ESIT – a digital twin of air surveillance infrastructure

Abstract

Civil air traffic surveillance and management depends heavily on the use of aircraft transponders replying to radio interrogations from cooperative surveillance systems such as secondary surveillance radars, multilateration and collision avoidance systems. A high number of such surveillance systems combined with a high aircraft density can cause a large quantity of interrogations, therefore having a detrimental impact on transponder behavior and sometimes even causing a loss of replies and thus a deterioration in aircraft surveillance. In order to ensure the safe operation of European air traffic management, the European Commission obligates the member states to take measures to ensure that the transponder load in their air spaces does not exceed a safe level. EUROCONTROL, in its function as network manager responsible for supporting the monitoring, management and coordination of the scarce spectrum resource, has contracted the implementation of a European simulation tool to Austro Control in collaboration with SeRo Systems and the Institute of Microwave and Photonic Engineering as subcontractors. This paper examines the development and use of this tool as well as the functionality it provides.

Zusammenfassung

Das reibungslose Funktionieren der zivilen Flugsicherung ist stark davon abhängig, dass die in den Flugzeugen verbauten Transponder zuverlässig auf die Abfragen von kooperativen Überwachungssystemen, wie Sekundärradar, Multilaterationssystemen und flugzeugbasierten Kollisionswarnsystemen, antworten. Aber eine große Anzahl solcher Abfragesysteme zusammen mit vielen Flugzeugen führt zu vielen Abfragen, die wiederum die Transponder so stark belasten können, dass Transponderantworten verhindert werden. Um einen sicheren Flugverkehr zu gewährleisten, hat die Europäische Kommission ihre Mitgliedsstaaten dazu verpflichtet, Maßnahmen zu treffen, sodass die Belastung der Transponder in den jeweiligen Lufträumen ein sicheres Ausmaß nicht übersteigt. EUROCONTROL, in der Funktion als Netzwerk-Manager zuständig für die Unterstützung bei Monitoring, Verwaltung und Koordination des beschränkten Ressource-Spektrums, hat Austro Control – zusammen mit SeRo Systems und dem Institut für Hochfrequenztechnik – mit der Implementierung einer Simulationssoftware zur Bestimmung der Transponderbelastung beauftragt. Dieser Artikel beschreibt die Entwicklung und die Funktionsweise dieser Software.

1 Introduction

1.1 About secondary surveillance radar and aircraft transponders [1]

Secondary Surveillance Radar (SSR) – in its inital form, also called Air Traffic Control Radar Beacon System (ATCRBS) – was introduced in the mid-1960s and is the backbone of civil airtraffic surveillance today, while also being used by the military for identification friend or foe (IFF). As is indicated by the name, the SSR measures an aircraft’s position using the radar method: range by echo-delay and bearing by a rotating antenna with a narrow beamwidth.

But unlike a so called primary radar, the SSR does not use passive reflections but the active replies of an instrument mounted aboard the aircraft – the transponder. Such systems are also called cooperative surveillance systems since they depend on the cooperation of the aircraft. This allows the implementation of a simple communication system, as the transponder can encode additional information – like aircraft identification or altitude – into it’s reply. Furthermore, the SSR has a better link budget and therefore a larger range than a primary radar. The radar is capable of different modes of transmission – also called interrogations – where each mode “asks” the transponder for a different piece of information.

Originally the following modes were used:

• Mode 1: military only

• Mode 2: military only

• Mode 3/A: civil and military transponder/aircraft identification

• Mode C: aircraft altitude

The encoding of the different modes is quite simple, 2 pulses of $0.8$ $\upmu$s length each with a spacing ranging from 3 to $25$ $\upmu$s, where the length of this interval determines the mode. In order to facilitate worldwide use of this system, it was decided, that all radars should interrogate at $1030$ MHz (uplink) and the transponders would reply at $1090$ MHz (downlink).

While this measure ensures the needed interoperability, it also comes with the danger of interference:

• If two or more interrogations are received simultaneously by a transponder, the transponder may not be able to decode any of those (garbling).

• A transponder can only reply to one interrogation at a time and due to power constraints on board of an aircraft, the transponder even needs a recovery time after a reply.

• Additionally, the radar receiver may not be able to decode replys due to garbling.

Addressing some of these issues, Mode S (S for selective) was developed and introduced as a countermeasure. Since a transponder replies to each decoded interrogation, a lot of redundant and superfluous messages are generated. In Mode S the radar interrogates transponders selectively by transmitting a unique address as part of the uplink message so that only the transponder the address belongs to replies, significantly reducing the overall number of replies.

As another advantage Mode S provides a much better data-link capability compared to to the original SSR (for which the term Mode A/C is used), allowing the extraction of additional information, e.g. transponder capability, speed, call-sign,….

Apart from SSR, other aeronautical systems use the same frequencies and protocols as well:

• TCAS (traffic collision avoidance system) onboard of the aircraft as an implementation of ACAS (airborne collision avoidance system) which allow aircraft to determine each others position, exchange information and take evasive maneuvers.

• Ground-based multilateration (MLAT) systems for aerodrome and especially en-route (WAM … wide area MLAT) surveillance as an alternative to radar. Ideally MLAT could measure aircraft positions by means of a TDOA (time difference of arrival) algorithm without additional interrogations at all, but in reality some information must be extracted actively, for operational and technical reasons.

• ADS‑B (Automatic Dependent Surveillance-Broadcast) which is the intended future surveillance technology. Using ADS‑B aircraft transmit their positions and other information on a regular basis in so called squitter messages entirely without the need for interrogations. ADS‑B can use different physical channels and protocols, with one of them being the SSR downlink (also called 1090 extended squitter).

All of the above makes it even more important to ensure that an aircraft replies to interrogations with a high probability and that those messages are detected correctly. The higher the transponder load (number of received and to-be-decoded interrogations, number of replies, percentage of occupancy), the more likely it is, that interrogations are missed and not replied to.

1.2 Addressing the problem

The efficiency and safety of the European Air Traffic Management (ATM) Network depends on the flawless operation of all surveillance systems.

Because of the aforementioned garbling or occupancy a transponder may not reply to an interrogation at all or its reply may not be detected. Subsequently, this could, at worst, lead to transponder outage resulting in a temporary loss of aircraft surveillance (i.e. the aircraft is not visible on the controller’s screen for a short time). This, in itself, is not necessarily an issue due to re-interrogations and because an aircraft is typically covered by more than one radar which – in combination with tracking algorithms – allow smoothing the aircraft trajectory and interpolating missing positions. However, an overload for an extended period may potentially lead to the disappearance of the aircraft on the air traffic controllers display. The large number of surveillance systems deployed in Europe combined with the high density of aircraft has led to an increased rate of interrogations received by aircraft transponders, which makes it possible to cause such an outcome. While the pandemic brought about a collapse in civil airtraffic, the number of flights and transported passengers, while not having reached the level of 2019 yet, is again rising worldwide [2]. There are areas where the traffic is even higher than 2019, e.g. in Austria and some other regions due to traffic diversion made necessary by the Ukraine war.

Although the introduction of Mode S reduced the number of replies of a single aircraft, the number of Mode S interrogations on the other hand depends merely on the number of aircraft in range of the radar and can even increase compared to Mode A/C.

The increasing flight density – together with an also increasing demand for extraction of aircraft data – has, of course, called attention to the problem of excessive load on aircraft transponders and surveillance infrastructure, which has always been of interest for the responsible institutions such as EUROCONTROL and air navigation service providers (ANSPs) [3].

The European Commission also recognized this and issued regulatory requirements which obligate Member States to check the transponder load in their airspaces and to take measures to reduce and limit interrogation rates to a safe level [4,5,6] and [7].

This can be addressed by undertaking measurement flights, during which special transponders count the number of interrogations and replies and measure the transponder load. This, however, only determines the status for a specific duration of time and for a single flight path.

Another possibility to tackle this problem is by computer-based simulation. A digital twin of both the ground infrastructure as well as the whole of the aircraft simulates all transmitted messages and calculates the aforementioned results. Contrary to a single measurement flight the simulation can calculate the load parameters over a larger volume and is limited only by the calculation resources and the accuracy of the used model. Furthermore such a tool supports other usecases like assessment of changes to surveillance systems configurations, long-term systems evolution strategies or strategic and regulatory decisions regarding aircraft and unmanned aerial vehicle (UAV) equipage. As far back as the year 2000 EUROCONTROL had already developed a simple model using Excel Visual Basic (VBA) called the VBA RF model.

Starting 2018, the Institute of Microwave and Photonic Engineering (IHF) of Graz University of Technology designed a software tool to calculate transponder load parameters in the Austrian airspace for the Austrian ANSP Austro Control GmbH (ACG) [8]. At the end of 2020 EUROCONTROL invited tenders for a European simulation software [9]. The call for bids was won by ACG, with IHF and SeRo Systems GmbH as subcontractors, and the development of the tool later named “European Simulator of Surveillance Interrogators and Transponders – ESIT” began in 2021 [8].

1.3 Contents

This paper intends to give a general overview of ESIT and its environment.

Sect. 2 addresses the issue of the transponder load and its influence on the detection of aircraft.

In Sect. 3 a short overview is given of the history of development for ESIT.

Sect. 4 provides a short insight into the overall ESIT environment and some selected fundamental features and concepts, while Sect. 5 gives more detail on the discrete-event-simulation module.

Sect. 6 presents some exemplary results, and Sect. 7 provides an outlook on the finalisation of the project and possible future improvements.

2 Transponder load

The transponder load is determined by the number of interrogations the transponder receives and by the number of replies it has to transmit in response to the interrogations. Additionally, there are the ADS‑B and TCAS squitters transmitted several times per second.

While a transponder is processing a decoded interrogation and possibly replying to it, the transponder is occupied and cannot successfully receive other messages arriving during this period. Furthermore, two or more messages received simultaneously will interfere with each other and impede decoding – as a result, garbling occurs.

This load is not only caused by relevant interrogations the transponder must reply to but also by non-addressed roll-calls or locked-out all-calls.

The important factor is the duration for which the transponder is occupied after the reception of an interrogation. An overview of the different occupancy times can be found in appendix M of [10].

Factors contributing to the occupancy of a transponder are the number and the configuration of the ground-based surveillance interrogators (SSR and MLAT) as well as the number of transponders due to traffic density.

Additionally, a transponder cannot receive messages while a co-mounted TCAS system is transmitting interrogations, adding further to the transponder load.

The higher the transponder load becomes, the higher the percentage of time for which it is occupied and therefore the higher the chance will be that it may not be able to detect and respond to genuinely relevant interrogations. While a single “miss” in itself is not necessarily an issue due to re-interrogations and the combined use of multiple radars to synthesize an air situation display, an overload for an extended period may potentially cause the disappearance of the aircraft on the air traffic controllers display. Furthermore, those re-interrogations increase the load even more causing a snowball effect.

In addition to transponder occupancy, an aircraft transponder that is subject to too many interrogations may not be able to respond to all interrogations once the number of triggered replies exceeds its capability. ICAO specifies in [11] the minimum reply rate capability of transponders.

3 History

IHF began to deal with this topic in 2018, when it was commisioned by ACG as a contractor to the Swiss air navigation service provider Skyguide to prepare a closed study and software tool to analyze the load of the $1030/1090$ MHz band with the focus on the influence of additional WAM systems in the vicinity of two major airports in Switzerland – specifically the Zurich and Geneva. While the first part of this study was the development of a simple spreadsheet-based occupancy calculation, the second part included the implementation of a simulation application, which would become the software-tool TOpAs (Transponder Occupancy Analysis).

The spreadsheet provides a purely statistical calculation of the occupancy of a single transponder, which can be either a Mode A/C-only or a Mode S transponder. It assumes that the transponder is interrogated by a given number of Mode A/C-only radars, Mode S radars and MLAT systems without including any geographical information like distributon of the interrogators in regard to the transponder. The not-addressed Mode S roll-call interrogations also received by the transponder are implemented by adding a user-defined number of Mode S-equipped aircraft spread uniformly across the volume. It is a statistical calculation in the manner that all interrogations are averaged over the rotation period of the simulated radars and the repetition interval of the mode-interlace-pattern (MIP) sequence of the simulated MLAT systems. Therefore situations like 2 or more radars “looking” at the transponder at the same time or overlapping interrogations causing garbling are not taken into account. Fig. 1 shows an example of the result table produced by the worksheet.

Fig. 1

Screenshot of the result page for a Mode S transponder

While this tool has helped to identify all the parameters influencing the transponder occupancy, it is very limited in its application. The main shortcoming is that it does not provide any geographical distribution of transponder loads.

The TOpAs application came closer to being a true digital twin of the situation on the ground and in the air. It already uses many features which later were also implemented in ESIT, like geographical positions of the ground infrastructure, a real-world aircraft scenario and a grid of simulation transponders to calculate the spatial distribution of metrics such as transponder occupancy and reply rates. Other such features are: usage of transmitter screening information, Mode S surveillance- and lockout-coverage (but only defined by maximum ranges), usage of real antenna-diagrams and the interrogator sidelobe-suppression (ISLS) mechanism. Detection of an interrogation by the transponder is based on a simple threshold.

Like the spreadsheet, TOpAs is a statistical simulator, additionally having the ability to determine statistical distributions of the results by simulating only a fraction of the rotation period of a radar (e.g., 1 second) instead of the whole and by repeating this for multiple simulation runs with different random azimuth angles. This method is depicted in Fig. 2

Fig. 2

Improved statistical simulation in TOpAs

An example of the output of TOpAs – the calculated geographical distribution of Mode S transponder occupancy – is shown in Fig. 3. The triangles in the graphic are real aircraft (brown for aircraft equipped with Mode S transponders and inverted red for Mode A/C-only transponders) and the circles are the position of SSRs (blue for Mode S and red circles are Mode A/C-only).

Fig. 3

Example of a result in TOpAs

Still, TOpAs has serious limits – mainly due to its implementation in Python – regarding it’s scalability to a simulation for the whole of Europe.

Both the spreadsheet tool and TOpAs only simulate the transponder load (interrogations, replies and occupancy) and neither considers the $1090$ MHz receivers.

The development of ESIT began in 2021 when ACG won the call for bids together with its partners and subcontractors SeRo and IHF.

4 About the ESIT system

ESIT is a cloud-based environment consisting of database and process management, access control and a web-based graphical user interface for configuration and display of results (developed by SeRo Systems) and 2 simulation backends, the discrete event simulation and the statistical simulation (developed by IHF).

The simulations are based on templates for airborne (distribution of aircraft/transponders) and ground scenarios (ground infrastructure, like radars), which are called baselines. Baselines can only be edited by administrators and are typically generated from other data sources such as an external tool or database, but the ordinary users can instantiate scenarios from these baselines, which they can change and also share with other users. Those scenarios are the configurations and therefore the input data for specific simulations.

Some of the information can be quite sensitive (e.g. military radars), therefore it is necessary to ensure that the user can only access permitted data, which depend on the role and affiliation of the user, while the simulation backend must have access to all data needed for the calculation.

A typical scenario may consist of a few hundred interrogators, a few thousands of aircraft and up to tens of thousands simulation transponders (see Sect. 4.4.3). Thus, the requirements on processing power and memory resources can be quite large.

4.1 Cloud Integration

As specified by EUROCONTROL, the underlying environment is Microsoft’s Azure cloud. The cloud supports the scalability needed for such a computation-heavy application. A simplified block diagram of ESIT’s cloud architecture is shown in Fig. 4.

Fig. 4

Overview of the ESIT cloud architecture

The orchestration of the containers of the ESIT software (web interface, simulation workers,…) is taken care of by an Azure Kubernetes Service (AKS) cluster. AKS is also responsible for the scaling of simulation workers according to the number of queued simulation jobs.

User access to the system and the data is only possible through an authenticated REST API, which enforces access control lists and provides all requested data to the front-end running in the user’s web browser.

Each running simulation process is encapsulated in it’s own instance of a simulation worker job.

4.2 User interface

The user interface (UI) allows external users to access and use ESIT. As mentioned above, ESIT uses a web-based UI. Therefore, the user only needs a web-browser which he has to connect to the ESIT server.

After log-in, the UI provides five views:

• 1. Management of the above mentioned baselines.

• 2. In the second view these baselines can be instantiated to so-called scenarios.

• 3. Once the scenarios are ready, users can switch to the simulation view which allows them to configure and launch a specific simulation based on the selected scenarios (see Fig. 5).

• 4. Each simulation job is put in a queue where it waits until resources become ready. As soon as a simulation is finished, the user gets notified via email and can

• 5. view and download the results.

Fig. 5

Screenshot of the ESIT UI

Depending on the role and organization of the user, the actions and information that are available through the UI vary. For instance, ordinary users are only able to access detailed information of surveillance interrogators (such as their exact locations or configuration) if that information was explicitly shared with them.

4.3 Simulation Backend

The simulation backend itself is written in Python and C++ (see Fig. 6) and encapsulated in each worker instance.

Fig. 6

Overall structure of the simulation backend: division between Python- & C++-code

The backend provides 2 different simulation modules, which the user can select for a simulation run:

• The discrete event simulation is a time-driven algorithm which takes the actual chronological sequence of all messages and the azimuthal positions of the rotating radar antennas into account. This makes it possible to also test for garbling (overlapping of messages).

  The principle is depicted in Fig. 7 and the module is described in more detail in Sect. 5.

• The statistical simulation on the other hand only regards the average number of events per second and therefore cannot simulate garbling. Instead, this is taken into consideration by applying a probability-of-detection factor.

  Fig. 8 shows the basic method of operation.

Fig. 7

Discrete event simulation in ESIT

Fig. 8

Statistical simulation in ESIT

4.4 Requirements, features & limitations

4.4.1 Systems

The simulation has to take into account and simulate the behaviour of the following ground-based systems:

• Mode S radars

• Mode A/C radars

• MLAT systems

• ADS‑B receivers

Apart from that, different versions of TCAS will, of course, be simulated too.

Antenna visibility information will be taken into account for ground stations if screening files are provided in supported formats (either in EUROCONTROL’s older SALADT [12] or in the GeoTiff format). If no screening file is provided, line-of-sight coverage without terrain obstacles is assumed.

For Mode S interrogators the strategy of register extraction (maximum valid data ages) has to be defined together with the coverage defined either by sectors (bounded by minimum & maximum azimuth angles, ranges and altitudes), segments (bounded by geographical polygons and altitudes) or European Mode S Station Coverage Maps (EMS) files [13]. There are definitions for surveillance-, lockout- and datalink-coverage.

4.4.2 Stationary behaviour

While ESIT – in its discrete event simulation mode – does incorporate the chronological sequence of interrogation and replies, it only simulates a stationary case, i.e. aircraft do not change their positions for the entire duration of the simulation. Furthermore, all transponders have already been acquired by Mode S interrogators in their surveillance coverage and – in case of Mode S radars – locked-out for all-call replies in their lockout coverage. Parameters like aircraft speed, altitude rate or heading are still taken into account for the determination of the interrogation rate used in TCAS transmitters.

4.4.3 Simulation transponders & receivers

In order to not only determine the geographical distribution of the transponder load parameters (such as received interrogations, number of replies, occupancy) of actually existing aircraft transponders, a net of so called simulation transponders is introduced.

These sim-transponders represent measurement probes. They behave in many respects like real transponders: they are interrogated (even by roll-call interrogations), they are occupied and they will reply to the interrogations. But those interrogations addressed to the sim-transponders and their replies are only counted and not really “transmitted”, because they would heavily distort the simulation results of the real existing aircraft transponders and the receivers.

The sim-transponders are positioned in a uniform grid with constant distances in latitude and longitude. The grid can also be 3‑dimensional, in which case the altitudes do not need to be spaced evenly.

Similar to the simulation transponders a grid of simulation receivers – also called passive observation points – can be defined. These are $1090$ MHz receivers for calculating the distribution of the received transponder replies at ground level.

5 Discrete event simulation

The ESIT simulation core is divided into three parts: the correlation and the two alternative simulation modules – a discrete event simulation (Module 1) and a statistical simulation (Module 2). This paper deals with the discrete event simulation.

Discrete event means that every message (interrogations, replies,…) is processed individually. That means, that Module 1 really emulates the mode of operation of every single interrogator, transponder and receiver, together with incorporating the propagation delays in order to generate the chronological sequence of the messages at each processing step with the correct time stamps.

This will be discussed in Sect. 5.2 in more detail. First, however, the correlation is described, which is needed by both modules.

5.1 Correlation

The correlation determines which interrogator-transponder and transponder-receiver combination needs to be simulated later on. In order to accomplish this, first the distance, the azimuth and the elevation angle between interrogator (or receiver) and transponder are calculated. The power received by the transponder is then calculated taking into account

• the output power of the transmit amplifier,

• losses in the transmitter,

• the transmitter’s antenna gain in the direction of the aircraft,

• and the freespace path loss for $1030$ MHz.

The decision threshold is not the minimum transponder level (MTL), but the minimum power which could cause garbling in the transponder, which is significantly below the MTL.

For the downlink the parameters are

• the equivalent isotropic radiated power (EIRP) of the transponder,

• the freespace path loss for $1090$ MHz,

• the receiver’s antenna gain in the direction of the aircraft,

• the receiver’s sensitivitiy

• and losses in the receiver.

In addition, the range is needed for the propagation delay.

For radar transmitters and receivers the correlation is done for each pointing angle of the rotating antenna separately. The chosen azimuth angle stepsize is 0.05° to ensure a good resolution on the one hand and to be compatible with radar sectors of 11.25° on the other.

Another important step during the correlation process is the implementation of the TCAS interference limiting and threat assessment algorithm. The power and number of TCAS interrogations depends on the number of other TCAS-equipped aircraft in the vicinity [14]. Furthermore newer TCAS versions allow passive surveillance of another aircraft to some extent, if it does not represent a near-term collision threat [15].

Since the simulation is stationary, the correlation only needs to be done once before the simulation.

5.2 Simulation program flow & algorithm

For the discrete event simulation it is required that the simulation algorithm is time driven and that every event – be it an interrogation, reply or suppression pulse – has an unambiguous time stamp. The sequence of the simulation itself is clocked with a step of $1$ $\upmu$s, which corresponds to a distance of $300$ m, thus being short enough to ensure that any reaction triggered by the processing of an event in the current clock cycle will happen only in later cycles (e.g. an interrogation transmitted at $0$ $\upmu$s will not be received by any transponder before $1$ $\upmu$s). Contrary to the clocks, for the time stamps of the events a precision of $1$ ps is used. This is necessary to ensure the required accuracy. E.g. for correct decoding it is necessary to know which one of two interrogations is received first by the transponder. For the time stamps a 64-bit integer format was chosen instead of a double-precision floating-point format to eliminate rounding and similar issues.

Within a single clock cycle, the necessary operations are divided into 3 separated steps (see also Fig. 9):

• Step #1: interrogating by the the $1030$ MHz transmitters

• Step #2: receiving & replying by the transponders

• Step #3: receiving by the $1090$ MHz receivers

Fig. 9

Simplified structogram of the simulation

The steps are decoupled from each other by two sets of buffers. One could argue that the buffers play the role of the propagation, they save the messages between transmission and reception. The multiplicity of the first set is determined by the number of transponders (real and sim-transponders) and a single buffer holds the uplink messages received by one transponder. The size of the second set is determined by all receivers and in those the received downlink messages are stored for each $1090$ MHz receiver.

The simplified processing in these steps is shown in Figs. 10–12 respectively.

Fig. 10

Simplified structogram of the processing in the interrogator model

Fig. 11

Simplified structogram of the processing in the transponder model

Fig. 12

Simplified structogram of the processing in the receiver model

Since the writing to the buffers is asynchronous – i.e., first one transponder creates it’s replies then the others, in a sequence defined by their order of creation – the messages have to be sorted by their time of reception to ensure causality in the later processing.

The looping over all interrogators, transponders and receivers could be parallelized if the access to the buffers is synchronised since these buffers are the only shared resources.

5.2.1 Step #1 – interrogating

In the course of this step all the interrogations are created and “transmitted” to the transponders which are able to receive them. This is accomplished by looping over the interrogators and determining for each interrogator if an interrogation is to be done in the present clock cycle and – if so – which type of interrogation to send to which transponder. In case of all-calls (Mode A, Mode C or Mode S-only all-call), the interrogation is sent to all transponders with attributes, such as stochastic acquisition or lockout-override, set according to the interrogator’s strategy. A roll-call interrogation requires different processing. If it is addressed to a real transponder it is sent to said transponder but also to all non-addressed ones, regardless of those being real or sim-transponders. On the other hand, a roll-call addressed to a sim-transponder, is only sent to this sim-transponder.

In case of Mode S interrogators, the interrogations in a single roll-call period also have to be scheduled beforehand. This scheduling algorithm must optimize the order of interrogations to fit in as many transponders as possible, while at the same time it has to make sure, that the replies will not cause garbling on reception. This has not been made easier by the fact that the radar manufacturers do normally not publish how they implement this. A solution has been derived based on a recorded interrogation sequence of a real radar.

5.2.2 Step #2 – transponder processing

The processing of the received uplink messages includes checking for received power, sidelobe suppression and garbling. Garbling occurs whenever a part of one message overlaps with a part of another.

A successfully decoded interrogation needing a reply (e.g. not locked-out all-call or addressed roll-call) will trigger a reply, but only in real transponders. In simulation transponders replies are only counted, but not sent to the receivers.

5.2.3 Step #3 – receiving

The processing at the $1090$ MHz receivers is similar to the transponders without the need to send replies.

On a successful reception of a Mode S roll-call reply the next instance of extraction of the received data (altidude, identification, BDS register content) needs to be updated. If there is no reply to a previously sent roll-call interrogation, a re-interrogation must be triggered.

6 Results

There is quite a variety of different results the simulation is able to calculate:

• interrogations received by a transponder, split by mode, uplink format (UF, in case of Mode S) and interrogator, whether it was decoded/not decoded, addressed/not addressed,…

• replies of a transponder, split by mode, downlink format (DF),…

• occupancy of a transponder, split by interrogator type, mode, UF/DF, addressed/not addressed,…

• replies received by a $1090$ MHz receiver, split by mode, DF, decoded/not decoded,…

• probability of detection, split by mode, DF, power, range,…

• …

In total there are more than 500 different variables.

Depending of the simulation module peak-, sum-, average-values and time series are generated.

As an example for a simulation result Fig. 13 shows the heatmap of the number of Mode S interrogations in a 0.1 by 0.1° grid in Cental Europe caused by Austrian and neighbouring radars and the Austrian WAM. Below the heatmap the results of all transponders are presented in table format.

Fig. 13

Exemplary Result as shown in the ESIT UI

7 Outlook

With the factory acceptance tests completed at the end of 2023, further thorough testing and calibration based on verifiable test cases and real-world measurements will initiate the final launch process.

ESIT is intended to go operational in 2024. It will then be available to the member states of EUROCONTROL as a cloud service, and those will also be responsible for keeping the necessary interrogators’ database up to date.