Space Surveillance and Tracking - SST Segment

Space junk is now one of the principle threats to orbital satellite systems, on which we depend for a multitude of essential services: from meteorology to the global transport of goods and passengers.

It is estimated that more than 700 000 dangerous debris objects are in Earth orbit and have the potential to damage or destroy operational satellites. For many missions, the risk of losing a mission through the impact of space debris is considered to be the third highest risk, after the launch and deployment risks.

In order to avoid the consequences of space debris, we need to know where the debris objects are, which means developing technologies related to surveillance radars and telescopes. As part of the European Space Situational Awareness Programme, ESA is designing a system to track debris and alert satellite operators when evasive action may be necessary.

About tracking space debris

Space Surveillance and Tracking (SST) is the ability to detect and predict the movement of space debris in orbit around the Earth. The data generated through an SST system can be used to actively protect space-based infrastructure, such as Earth observation satellites or navigation systems, from colliding with the ever-increasing cloud of man-made space debris.

Any SST system can be considered – in a very simplified way – as a 'production line' for observation data. Sensors, such as telescopes or radars, look at the sky and produce images of the Earth-orbiting objects that they see. These images are then transformed into plots that describe the path or trajectory of any particular object. Then, the plot must be examined to determine if it is showing a new object, or one already known to the system.

If the object is one that has already been seen, then the observations are used to update the record for that piece of space debris. If the detected plot shows a new object, then the rest of the sensor network is used to try and see this newcomer again and obtain better data of its orbit. It is then added to the catalogue together with all the other observed objects. The total number of objects can be as many thousands of objects, and they all must be seen on a regular basis and very accurately. In this way, the 'production line' converts observations into a catalogue.

ROLEDetect, catalogue and predict the movement of objects orbiting the Earth
COORDINATION CENTRESSTC - Space Surveillance Test & Validation Centre, ESAC, Spain
  • Now: existing European radar & optical systems that can scan near-Earth and geostationary orbits
  • Future: dedicated surveillance radar supported by optical sensors, all coordinated with international partners
+ Prediction of potential collisions between orbiting satellites and debris; reentry analysis; detect on-orbit explosions; assist missions at launch, deployment and end-of-life; reduce cost of space access +

SST Segment

The core of the SST Segment is the catalogue; this contains information on everything that has been detected in orbit. In order to produce this catalogue, it is necessary to:

  1. Reconstruct object orbits from the data that are produced by the sensors (orbit determination)
  2. Check to see if this object has already been seen and is already in the catalogue (correlation)
  3. Monitor the data in the catalogue so that sensors can be tasked to update the information when needed
Future SSA radar system - artist's impressionAccess the image

Once a comprehensive catalogue of all potentially dangerous objects that orbit Earth has been created, this is maintained so that this data can be used by a diverse range of end customers. The main use will be for the prediction and warning of collisions between operational satellites and other satellites or pieces of space junk. The system will be designed to automatically calculate the possibilities of collision between the thousands of objects in the catalogue and then warn satellite operators of any potential risks. This warning contains information which will help the satellite operator make the best decision of how to protect their satellite, by maneuvering out of the way, in the most efficient way; saving fuel, safeguarding operational time and extending the life of the satellite.

Observing reentries

Another application for the gigabytes of data in the catalogue is to detect when large pieces of space debris reenter the atmosphere. If these pieces are big enough, they could potentially survive to hit the Earth’s surface and pose a risk to people, industry and national infrastructures. With the right information generated by a future SST system, warnings can be given to national governments of the potential so that they can take a balanced response to this threat and ensure the required level of safety.

Boosting science for cubesats

In addition to these two important applications, the catalogue and sensor network within the SST system can be used to detect when explosions of defunct satellites or rocket bodies happen on orbit, enabling a rapid evaluation of the new situation and helping satellite operators take contingency measures. The data can also be used to help small satellites, which do not have a dedicated tracking network, to fix where they are, thus assisting scientists and researchers run their missions without having to procure an additional expensive system.

SST team

Space Surveillance Test & Validation Centre (SSTC), ESAC, SpainAccess the image

ESA's Space Surveillance and Tracking team is based at the Space Surveillance Test and Validation Centre (SSTC) located at ESAC, ESA’s European Space Astronomy Centre, and is supported by an extensive network of scientists and engineers across Europe. The team are also supported by experts within the Agency as well as by contributions from existing European centres of expertise.

The SST precursor services are taking advantage of the numerous resources that exist across Europe. For data acquisition, data from radar and optical systems will be tasked to obtain both routine and ad hoc observations of both satellites and debris. These sensors are provided by national space agencies, European defence ministries, academia and private entities.

Since space surveillance is a global concern, the SSTC also works closely with international partners. ESA has a long history of technical cooperation with agencies outside of Europe; the availability of international channels – such as the data provided by the US Air Force – greatly adds to SSTC capabilities.

SST system development overview

Test and validation radar Access the image

During the 2009-12 Preparatory Phase of ESA's SSA programme, important steps were taken towards building the foundations of a future European capability. These included the development of precursor applications to serve as a test bed for the novel techniques and algorithms needed for the SST system that will collate and deliver orbital object data both accurately and affordably. In parallel, the actual performance of a future system was calculated, together with the design and distribution of the sensors that are necessary to scan the skies for potential hazards.

In the course of the first phase of the programme, a lot was accomplished to set the foundations for future, incremental developments towards a fully operational system. Currently, the framework for the data processing chain – transforming sensor observations into catalogued objects – has been developed. This is complemented by the generation of ESA’s first user applications, which provide both conjunction prediction and re-entry warnings to interested parties. Both of these systems are accessible at

SST segment activities 2013-16

The SST segment activities for the second period (2013-16) of the SSA programme are divided into seven specific domains:

  1. Evaluation of the enhancements required to improve the systems developed in the SSA Preparatory Phase
  2. Development of SST systems
  3. Research and development within the area of satellite laser ranging (SLR)
  4. Research and development within the areas of optical surveillance
  5. Research and development for the security aspects of the SST system
  6. Enhancement and exploitation of the deployed IT infrastructure
  7. Technical support specific to SST activities

These activities will be carried out in a step-wise approach during the next phase (Period Two) of the SSA Programme, seeking to correctly understand the conditions for each subsequent step and reducing the potential risk on time, scope and financing as the programme progresses.

The sensors

EISCAT facilities SwedenAccess the image

The sensors used in SST can be divided into two main types: surveillance and tracking.

Surveillance sensors

A surveillance sensor is the workhorse of a surveillance system. It provides the data for both the initial catalogue development (the so-called ‘cold start’) as well as the day-to-day maintenance of the catalogue.

The main difference between the tracking and surveillance sensors is that the surveillance sensor sees a very large area of the sky at one time. It is also not actively looking for objects, but rather passively (which counter-intuitively can be 'active') waiting for debris – any debris – to pass over. Once it detects something passing over it, the data related to this pass is processed and passed to the catalogue maintenance system.

In this way, the surveillance sensor creates a ‘fence’ that is triggered by any object passing through it. No prior information is needed by the sensor to generate new data regarding any specific debris object and the system therefore does not need to be ‘tasked’ to look out for an object. In reality, the fence can also be generated using an active sensor scanning the sky with a frequency that ensures nothing will be missed. This is the case for radar systems which quickly scan across a path. It doesn't look in all directions at all times, but still forms an effective fence.

Through the use of surveillance sensors, a catalogue can be built up. The precision of this catalogue will not be very high initially, although the design of the surveillance network should be such that the eventual precision using just the surveillance assets will be enough to give a reliable warning of potential collisions with operational satellites. When the warning is triggered, then comes the turn of the tracking sensors to refine the orbit of this debris and provide the precise information that satellite operators need to plan their manoeuvres.

Tracking sensors

Tracking sensors usually have a very small field of view; given a fixed detector performance, the smaller the field of view, the more precise the locations of the objects detected within this field of view can be known. This is particularly useful when you want to increase the precision of an object's location when you already have some orbital data, such as when a catalogued piece of debris (whose orbit is roughly known) may collide with an operational spacecraft. You simply take this rough orbit and set your tracking sensor to point along this orbit at the position you think the debris should be. When you see the debris, you can then obtain a more precise orbit fix – since your detector is looking at a very small region of space and so has a high precision.

The problem is – of course – that since you only see a small area of the sky, if the error on your rough orbit is too high, you might not see the debris at all (it might slip by outside your field of view). It also makes these sensors very inefficient (read: almost useless) for the build-up a catalogue of objects. Since the view is small, it is difficult to trap new objects, unless you are very lucky. Even then, given the small view, you only have a very short reading as the debris passes across the sensor. This results in an initial orbit guess (orbit determination) which can have very high errors. For the development and maintenance of a catalogue, an effective system also needs a surveillance sensor.

Governance and data policy

In order to ensure that the right people receive the right information at the right time, a system of governance and data management is needed. This will check the user who is asking for information to make sure that they are authorised to receive data about a specific object and protect the catalogue and the applications from unauthorised modification, substitution or use.

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