systems available for detecting "silent" targets besides your eyes and position reports. The military obviously operates such system though.

In my very personal experience in Germany at low level and in the pattern an estimated 5% of the aircraft fly with the transponder turned off. Please keep this risk in mind at all times.


The Traffic Collision Avoidance System (TCAS) is an on board system which actively interrogates all transponders (Mode A/C/S) in range by sending appropriate signals on 1030 MHz. It will then process the answers and extract the following information:

First the time delay between the interrogation and the answer allows the TCAS to calculate the distance to the target. Second the TCAS has got an antenna which can detect the relative angle to the signal. Third for Mode C or S it can read the pressure altitude of the other aircraft. With this information the TCAS system can locate all transponder equipped traffic in the vicinity of your aircraft.

Basic TCAS I systems available for small general aviation aircraft will only display the targets and issue Traffic Advisories. A real TCAS II (very expensive) as installed in airliners etc. will also issue vertical Resolution Advisories. If the target aircraft is also TCAS II equipped, it will try to coordinate the advisories with the other aircraft.

Below a short list of popular active system but there are many more and I suggest you do your own research. All are for fixed installation only and currently the absolute lowest cost options seems to be the LYNX NGT-9000 with active traffic option starting around 6300 USD plus installation.

LYNX® NGT-9000 with active traffic option
Avidyne SkyTrax600
Garmin GTS800

Passive Traffic Systems

Passive traffic systems rely on either ground secondary surveillance radar or other aircraft TCAS to trigger answers from Mode A/C/S transponders. It will receive those answers and generated traffic information from them. As a result the following information is available:

If the transponder is Mode C or S the pressure altitude of the other aircraft is received. But then there is no precise way to determine the distance. Most systems will guess the distance by using the signal strength. But obviously that is not very precise as different transponders transmit at different power levels and the location of the sending and receiving antennas make a big difference.

Finally all currently available passive traffic system only use an omni directional antenna and thus can not determine the bearing to the target. There used to be only the Zaon PCAS which used a directional antenna but that device is long out of production.

Also passive traffic system can not operate in areas without stations triggering the transponders in the first place. So in valleys or over the ocean passive systems will usually not work.

To sum it up passive traffic systems can detect other transponder equipped aircraft, but will not display precise range and no bearing. So all you get is the information there is some aircraft at a certain altitude in the area but its position is unknown. Such targets are usually called "omni directional targets".

In my personal opinion in cruise this might be enough to trigger some avoidance action. In proximity to the airport pattern etc. this is not enough to trigger some action besides a good look out and a radio call asking others for their positon. But wouldn't you do that anyway close to the pattern?


FLARM is a proprietary system owned by FLARM Technology AG, Switzerland. The idea is to have a device which broadcasts its own GPS position and altitude using the license free frequency 868 MHz (in Europe). The signal is proprietary and should only be received by FLARM receivers. The receiver can then compare its own position with the GPS position of the other aircraft.

FLARM was originally designed for gliders and contains special algorithms which determine when a traffic advisory will be issued. Thi is important for gliders which often fly much closers to each other compared to powered aircraft.

The two big drawbacks of FLARM are first its limited transmission power. It will transmit usually at 0.025 Watt (a little more in some reagions). Just for comparison a normal Mode S transponder will transmit at 250 Watts. That is not a typing error, so a normal transponder uses ten thousand tims as much transmit power as FLARM uses. Obviously the effective range of FLARM is very limited. Second it is a proprietary system. So only transmitters manufactured or licensed by FLARM should use the system.

In real life many gliders are equipped with FLARM while its use on powered aircraft is limited and will depend on the region you fly in. Usually FLARM device will transmit and receive but lately a few solutions like the SkyEcho II with a software license have surfaced which will receive only. In my personal experience if you are worried about gliders install a FLARM but expect very limited detection range.

Glider activity will obviously vary a lot by geographic region. For example in Croatia I have not yet seen a single one while southern Germany is full of them in the summer.

FLARM Website

ADS-B 1090 MHz / Mode S ES

Mode S extended squitter transponders will broadcast their own GPS position and altitude (and some more information) on 1090 MHz. This is an extension of the Mode S transponder technology but please note that only some Mode S transponders will transmit this signal. The signal is then received by an installed or portable device and the traffic information is displayed on a variety of systems.

Such receivers can be manufactured at very low cost and due to the massive transmit power of the Mode-S transponders the detection range is very good. So as long as the target is 1090 MHz ADS-B out equipped this is an excellent tool for collision avoidance.

Unfortunately only a small fraction of the European GA fleet is 1090 MHz ADS-B out equipped. Airliners and bigger jets are nearly all equipped and the faster and newer the GA aircraft the more likely it will be equipped. But so far for there is no 1090 MHz ADS-B requirement for small GA aircraft and the typical aeroclub Cessna 172 will not be equipped.

My very personal observation in Germany is that an maximum of estimated 10% of the small GA fleet is equipped.

In the USA ADS-B out is mandatory for aircraft since 2020 but please note this is in the USA only and this does not apply to Europe (in the USA 978 MHz ADS-B out might be used as an alternative below 18000 ft).

Nearly always the 1090 MHz transmitter will be installed permanently in the aircraft and be part of the Mode S transponder. The big exception is the SkyEcho II device which is legal in the UK only. It will broadcast 1090 MHz ADS-B without Mode S functionality and is portable. As an exception such a device can be detected by ADSB 1090 MHz receivers but not by classic TCAS while all other Mode S ES transponders are also visible to classic TCAS.

Some popular 1090MHz ADS-B out products:
Avidyne AXP340
Garmin GTX345 ADS-B in/out transponder
SkyEcho II

ADS-B 1090 MHz ADS-R / TIS-B

Theoretically ATC radar could broadcast traffic information from its systems to other aircraft over the 1090 MHz Mode S / ADS-B frequency. This is used in the USA but apparently implemented nowhere in Europe.


In the USA aircraft flying below 18000ft can install an ADS-B out transponder operating on the 978 MHz / UAT frequency instead of the Mode S extended squitter on 1090 MHz. Low cost portable and installed receivers can then receive those reports similar to the 1090 MHz receivers. But 978MHz transponders are not legal in Europe so far. ADS-R / TIS-B rebroadcasts are also available on 978 MHz in the USA only.


Pilot Aware is another system similar to FLARM. It also transmits on a license free frequency 869 MHz with similar transmit power limitations. In addition to FLARM the PilotAware system also operates ground stations which rebroadcast traffic information in a similar way to ADS-R/TIS-B but again only with very low transmit power and limited geographic coverage. So far this system seems to be mostly used in the UK.

PilotAware Website

Internet Based Systems

Some systems try to rebroadcast traffic information from sources like flightradar24 or similar networks to aircraft in flight. But this will only work as long as the internet connection in flight is very stable which will nearly never be the case in my personal experience. Also flightradar24 and similar system have difficulties tracking targets at low level like in the pattern. Unfortunately this is where you will probably need the information the most.

My Personal Conclusion

If you are worried about collisions with other powered aircraft, currently in Europe there is no alternative to an expensive active traffic system. Mode A/C/S transponders are the only system which is widely installed in powered GA planes of all sizes. Passive systems deliver just not enough information.

Some say it brings up your alert level but I am on high alert at low altitude anyway and after many years with an active system I did experience how difficult it is to spot aircraft even if bearing and distance are known. So without that information such traffic warning is of limited use to me personally and I would not spend serious money on a passive system.

If you are worried about collisions with gliders get a FLARM system. Many gliders will fly with FLARM only so this is the only way to detect them. But expect limited range which might be a problem especially if you have a fast aircraft.

1090 MHz ADS-B receivers are great entertainment to see all the commercial traffic around you with many details like speed etc. Also devices like the UK SkyEcho II can only be detected with a 1090 MHz receiver. For collision avoidance their use is still very limited unless more aircraft get equipped but the receivers are very low cost. If it is cheap personally I would get a 1090 MHz ADS-B receiver but not spend serious money on it.

PilotAware and 978 MHz ADS-B are basically not used where I fly but might become relevant some day.

I am sorry for the disappointment, as the most important solution for European traffic avoidance is unfortunately still an expensive active traffic system. In my opinion all the cheap solutions will not really do the trick.

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