Frequently asked questions


EU SST stands for European Union Space Surveillance and Tracking. An SST system is a network of ground-based and space-based sensors capable of surveying and tracking space objects, together with processing capabilities aiming to provide data, information and services on space objects that orbit around the Earth.

EU SST fosters the competitiveness, efficiency and innovation capacity of the European space industry and research institutions by means of open tenders, following the principle of "best value for money".

Therefore, the European industry and research institutions are involved in activities such as data provision, functioning of Operations Centres, development of services, and R&D. Currently, around 80% of the total funding of EU SST is used for this purpose.

More information on the R&D activities performed within EU SST can be found here.

The SST Cooperation, composed by the Member States of the SST Partnership and EUSPA, does cooperate with international organisations, space agencies, and other SSA initiatives. There are also links between the Member States of the SST Partnership and the European Space Agency (ESA), as those Member States are also members of ESA, although ESA is not involved in EU SST.

In addition, EU SST contributes to global burden-sharing and international cooperation in the SSA domain, in particular with the United States of America.

For enquiries or requests for information regarding EU SST, you can contact the EU SST Front Desk. The contact details are available here.


EU SST aims at having full coverage of the GEO and MEO regions for objects larger than 35 cm by 2023. For the LEO region, EU SST aims at covering almost 100% of objects larger than 50 cm and approximately 20% of the objects larger than 7 cm.

EU SST seeks to increase its autonomy at all orbital regimes. To this end, its sensors network is under continuous upgrade and expansion, which improves the EU SST coverage and orbital information accuracy and timeliness.

While there is processing of data across all three functions of EU SST, the role of the Processing function is to collect, store and share all measurements from the sensors contributing to EU SST through the EU SST Database and to use this data to generate the EU SST Catalogue in the future.

The German SSA Centre (GSSAC) is responsible for the EU SST Processing function, in particular for the development, maintenance and operation of both the EU SST Database and the future EU SST Catalogue.

The EU SST Database is a data-sharing platform that connects all national operations centres (OCs) of EU SST. Each OC can upload and download data and information. Apart from storing measurement and orbit data, the Database offers the capability to provide an overview of the network of sensors contributing to EU SST (including information on availability, maintenance, operational status, and sensor parameters) and support the operational daily work regarding tasking requests, where an OC can ask for additional data on specific events such as conjunctions, re-entries or fragmentations.

Every day, the operational sensors contributing to EU SST generate hundreds of thousands of measurements of space objects in all orbital regimes. These measurements are shared through the EU SST Database and are processed into orbits by using a single processing chain. The shared data constitutes the basis for a future EU SST Catalogue of space objects, which will be used as common basis for the provision of EU SST services.

At the moment it is not foreseen that the EU SST Catalogue will be made publicly available once it is being used operationally for the provision of EU SST services. However, discussions are ongoing to make a subset of the EU SST Catalogue publicly available at a later stage.

The Service Provision function is in charge of providing today three SST services to users through the EU SST Portal: Collision Avoidance (CA), Re-entry Analysis (RE) and Fragmentation Analysis (FG). 

The French (FR-SSA) and Spanish (S3TOC) Operations Centres are responsible for the Collision Avoidance service in a hot redundancy scheme, while the Italian Operations Centre (C-SSA) is in charge of the Re-entry Analysis and Fragmentation Analysis services. Furthermore, EUSPA as the EU SST Front Desk is developing and operating the EU SST Portal and providing a Helpdesk to support users.

EU SST provides flexible, tailor-made and responsive services, based on autonomous data (from EU SST contributing sensors) whenever possible, complemented by external data when own measurements are not available. In particular:

  • The CA service allows users to configure the thresholds for risk-level categorisation and adapt them to their concept of operations. It performs Probability of Collision (PoC) sensitivity analysis following the Scaled PoC method, which is considered more accurate than the standard PoC calculation as the first one is able to extrapolate and predict the evolution of the risk for a given conjunction. Besides, the service also provides support to users to mitigate the collision risk and define Collision Avoidance Manoeuvres (CAMs).
  • The RE service monitors and updates on re-entries potentially impacting areas of interest as pre-defined by users.
  • The FG service allows users to characterise a fragmentation event as soon as it is confirmed, and when available, it provides information regarding the fragments’ distribution and evolution.

More information about each service can be found in the EU SST Service Portfolio.

EU SST services are provided free of charge.


The EU SST CA service establishes three levels of risky events:

  • High Interest event (HIE), with ALERT level (the highest level of risk – a mitigation action will be recommended if the risk is in the near future);
  • Interest event, with WARNING level (medium to low level of risk – a mitigation action may be required if the risk increases);
  • Information event, with INFO level (low level of risk).

The criteria and thresholds to define each level of risk are based on time to close approach, Scaled Probability of Collision (PoC), and/or geometry (radial and total miss distances). The thresholds used to provide the service are defined in the Service Configuration Document agreed between the user and EU SST. They are defined by the user in coordination with the responsible OC, tailored to each user’s needs and considerations.

For a more comprehensive explanation of the criteria used in the CA service, please refer to the EU SST Service Portfolio, publicly available here.

The purpose of the Service Configuration Document is to define the operational and technical interfaces between the users and the EU SST Cooperation for the provision of the Collision Avoidance service. An example of this document is available here.

EU SST provides recommendations to support an operator in case of potential collisions, and within these, it can provide different possible mitigation actions to reduce the risk. However, operators are solely responsible for following EU SST recommendations and deciding (or not) to perform a mitigation action, and EU SST cannot be held responsible for damages caused to spacecraft (according to Art. 55 par. 3 of the Regulation (EU) 2021/696 of the European Parliament and of the Council.)

In the first semester of 2021, more than 7,000 close approaches were detected in the three orbital regimes. Out of this number, nearly 190 were considered High Interest Events. Those are events with a high level of risk that require to be further analysed and monitored and that trigger all the mechanisms to activate the EU SST contributing sensors to obtain more accurate data.

Regarding orbital regimes, most of the HIE happened in LEO (nearly 140), followed by the GEO events (40) and the MEO ones (10) where for the first time a Galileo spacecraft made a collision avoidance manoeuvre. More details about the support of EU SST to Galileo can be found here.


In the first semester of 2021, there were 26 events in which at least one EU Member State was included in the re-entry window released (out of 31 events for which EU SST published products). Nonetheless, this does not imply that these objects reached the ground (they could have burned up in the atmosphere), or that the final impact point was on Europe, given the inherent uncertainties in the estimates.

As of July 2021, a total of 3,656 payloads, 3,995 rocket bodies and 17,889 pieces of orbital debris have re-entered into the Earth’s atmosphere1. Since the beginning of the Space Age, this corresponds to an average of one intact object (either payload or upper stage) re-entry every three days, and one piece of debris re-entry every 31 hours.

EU SST monitors the re-entry of rocket bodies and other large objects.

EU SST monitors the uncontrolled re-entries of man-made space objects. The monitoring of natural space objects (such as Near-Earth Objects) belongs to a different area of Space Situational Awareness.


On average, EU SST detects six fragmentation events per year.


Yes. The EU SST services are available free of charge to all registered users.

The EU SST services are provided upon request to all EU Member States, EU institutions, spacecraft owners and operators, and other public and private entities. The current groups of users are defined by the Regulation (EU) 2021/696 of the European Parliament and of the Council.

Access to the EU SST services requires registration in the EU SST Service Provision Portal, followed by an approval process.

To become a registered user and be able to access one or more EU SST services, it is necessary to fill in the registration form available here and agree with the Privacy Statement and Terms and Conditions of Use of the EU SST Portal. After the registration, an email will be sent to confirm the provided email address, upon which the registration request will follow an approval process. If the registration is approved, the user account will be activated. The user will then receive an email confirming the approval, and from that moment the access to the requested EU SST services will be granted.

Information uploaded by the users (such as satellites ephemerides) to the EU SST Portal is only applicable to the CA service and is only made available to the OCs responsible for the provision of that service and the EU SST Front Desk. SST information stored in EU SST Portal is protected in accordance with the applicable EU security regulations and national laws and regulations.

1Space-Track Organization, Box Score, accessed on 3 August 2021 at

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