Project ESIT

European Simulator of Surveillance Interrogators and Transponders

Aircraft in clear sky
© Schreiber
Antenna of an airsurveillance radar
© Noël Magis -
Radardisplay of an airteaffic-controller
© Austro Control GmbH

ESIT is a research and development project awarded by EUROCONTROL. The Austrian air navigation survice provider (ANSP) Austro Control GmbH. won the call for tenders of this project and carries it out together with the German company SeRo Systems GmbH. and the IHF as sub-contractors.


Today civil air traffic surveillance is based mainly on the information provided by secondary surveillance radars (ATCRBS ... Air Traffic Control Radar Beacon System). This method of determining the aircraft positions depends on the active replies of so called transponders installed in the aircraft to interrogations by the radars. This allows also the transmission of additional information from the aircraft to the radar receiver. The interrogations are done at a frequency of 1030 MHz and the replies use 1090 MHz. The use of fixed frequencies assures the international interoperability but allows interference to radars and transponders.

© IHF/TU Graz

The situation is made worse due to other systems using the protocol and the frequencies of the secondary radar. Those are TCAS (Traffic Alert and Collision Avoidance System ... air-based collision warning and avoidance system), MLAT (multilateration, TDOA .. time-difference-of-arrival positioning) and ADS-B (Automatic Dependent Surveillance-Broadcast).

The two figures below show simplified example scenarios for ground based interrogations ...

© IHF/TU Graz

... and TCAS interrogations.

© IHF/TU Graz
Airtraffic controller's position
© Austro Control GmbH

All these interrogations are received, decoded and - if necessary - answered by the aircraft transponders. The replies are again received by the interrogation ssystems and the aircraft position is determined. A large number of - possibly simultaneous - interrogations may result in transponder overload, which means that single interrogations cannot be decoded and replied to any longer. This could result in the aircraft vanishing - for a short time - from the involved sensor.

Additionally modern transponders will transmit signals independently from any interrogation (ADS-B messages or squitters).

This results in a high density of signals, both at 1030 and 1090 MHz.

View of an airtraffic control-room
© Austro Control GmbH

For a thorough planning of air surveillance infrastructure (e.g. when introducing a new system) it is quite important to know in advance, how large the impact on transponder load will be. Especially multilateration-systems can - due to their mode of operation - produce higher loads than radars.

Because of interrogations by other aircraft (TCAS systems) and because of selective Mode S interrogations the traffic density itself has also an impact on the transponder load.

© Schreiber


Based on the research project TOPAS and the resulting simulation tool, a digital twin is being developed incoperating all possible interrogations and replies  at 1030 MHz and 1090 MHz, respectively. This includes the following Mode A/C and Mode S interrogation systems:

  • Mode A/C radar sytems
  • Mode S radar systems
  • Multilaterationssystems
  • TCAS

Beside those also differnt transponder-types have to be taken into account.

Research approach

ESIT has to implement the following features not implemented in TOPAS:

  • Variation in time of interrgations and replies with a resolution of 1 µs in order to detect mutual interference (garbling)
  • Re-interrogations
  • Improved scheduling of the Mode S interrogations (taking the maximum allowed ages of the different information into account)
  • Modelling TCAS's interference mitigation mechanism
  • Determination of the spatial distribution of the reply density

Additionaly, scaling the algorithms from the Austrian airspace to the European one is quite a challenge because of the much higher number of radars, among other things.

ESIT will be deployed in a cloud-environment and will offer the users a web-based user-interface, which allows a per-user configuration of the simulation-runs and which can display the results in various ways. The figures below show heatmaps generated by TOPAS (showing the transponder occupancy, the number of Mode S replies to interrogations by radars and a MLAT-system and the coverage of a Mode S radar including the areas of different interrogations).

© IHF/TU Graz

IHF is developing the simulation core and our partner Sero Systems is responsible for the webserver environment including the necessary databases and the user interface.

Austro Control is responsible for the project management and provides further technical knowledge.

Press release and publications

The media department of Graz University of Technology has released a statement regarding this research project. This can be found in the news section of the university homepage.

Simultaneously the daily newspaper "Die Presse" conducted an interview with Helmut Schreiber and published an article in the science part of the issue of Saturday, January 22nd 2022 (in German, behind paywall)

The conference paper presented at the IRS 2023 (International Radar Symposium) in Berlin can be accessed on IEEExplore.

Furthermore an open access paper for the journal e+i (Elektrotechnik und Informationstechnik) can be read here.

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

Helmut Schreiber
Dipl.-Ing. Dr.techn.
+43 316 873 - 7930
+43 316 873 - 7941
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