top of page

Question: What are Mode C/S transponders and how do they work compared to ADS-B?

Updated: Oct 31

Preliminary Note:

This note is inspired by our response to a personal question from one of our loyal readers and users. It has been intentionally simplified. It is by no means a detailed technical note, but it straightforwardly sheds light on this delicate aspect of our light aviation, often misunderstood.

SafeSky wishes to express its gratitude to the pilots who have contributed to the improvement of this article through their insightful feedback. Thanks to their input, we have been able to make several adjustments to provide you with the most accurate, albeit simplified, information possible. Among them, Steve UZOCHUKWU, a graduate in Avionics from Queen Mary College, London University, deserves special mention. He has reminded us of the importance of being very precise when addressing such important subjects.

Mode C/S Transponders: How do they work?

Mode C/S transponders transmit on the 1090 MHz frequency, just like ADS-B. However, the main difference lies in the amount of information transmitted. The message from a Mode C/S transponder is much less detailed than that of ADS-B.

In the message from Mode C/S transponders, precise location is missing although both types will give a standard pressure altitude for the other aircraft. Consequently, even modern situational awareness systems capable of receiving radio waves can only roughly estimate the distance and the location between two aircraft. The information received might be something like: "intruder aircraft within a radius of 5Nm around my position, similar altitude." This is a very basic alert and tends to make pilots nervous, as they do not know where to look. The other aircraft could be anywhere in a 360 degree arc.

Our aircrafts respond to secondary air traffic control radars via transponders (C/S mode). The radars send queries at 1030 MHz and receive responses at 1090 MHz. In the S mode, the radar receives signals without direct GPS positioning data. Aircraft location is determined by the radar using information on time, distance, and azimuth of the returned signals.

Some installations (secondary radars) are capable of decoding and interpreting ADS-B, adding better accuracy in aircraft location.

Hence the importance of ADS-B, which offers much completer information, significantly enhancing safety by accurately positioning traffic. This occurs simply because the transponder is connected to a GPS, allowing the transmission of the aircraft's exact coordinates in three dimensions.

And what about SafeSky?

SafeSky can provide part of the Mode S transponder information thanks to multilateration made possible by the presence of ground-based traffic reception antennas. The lack or absence of these ground stations means there can be no multilateration and therefore no traffic reported on SafeSky or other equivalent platforms.

That's why SafeSky is actively involved in a collective effort with authorities, associations, and other organisations concerned about flight safety in order to establish a semi-professional network of ground stations specifically dedicated to general aviation, preferably positioned at airports.

This network would increase visibility of traffic near airfields, where the risk is highest. We are working closely with authorities, IAOPA, and drone-related movements to make this solution a reality.

ADS-B Transponders: Good to know.

Most modern transponders, such as the Trig TT21, can transmit in ADS-B if they're connected to a GPS. A simple cable costing a few euros is enough to make you visible/able to transmit in ADS-B. This connection can usually be made via EFIS like the G3X or Dynon, or even via a Garmin 650. However, this work must be carried out by a qualified person or during the radio and transponder check, which must be performed regularly.


In (social) media, there are many evaluations and comparisons of systems that detect nearby aircraft, albeit with varying degrees of effectiveness. This exercise is undeniably positive, and the sometimes predominant position of C/S mode in these evaluations is not surprising, as it is currently the only technology recognised by the ATCs.

  • Firstly, the C/S mode transmits a lightweight message, with no GPS data except altitude, which sets it apart from ADS-B. It is the secondary radar that is capable of positioning the traffic with a bearing, but only for itself. Also, without that initial query from the (primary) radar, the transponder is silent and says nothing to any system. This is in contrast to ADS-B, where the unit is always sending out GPS location data.

  • Additionally, traffic detectors (TAS) capable of identifying Mode C/S only provide proximity information, based on the radio signal reception time. This results in a rough distance approximation, leaving pilots in a state of unsettling uncertainty.

  • Furthermore, radio waves cannot penetrate certain obstacles such as mountains, limiting aircraft visibility in specific regions. Although we have little chance of colliding with an aircraft on the other side of the mountain, it is useful to be aware of the position of other aircraft, in order to anticipate traffic when crossing ridges, for example.

What we often overlook in this equation is the importance of effectively sharing one's position during flight. As mentioned earlier, a simple cable can enhance your transponder's efficiency, propelling it into the modern era by sharing precise location data via ADSB.

At SafeSky, we have always advocated for a hybrid approach that combines RADIO and MOBILE INTERNET. The strengths of one compensate for the shortcomings of the other.

The fundamental rule of visual flight remains "SEE and AVOID," but in increasingly crowded skies, it is equally crucial to be seen by others.

This is the pilot's responsibility towards their environment.

Recent Posts

See All

As summer fades, autumn reveals a new chapter in aviation: Starlink promises celestial connectivity, U-Space shakes up general aviation, and some nations face risky skies. We're eager to embrace these

bottom of page