Investigations of the Feasibility of a
European Space Surveillance System
Heiner Klinkrad1, Therese Donath2 and Thomas Schildknecht3
1
ESA Space Debris Office, Darmstadt, Germany
2
ONERA, Châtillon, France
3
AIUB, University of Bern, Bern, Switzerland
Abstract
Europe presently does not have an autonomous space surveillance capability which can
meet user requirements. Critical operations, including conjunction event forecasts and reentry predictions of risk objects, completely rely on information of the two-line element
catalog provided by the US Space Surveillance Network (SSN). This information in many
instances is incomplete, of limited detail and of insufficient accuracy to perform reliable orbit
predictions and risk assessments.
Since several years the European Space Agency has been investigating the feasibility of a
European space surveillance system, as part of a more comprehensive system for space
situational awareness. Following an overview of existing European assets, the present paper
will outline a ground-based space surveillance concept consisting of a bi-static LEO radar
with a continuous wave (CW) phased-array transmitter and a separate phased-array receiver,
and a global network of tracking and tasking telescopes to cover the MEO and GEO regions.
The anticipated performance of this system, derived from radar and telescope system models
and from simulations of the data processing chains, is close to that of the US SSN in terms of
detectable size and catalog completeness in the different orbital regimes.
Introduction
At present, the only nations with an operational space surveillance capability and with a
routinely updated space object catalog are the USA and Russia. In early 20061 the number of
unclassified objects in the catalog of the US Space Surveillance Network (SSN) was on the order of
10,000. These catalog objects are typically larger than 10 cm in low-Earth orbit (LEO, below 2,000
km altitude), and larger that 1 m in geostationary orbit (GEO, at altitudes close to 35,786 km). This
is due to sensor sensitivity, which decreases by 1/16 when doubling the target distance to a radar,
and with 1/4 when doubling the target distance to a telescope. Hence, radars are primarily used for
LEO, and optical systems are mainly used for GEO surveillance and tracking. With 75.7% the vast
majority of catalog objects reside in the LEO region, below 2,000 km altitude. Another 8.7% of the
catalog orbits are in or near the GEO ring at 35,786 ± 2,000 km altitude, within latitudes of ±17°.
The remainder of the catalog mainly belongs to the medium Earth orbit region (MEO), which also
contains the near-circular, semi-synchronous GPS and GLONASS constellation orbits near 20,000
km altitude.
Peak concentrations of catalog objects exist at altitudes of 800 km to 1,000 km, and around
1,400 km. Peaks in the latitude distribution are located between 65° and 82°. As a consequence, a
zenith staring electronic fence deployed in Europe at 50°N is still able to observe almost 80% of the
entire US SSN catalog population. Due to the sparsely populated inclination bands below 50°, the
1
All population data in this article shall refer to Jan. 2006
coverage would only improve by about 5% when moving the fence 20° to the south. On the other
hand, it would decrease by 20% when moving it 20° to the north. Hence, European latitudes are a
good compromise between possible coverage of the orbit population and frequent station passes.
Requirements of a European Space Surveillance System
”Space surveillance” can be defined as the routine, operational service of detection, correlation,
characterization, and orbit determination of space objects. Some required core functions and
associated performance criteria of a future European Space Surveillance System (ESSS) were
defined by a Space Surveillance Task Force. These ESSS functions comprise the following
elements:
• full coverage of LEO, GEO and 12 hour, near-circular MEO orbits; limited coverage of orbits
outside these regions
• autonomous build-up and maintenance of a catalog of all observable space objects
• detection, tracking, orbit determination, target correlation, and physical characterization for
objects in LEO, MEO and GEO with a reliability and sensitivity matching the one of the US
Space Surveillance Network (d > 10 cm in LEO, and d > 1 m in GEO)
• estimation of orbit maneuvers
• detection of on-orbit break-up events and correlation with the source object(s)
These requirements were used in work statements for ESA contracts with European industry to
define sensors and to devise data processing schemes of a European Space Surveillance System
which meets these criteria. The major findings of these studies are summarized in this paper (see
[1], [2], [3], [4], [5]).
Existing European Sensors
Existing Radar Sensors
Apart from the USA (supported by their Space Surveillance Network, SSN), only Russia has an
operational space surveillance capability (supported by their Space Surveillance System, SSS). In
the former case, the corresponding unclassified catalog comprises orbit and characterization data of
about 10,000 objects (status in Jan. 2006). In the latter case, the catalog comprises fewer data, due
to the lack of SSS sensors at lower latitudes. Also France has an experimental surveillance system:
the bi-static GRAVES installation (Grand Réseau Adapté à la Veille Spatiale). The related catalog
is limited to objects of typically 1 m size and larger in low Earth orbits (LEO), with a total count of
about 2,200. In addition, there are European sensors which track known objects with higher
accuracy, and sensors which acquire detailed statistical information on small-size objects (see [6],
[9]).
The most powerful space surveillance sensor in Europe is located at Fylingdales (UK), and is
operated by the British military. Most of its activities are geared to the US Space Surveillance
Network (SSN) early warning and space surveillance mission. The Fylingdales complex consists of
a high performance 3-face, phased-array radar operating in the UHF-band. A second facility, which
is associated with the US SSN, is the Norwegian Globus II radar. It is located at Vardø, at the
northernmost tip of Norway. Globus II is an X-band mono-pulse radar, with a 27 m parabolic dish
antenna, housed in a radome of 35 m diameter. Due to the special bi-lateral agreements between the
US SSN and the operators of Fylingdales and Globus II, data from these sites have not been
available for unclassified use within Europe.
The French GRAVES system is presently the only European installation outside the US SSN
which can perform space surveillance in its classical sense. GRAVES is owned by the French
Ministry of Defense, and is operated by the French Air Force. The concept of GRAVES is based on
VHF transmitters with four planar phased-array antennas of 15 m × 6 m each, which are located
near Dijon. These tilted antennas are arranged in a south-facing semi-circle to deploy a conical
detection fan up to altitudes of about 1,000 km. Objects which pass through the detection volume
reflect a fraction of the transmitted power, which is then received by a planar phased array of dipole
antennas, arranged in a circular area of 60 m diameter, located at Apt, 380 km south of the
transmitter. The GRAVES system determines orbital element sets from measurements of direction
angles, Doppler, and Doppler rates for a large number of simultaneous targets. GRAVES produces
a ”self-starting” catalog which can be autonomously built up and maintained. The detection size
threshold up to 1,000 km altitude is on the order of 1 m. The system has started its routine
operations in 2005.
The German FGAN Radar belongs to the Research Establishment for Applied Science at
Wachtberg. It is a mono-pulse tracking and imaging radar (TIRA) with a parabolic dish antenna of
34 m diameter, housed in a 49 m diameter radome. The radar uses L-band for tracking at 1.333
GHz, with 1 MW peak power, and Ku-band for Inverted Synthetic Aperture Radar (ISAR) imaging
at 16.7 GHz, with 13 kW peak power. In its tracking mode, the TIRA system determines orbits
from direction angles, range, and Doppler for single targets. The detection size threshold is about 2
cm at 1,000 km range. For statistical observations this sensitivity can be enhanced to about 1 cm,
when operating TIRA and the nearby (21 km away) Effelsberg 100 m radio telescope in a bi-static
beam-park mode with TIRA as transmitter and Effelsberg as receiver. TIRA’s range-Doppler ISAR
imaging in Ku-band produce images with range resolutions better than 7 cm.
DGA/DCE, the Systems Evaluation and Test Directorate of the French Ministry of Defense, is
operating several radar and optical sensors throughout France. The most powerful of these systems,
Armor, is located on the tracking ship Monge. The two Armor C-band radars (5.5 GHz, 1 MW peak
power) are dedicated to tracking tasks, based on high resolution angular and range data.
The Chilbolton radar is located in Winchester/UK. It is operated by the Rutherford Appleton
Laboratory (RAL). This monopulse S-band (3 GHz) radar with a 25 m parabolic dish antenna is
mainly used for atmospheric and ionospheric research. With a planned upgrade the radar will be
able to track LEO objects down to 10 cm sizes at 600 km altitude.
EISCAT is a network of European Incoherent Scatter Radars, with sites at Tromsø/Norway,
Kiruna/Sweden, Sodankylä/Finland and Longyearbyen/Svalbard. The EISCAT system is mainly
used for high latitude ionospheric research. Its radar echoes, however, also contain information on
LEO space objects. The Tromsø transmitter/receiver site with its 32 m antenna is able to detect
objects down to 2 cm sizes at altitudes of 500 to 1,500 km. Since these measurements are
insufficient to determine complete orbits, EISCAT is only of limited value for space surveillance,
but it may contribute to space situational awareness.
Existing Optical Sensors
Optical sensors are mostly used to observe objects beyond LEO altitudes, particularly near the
important GEO ring. This is due to the higher sensitivity of telescopes at long ranges, as opposed to
radars. However, as a disadvantage versus radars, optical observations are constrained in their
useful observation times with regard to target illumination, observer location and meteorological
conditions. Europe has several optical systems that could contribute to a surveillance and tracking
network (see [6], [9]).
ESA operates a Space Debris Telescope of 1 m aperture with a 0.7° field of view (FoV), which
is located on Tenerife. It uses a 2 × 2 mosaic of CCDs of 2048 × 2048 pixels each, with a detection
threshold of +19 to +21 visual magnitude (corresponding to 15 cm objects at GEO altitudes). The
telescope covers a sector of 120° of the GEO ring. From single observations, initial orbits can be
derived which are generally adequate for re-acquisition of the object within the same night, and
which can then be successively improved.
CNES uses observation time of the TAROT telescope (Télescope à Action Rapide pour les
Objets Transitoires) to survey the GEO ring. TAROT’s primary mission is to detect the optical
afterglow of gamma-ray bursts. The telescope has a 25 cm aperture, and a field of view of 2° × 2°.
It is equipped with a CCD of 2048 × 2048 pixels for detections and follow-up measurements of
objects up to visual magnitude +17 in and near the GEO ring. A companion telescope, TAROT-S
has been deployed in Chile.
Since 2006 the British National Space Centre (BNSC) sponsors the Starbrook wide-field
telescope as an experimental survey sensor. The telescope is located at Troodos/Cyprus, has an
aperture of 10 cm, a field of view of 10° × 6°, and a CCD of 4008 × 2672 pixels. It can detect GEO
objects down to 1.5 m sizes (visual magnitude of +14).
The Astronomical Institute of the University of Bern (AIUB) is operating the ZIMLAT
telescope, with an aperture of 1 m, and a field of view of 0.5°. From its location at Zimmerwald it
covers a sector of 100° of the GEO ring. A CCD of 2048 × 2048 pixels allows detecting objects up
to visual magnitude +19. The primary applications of ZIMLAT are astrometry and Laser ranging.
However, up to 40% of its night time observations are used for follow-ups of GEO objects
discovered by the ESA telescope. ZIMLAT was complemented in 2006 by the 20 cm ZimSMART
telescope (Zimmerwald Small Aperture Robotic Telescope). It is dedicated to GEO survey, using a
CCD of 3056 × 3056 pixels with a field of view of 4.2°.
The existing radar and optical sensors have already demonstrated their suitability as stepping
stones for a future European space surveillance and space situational awareness system.
Feasibility Assessment of a European Surveillance System
Space Surveillance denotes the task of systematically surveying, tracking and correlating all
objects above a certain size, and of maintaining a catalog with timely updates of orbital and
physical characteristics of these objects.
While some European radar and optical facilities exist for tracking and imaging space objects,
Europe has no systematic, operational capability for space surveillance, and it is hence strongly
dependent on external information, predominantly originating from the Space Surveillance Network
(SSN) of the USA.
Ground-Based LEO Surveillance Concept
About 69% of the US SSN catalog is related to LEO objects, in altitudes below 2,000 km. A
survey of existing sensors, in combination with findings from several ESA studies (see [1], [2], [3]),
has led to the following core recommendations for a European Space Surveillance System (ESSS)
radar design to survey the LEO region (see Fig. 1):
• radar design: bi-static continuous-wave radar operating at 435 (alternative: 891 MHz)
• transmitter: 4 phased arrays of 26 elements each with 16 kW transmitted power
• receiver: 4 phased array receivers, of 1,500 elements each
A move to this frequency of 435 MHz was necessary due to regulatory problems in allocating a
600 MHz frequency slot (former recommendation). This change incurred an increase in the required
transmitter power by 35%, to 3.5 MW, and some added complexity in the radar design. A higher
frequency of > 891 MHz, outside the blocked frequency window, is under consideration to improve
the detection threshold. The achievable cataloging performance, however, remains close to the 600
MHz frequency option. It is expected that 98.6% of the US SSN Catalog and 96.1% of the more
complete MASTER-20012 debris environment population larger than 10 cm (see [7], [8]) can be
2
MASTER = Meteoroid and Space Debris Terrestrial Environment Reference model (ESA)
detected and cataloged for a radar range of 1,400 to 1,500 km (for 435 and 891 MHz). An extended
range of 1,700 km will increase the detection rate by less than 0.5%, due to the sparsely populated
altitudes in this regime. The simulations done for the performance evaluation are based on
ONERA’s Surveillance System Simulation (S3) software, run over a simulation time span of one
month.
7.5°
Transmitting
panel
Sca Sca Sca Sca Sca
n1 n2 n3 n4 n5
Sca
n6
20°
Performs
sequential
scanning
and electronic
beam forming
Receiving
panel
45°
Performs
simultaneous
digital beam
forming
Fig.1: Concept of the proposed bi-static LEO surveillance radar (image: ONERA).
For the location of the bi-static LEO radar two sites in Spain are recommended: Pico Villuercas
in Extremadura for the transmission, and the Arenosillo military base in Andalucia for the reception
(380 km south of the transmitter). Spain constitutes a near-optimal deployment location due to
sufficiently frequent sensor passes with acceptable observation gap times.
Ground-Based GEO Surveillance Concept
About 9% of the US SSN Catalog is associated with objects in or near the GEO region, with
detection size thresholds of about 1 m. Due to large distances to the targets the survey of this family
of orbits is normally done with optical instruments. In an assessment study two different types of
sensors were identified to be necessary (see Fig. 2):
• detection and tasking: 0.5 m telescopes with a wide field-of-view of 3° × 3°
• survey: identical 0.5 m telescopes (alternative: 1 m aperture with a field-of-view of 1.2° × 1.2°)
The GEO sensors should be uniformly distributed in longitude, at sites of low latitude, with
acceptable meteorological and seeing conditions. A first network of sensors could consist of 3 sites:
Tenerife, Perth, and the Marquesas Islands. Each site would be equipped with a 0.5 m detection and
tasking telescope, and with a 0.5 m survey telescope (alternative: 1m survey telescope). The
coverage of the GEO ring from these sites is about 85%. It can be extended to 95% when adding a
fourth site at Cyprus.
The proposed GEO survey strategy implies a continuous coverage of a stripe (or “fence”) of
±17º in latitude, centered on the equator. Each GEO object crosses this stripe once within 24 hours.
As a single sensor cannot cover this stripe continuously (due to visibility constraints and night time
limitations), a network of low latitude optical sensors equally distributed in longitude is required.
The observation strategy uses a combination of survey observations (searching for new objects) and
tasking observations (for initial orbit determination and orbit improvement).
Fig.2: Concept of a proposed GEO observation telescope (image: AIUB).
Ground-Based MEO Surveillance Concept
About 2% of the US SSN catalog population is related to MEO objects in the vicinity of the 12
hour, near-circular orbits of navigation satellite constellations. This small, yet important population
of space objects can be monitored down to size thresholds of 0.3 to 1.0 m by means of two
dedicated survey telescopes of aperture 0.8 m, with a field of view of 4.7° × 4.7°, located at
Tenerife and on the Marquesas Islands. The proposed 0.5 m GEO tasking telescopes could also
provide the needed MEO tasking capability. The US SSN catalog MEO population near the 12 hour
near-circular constellation orbits is expected to be cataloged to 95% within 2 months.
Surveillance of Orbits outside LEO, MEO and GEO
The share of objects in the US SSN catalog, which do not belong to the LEO, GEO, or nearcircular MEO class, is approximately 20%. In an operational space surveillance system the survey
of this class objects would have second priority in view of the limited return for the spent effort.
Surveillance Capabilities of On-Orbit Sensors
Preliminary assessments suggest that the most promising space-based telescope scenario would
be a survey of the GEO region from a Sun-synchronous low Earth orbit. The proposed telescope
has an aperture of 20 cm and a conical field of view of 6°. The detection threshold would be at
visual magnitudes of +15.8 in GEO. This observation concept was successfully demonstrated by the
SBV (Space-Based Visual) sensor of 15 cm aperture on the American MSX satellite. For a limited
time it contributed up to 20% of the SSN GEO catalog population (~2% of the total SSN catalog).
Assessment of Cataloging and Data Processing Performances
To assess the performance of a generic space surveillance system, an Advanced Space
Surveillance System Simulator (AS4) was developed (see [3]). It uses radar and telescope system
models to translate sensor-specific field-of-view crossings of orbital objects into estimated
instrument detection rates for a space object population according to ESA’s MASTER-2001 space
debris environment model (see [7]). This MASTER-2001 population consists of 17,800 ”real”
objects larger than 10 cm (as opposed to about 10,000 US SSN catalog objects of the same size), of
which approximately 55% are in the LEO regime.
The AS4-simulated catalog build-up process is self starting, with no initial information required.
In a test the simulator was applied to 20,000 LEO tracks, with 4,500,000 measurements, covering
93% of the observable LEO population larger than 10 cm. After 1 day of simulated radar operations
more than 90% of the test population was cataloged, with a steady state level of 98.5%. This
corresponds to 96% of the reference MASTER-2001 population in LEO. In 99% of all cases
correlation could be achieved within 9 hrs (mean) to 25 hrs (max.). Correlation errors could be
maintained at a level of about 1%. A minimum of one detection of 10 s duration is required for each
object in order not to loose it from the catalog. The final product is a database with identification
and characterization data for each unique object, estimated orbital parameters, and information on
the orbit determination uncertainty. The achievable orbit determination position accuracy is on the
order of 1 to 10 m in LEO, and 10 to 1,000 m in GEO and MEO regions.
Dual-Use Requirements on Space Surveillance Data
The term ”dual use” in context with space surveillance data refers to civilian users, and military
or state authorities. The requested information of the latter user community can be of a different
nature and/or of a larger extent than the data of the former users. The main requirements of a
European Space Surveillance System (status: 2006) can be summarized as follows:
• autonomous catalog build-up and maintenance capability
• extended object characterization attributes, including mission objectives and capabilities
• traceability of data and products at all processing levels
• well defined accuracy and timeliness of data products
• well-defined data policy concerning dissemination and sharing
• data security, confidentiality, integrity, and high availability
• procedures for tasking and prioritizing sensors in response to requests
• incorporation of available national sensor capabilities
The proposed space surveillance system can be considered as a first and major building block
towards a “Space Situational Awareness” (SSA) capability, where SSA can be defined as the
understanding and maintained awareness of the Earth orbital population, the space environment,
and possible threats.
Conclusions and Outlook
The feasibility studies on the system design and operational concepts which are summarized in
this paper were performed with the intention to define a modular space surveillance system, which
can be gradually composed of sub-system building blocks with proven, low-risk technologies, and
which can be used as a starting point for a more comprehensive space situation awareness system.
The proposed radar system for LEO surveillance could be deployed in Spain, with a transmitter
in Extremadura and a receiver 380 km to the south, in Andalucia. These locations meet all
requirements on radiation safety, power supply, and security. In a first step a demonstration radar
could be deployed, consisting of one array with 9 transmitters of 0.28 MW power in total, and one
reception array of 220 antennas. In a second step the single transmitter array could be upgraded to
28 elements of 0.87 MW power, and the single receiver array could be extended to 1,500 antennas.
Finally, the ESSS surveillance radar could be completed to its full capacity with 4 transmitting
arrays of 28 transmitters each (requiring 3.5 MW power), and with 4 receiver arrays of 1,500
antennas each. The 3-phase ESSS radar development is foreseen to extend over a time span of 5
years. The system is expected to detect, track, correlate and catalog 98% of the LEO population of
the US SSN catalog, with verifiable performances and accuracies. First operational test campaigns
might begin 3 years after the start of the development of a European Space Surveillance System.
The proposed optical surveillance system could initially be composed of 3 survey telescopes of
0.5 m aperture (alternatively 1.0 m), deployed at Tenerife (E), Perth (AUS), and the Marquesas
Islands (F). In this pre-operational configuration the 3 sensors can detect, track, and correlate 87%
of the US SSN GEO catalog. This coverage may be extended to 95% when including a fourth
telescope on Cyprus during the operational phase. Each of these telescopes should be co-located
with a 0.5 m tasking telescope. This network of 4 × 2 telescopes should be augmented by two
additional 0.8 m telescopes, dedicated to the survey of MEO objects near the 12 h navigation
satellite orbits. These sensors are proposed to be located at Tenerife and on the Marquesas Islands.
They can share the co-located 0.5 m tasking telescopes. The cataloging performance is expected to
reach 89% to 95% of the US SSN MEO catalog population for near-circular 12h orbits.
A promising concept for space-based surveillance of the GEO region appears to be a 20 cm
telescope deployed on a Sun-synchronous low Earth orbit at about 800 km altitude.
Existing national and ESA assets could be used to test and validate critical technologies and data
processing concepts during the development and deployment of a European Space Surveillance
System. They could subsequently be employed for dedicated surveillance and/or reconnaissance
tasks, for space situation awareness applications, and for dedicated national investigations and
database maintenance in a stand-alone or cooperative fashion.
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