Project TOPAS

Transponder Occupancy Analysis


© Noël Magis -

TOPAS is a common research activity with the Austrian air navigation service provider Austro Control GmbH. in order to determine the load of aircraft transponders. Those transponders are interrogated by various interrogation systems at 1030 MHz and reply at 1090 MHz. Typical interrogation systems are

  • SSR ... standard secondary surveillance radar (Mode A/C only)
  • Mode S radar systems
  • Multilateration-systems
  • ACAS ... airborne collision avoidance system

The higher the load of a transponder - due to a higher number of interrogations - the higher the probability that the transponder will not be able to reply to an interrogation. That would make the aircraft - at least for a short time - "invisible" for this sensor. The transponder occupancy is the percentage of time the transponder is occupied by decoding interrogations and transmitting replies. A thorough planning of the air surveillance infrastructure (e.g. in case of introduction of a new system like multilateration) needs also to know the impact on the transponder load. Especially multilateration systems may contribute to a higher amount due to their mode of operation.

The result of this project is on the one hand a theoretical study and on the other hand a set of tools to determine the transponder load.

Research approach

The load of a transponder is determined by 2 numbers :

  • Percentage of time the transponder is busy (occupancy)
  • Average reply rate

These parameters depend on various factors, like number, type and configuration of the sensors' transmitters which cover the transponder, but also the number of Mode S transponders selectively interrogated  and the transponder's properties. From this information the number of interrogations received by the interrogator is determined - both the interrogations addressed to the tronsponder itself , the interrogations addressed to other transponder and the "all-calls" which are broadcasts to all transponders. Based on this interrogations the occupancy and the reply rate can be calculated.

Mode S interrogation frame :

© IHF/TU Graz


The first result of this research project was a theoretical study determing and investigating all influencing factors on the transponder load. Based on this work 2 tools were developed which allow the calculation of occupancy and reply rates depending on sensor infrastructure and traffic situation.

Spreadsheet analysis

This is a simple assessment which considers only the number of transmitters and aircraft but not there spatial distribution. This will give therefore only approximated values but can show very fast how a change in a single parameter will influence the transponder load.

The figure shows a cutout of the spreadsheet :

© IHF/TU Graz

TOPAS Software Tool

The second tool developed is a software that needs and uses the real positions of the sensors and can also base its calculation on real air traffic scenarios (derived from a live recording). Furthermore complex interrogation strategies - different for each interrogator - can be used. Therefore the results of this tool will approximate the reality much closer. Other features to improve the output are

  • Consideration of coverage diagrams of the sensors
  • Calculation for multiple flight levels
  • Simulation of the sidelobe suppression mechanism
  • Introduction of a reply probability

The tool gives you a coloured map where the colour represents the actual value of the transponder load parameter at a given position.

Below is an example of the results which can be gained by the tool. The video visualizes the transponder occupancy during a whole day using real traffic numbers :

© IHF/TU Graz

Further research topics regarding air-traffic control you can find here.

In teaching the topic of air surveillance in general and particularly the function of secondary radars is covered by the following lecture :

Introduction to Radar Systems
Helmut Schreiber
Dipl.-Ing. Dr.techn.
+43 316 873 - 7930
+43 316 873 - 7941