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GMES SPACE COMPONENT
SENTINEL-1, -2 AND -3
Peter G. Edwards, Guido Levrini, François Spoto, Bruno Berruti, Kurt Büchler
European Space Research and Technology Centre (ESTEC), Keplerlaan 1, Postbus 299,
2200 AG Noordwijk (The Netherlands)
Emails: Peter.Edwards@esa.int, Guido.Levrini@esa.int, Francois.Spoto@esa.int,
Bruno.Berruti@esa.int, Kurt.Buechler@esa.int
ABSTRACT
In the frame of the Global Monitoring for Environment
and Security (GMES) programme jointly implemented
by ESA and EC, ESA is developing the Sentinel-1, -2
and -3 satellites as part of the GMES Space Component
(GSC) system. The Sentinel satellites will provide
dedicated services alongside national and other missions
which contribute to GMES with the following
instruments and technologies:
• Sentinel-1: C-band interferometric radar
• Sentinel-2: Multispectral optical imaging
• Sentinel-3: Wide-swath, low-medium resolution
optical and infrared radiometers and a radar
altimeter package
Each Sentinel mission is based on a constellation of 2
satellites in the same orbital plane. The lifetime of the
individual satellite is specified as 7 years (with
consumables for 12). The life cycle of the space
segment is planned to be in the order of 15-20 years. A
description of the satellites and their payload
complement with the planned performance data is
given.
1. THE SENTINEL SATELLITES AND GMES
The objective of the Global Monitoring for
Environment and Security Space Component
programme is to fulfil the space-based observation
requirements in response to European policy priorities
with a particular emphasis on the “fast-track” services
identified by the EC for early implementation. It aims at
the provision of satellite data for these fast-track
services from 2008 onwards and the development,
launch and operations of dedicated infrastructure
(Sentinel satellites) with the corresponding ground
segment developments. The first Sentinel launch is
planned for 2011.
Currently three fast-track services - on land monitoring,
marine services and emergency response - are defined
by the EC’s Fast-Track Implementation Groups. The
Sentinel satellites are tailored for these services. To
fulfill revisit and coverage requirements, to provide a
robust operational service and to be affordable, each
Sentinel mission is based on a constellation of 2
satellites in the same orbital plane. The lifetime of the
individual satellite is specified as 7 years (with
consumables for 12). The life cycle of the space
segment is planned to be in the order of 15-20 years.
The strategy for Sentinel procurement and replacement
over this period is being elaborated, but will likely result
in a need for 4 or 5 satellites of each type if the desired
robustness for the service that GMES will provide is to
be achieved.
2. TECHNICAL KEY ELEMENTS OF THE
SENTINELS
Technical key success criteria for GSC include, aside
from the obvious need to complete the individual
missions to specification, in schedule and within cost,
the following specific issues related to the purpose of
the programme:
• Use of robust, state-of-the-art, technologies which
will minimise development risk and maximise
production schedule reliability, and which are
aimed at maximised cost-efficiency especially for
spacecraft;
• Consideration of the operational basis and life-
cycle of the space segment (15-20 years) and
development of resulting strategies for
procurement, storage, reactivation and launch;
• Use of established (ESA) Earth Observation data
archives, with the aim to maximise the availability
of a long term coherent data set;
• Use and extension of existing ground segment
facilities, sharing them with national missions;
• Commonalization of key spacecraft elements where
possible ( e.g. avionics, software, ground segment
interfaces, tools) to minimise costs;
• Minimisation of the schedule to orbit of the first
spacecraft, to avoid (or reduce) data gaps.
1. SENTINEL-1
Sentinel-1 (Fig. 1) is an imaging radar mission in C-
band (5405 MHz) for marine and land monitoring,
aimed at ensuring continuity of data provision for user
services currently available from ERS2/Envisat,
particularly those initiated within the GMES Service
Element programme.
_____________________________________________________
Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)
Figure 1. Sentinel-1 Configuration
To provide guaranteed weather independent day-and-
night revisit time of 6-days globally and 2 days over
Europe and Canada the Sentinel-1 mission is based on
the simultaneous operation of a pair of identical three-
axis stabilised satellites phased by 180 degrees in a
common orbital plane, with a frozen Sun-synchronous
orbit of 693 km mean spherical altitude, a mean local
solar time at ascending node of 18:00 hours, and
14+7/12 revolutions per day.
The operational scenario foresees the development of
the first spacecraft within Segment 1 and the
procurement of an additional spacecraft within Segment
2 (to complete the mission for coverage reasons).
Further spacecraft procurement will be required in later
segments to cover the target operational service period,
currently estimated at 20 years. The mission is tailored
to 95% monthly averaged product availability
(excluding mission loss).
The Sentinel-1 payload consists of a synthetic aperture
radar operating in the C-band with a centre frequency of
5405 MHz. Sentinel-1 has four nominal operational
modes designed for inter-operability with other systems:
- Stripmap Mode (SM) with 80 km swath and
5x5 metre spatial resolution
- Interferometric Wideswath Mode (IW) with
250 km swath, 5x20 meter spatial resolution
and burst synchronisation for interferometry
- Extra-wide Swath Mode (EW) with 400 km
swath and 25x100 meter spatial resolution (3-
Looks)
These modes are available with selectable dual
polarisation (VV+VH, or HH+HV)
- Wave Mode (WV) with 20x20km swath.
Sampled image mode with low data rate and
5x20 meter spatial resolution.
This mode is available with selected single polarisation
(VV or HH)
Tab. 1 shows all measurement modes, with the primary
SAR imaging mode in colour.
Table 1. Sentinel-1 Data Product Characteristics Stripmap
Mode
(SM)
Interfe-
rometric
Wideswath
Mode (IW)
Extra-
wide
Swath
Mode
(EW)
Wave
Mode
(WV)
Swath
Width
80 km 250 km 400 km 20 km x 20
km
Polarisation VV+VH
or
HH+HV
VV+VH or
HH+HV
VV+VH
or
HH+HV
VVor HH
Spatial
Resolution
(Ground
Range
x Azimuth)
5 x 5 m,
single look
5 x 20 m
single look
25 x 100
m 3-
looks
20 x 5
single look
Sensitivity
(Noise
Equivalent
�˚)
-22 dB -22 dB -22 dB -22 dB
Radiometric
Stability
0.5 dB 0.5 dB 0.5 dB 0.5 dB
Radiometric
Accuracy
1.0 dB 1.0 dB 1.0 dB 1.0 dB
Ambiguity
Ratio
(Distributed
Target)
-22 dB -22 dB -22 dB -22 dB
With its greatly improved revisit (Fig. 2), coverage and
timeliness Sentinel-1 is designed for the provision of
guaranteed data services. A single main operational
mode, the Interferometric Wide Swath Mode (IW) is
designed to satisfy most currently known service
requirements thus avoiding conflicts and preserving
revisit performance. This provides robustness and
reliability of service while simplifying mission planning
and facilitating building up a consistent long-term data
archive. The routine operations would normally not be
interrupted but the system is designed to respond to
emergency requests to support disaster management in
crisis situations.
Data delivery to the end user will be within 1 hour from
ground station reception.
Figure 2. Average Revisit of a Satellite Constellation
with 250 km Swath in 12-175 Orbit
Tab. 2 presents an overview of the Sentinel-1 services.
Table 2. Sentinel-1 Services
GMES
Consolidated
Service
Sentinel-1 Contribution
Polar
Environment
Services
- Glacier and Snow
Monitoring
- Iceberg Monitoring
- Sea Ice Monitoring
- Oil Discharge
Monitoring
- Near Shore Ice
Complex
- Land Monitoring
- Lake Ice Monitoring
- River Ice Monitoring
Marine &
Coastal
Environment
Sea surface winds, currents & waves
Oil spill information services (surveillance,
drift forecasting)
Ship detection services for fisheries and
security
Land
Information
Services
Basic Land Cover
Soil Sealing Map
Forest
Monitoring
Services
Green house gas reporting
Sub-National Forest Information Updates
Mapping and Monitoring of Disturbances
(Clearing, Fires)
Land Cover & Forest Indicators
Geo-hazard
Risk
Management
Historical measurements of ground motion:
subsidence risk
Subsidence and landslide monitoring
(tunnelling project, water table change)
Geological engineering
Flood and
Fire Risk
Operational
Information
Services
Flash flood early warning
Floods rapid mapping
Flood risk analysis
Food Security
Information
Mapping ploughing time and acreage
Mapping planting time and acreage
Mapping cultivated area at harvest/during
growing period
Humanitarian Rapid mapping for out-of-area crises
Aid
operation
Cartography for development and
reconstruction planning
Key features of Sentinel-1:
- Mass 2.1 tons
- Power 5 kW
- Antenna size 10m x 1.4m
- Radar freq. 5405 Mhz
- Data Storage 2TB
- Downlink rate about 500 Mbps
- Near-polar sun synchronous orbit, 12-day repeat cycle
- 06:00 Local Time Descending Node (dawn-dusk)
- Launcher class Soyuz
2. SENTINEL-2
Sentinel-2 is a multispectral imaging mission. Its main
purpose is the continuity of Landsat, SPOT &
Vegetation-type data and the continuity to services for
multi-spectral high-resolution optical observations over
global terrestrial surfaces.
The Sentinel-2 mission (Fig. 3) is based on the
simultaneous operation of two identical satellites phased
by 180 deg. in a common orbital plane in a frozen sun-
synchronous orbit.
Figure 3. Sentinel-2 Configuration
resulting from Phase A/B1
The Sentinel-2 Multi Spectral Instrument (MSI) uses the
push-broom concept with an instantaneous swath width
of 285 km. The instrument features 13 Infrared bands,
10 in the Very Near Infrared (VNIR) and 3 in Short-
wave Infrared (SWIR). Resolution is 20 m in general,
except 60 m for atmospheric correction channels, and
10 m for 4 bands. Fig. 4 shows the spectral bands along
with their respective resolution.
Figure 4. 13 Spectral bands versus
spatial sampling distance
A possible optical configuration for the MSI is shown in
Fig. 5. This is the result of a definition study; the
detailed design has to be selected. The VNIR focal
plane technology will be CMOS or CCD, while for the
SWIR focal plane cooled MCT (Mercury Cadmium
Telluride) detectors (hybridised on CMOS read-out
circuit) will be used.
Figure 5. Possible MSI Overall Configuration
The baseline data acquisition scenario is based on the
permanent utilisation of 2 to 4 ground, with a data
downlink bit rate of 3x150 Mbps during overpass, and
an internal mass memory of 2 Terabits. A Real Time
transmission capability allowing direct data downlink is
also available. The downlink sub-system is a candidate
for commonalization with other Sentinels.
The services provided by Sentinel-2 are shown in the
following Tab. 3.
Table 3. Sentinel-2 Services
GMES Initial
Service
Sentinel-2 Features
Global
Change -
Land
Mapping services for monitoring urban areas
in Europe (urban sprawl, urban planning
modelling & forecasting, changes in urban
land use, environmental monitoring and
enforcement of urban planning discipline
Land cover & Comprehensive information services for
Land use
change
European users with respect to mainly
European policies (Water, Soil, Integrated
Coastal Zone Management, Urban
Environment, Spatial Development)
Monitoring
Forest area / forest area change map
Forest type map
Forest fragmentation
Food Security
early warning
Support to Crop and Food Supply Assessment
Agricultural mapping
Crop Yield assessment
Humanitarian
Aid
Appropriate and reliable application of
geographic information for humanitarian
organisations
Risk
Management
(flood and
fires)
Monitoring of floods, forest fires, volcano
eruptions, subsidence and landslides
Sentinel-2 will provide a large quantity of data, about 6
Terabits of data per day for each satellite. A significant
proportion of these data are however expected to be
cloud-covered, so that an efficient cloud screening is
required before disseminating data from the PDGS to
the service segment.
The satellite is designed to be largely autonomous and
simple to operate: continuous imaging mode over land,
with a period of 2 weeks without reprogramming need
under nominal operations, and acquisition of
housekeeping and science data telemetry over each
ground station pass. Similarly as for Sentinel-1, access
to the satellite is protected by telecommand
authentication, (partial) housekeeping telemetry
encryption and payload data downlinks are encrypted.
Key features of Sentinel-2:
- Mass 1000 kg
- Power 1175 W (GaAs Triple
Junction Cells, Li-Ion
Battery)
- Data Storage 2TB : lossy compression (wavelet
technology)
- Downlink rate about 450 Mbps (X-band)
- Near-polar sun synchronous orbit
- 10:30 Local Time Descending Node
- Launcher class: Vega
3. SENTINEL-3
Sentinel-3 implements 4 core operational missions in
continuity of existing ones, delivering:
- Sea surface topography (SSH) and, significant
wave height (SWH) over the global ocean to an
accuracy and precision exceeding that of Envisat
RA-2.
- Sea and Land surface temperature (SST, LST), at
least at the level of quality of the Advanced Along-
Track Scanning Radiometer (AATSR) instrument
- Visible and Infrared radiances (“Ocean Colour”)
for oceanic and coastal waters, determined to an
equivalent level of accuracy and precision as
MEdium Resolution Imaging Spectrometer
(MERIS) instrument data with complete Earth
coverage in 2 to 3 days, and co-registered with SST
measurements.
- Visible, Near Infrared, Short-Wave Infrared, and
Thermal Infrared radiances (“Land Colour”) for
land surface, with complete Earth coverage in 1 to
2 days, with products equivalent to those derived
from MERIS, A/ATSR and Spot VGT, together
with those from their combination.
The Sentinel-3 mission is based on a medium-size 3-
axis stabilised satellite flying in a sun- synchronous
frozen orbit at 799.8 km altitude, corresponding to 14 +
7/27 orbital periods per day. The orbit repeat period is
therefore 27 days, as required for sea surface
topography sampling. However, the altitude is close to
that of a 4 days short-repeat orbit such that, considering
the wide swath of the optical instruments, the optical
observations can achieve the desired rapid global
coverage. The local time at descending node (LTDN)
will be 10:00 to 10:30, which is the best compromise
between the constraints of the ocean and land colour
observations and those of continuity for the sea surface
temperature observations.
Figure 6. Sentinel-3 Configuration resulting
from Phase A/B1
In trade offs between instrument complexity and
development risk, and considering the robustness
required for an operational system, it has been
determined that the Sentinel-3 mission is best fulfilled
by a constellation of two satellites.
The payload complement of Sentinel-2 fulfils two tasks:
The Topography Mission with
- Bi-frequency Synthetic Aperture Radar Altimeter
(Fig. 7)
Heritage from CryoSat (SIRAL) and Jason, using
the Ku- (13.575 GHz) and C-band (5.41 GHz, for
ionospheric correction in a 320 MHz bandwidth),
with an antenna reflector of 1.2 m diameter. New
features are a SAR mode (along-track SAR
operation with a nadir resolution of ca 300m) and
open-loop tracking, resulting in an improved
monitoring of coastal ocean, ice surfaces and in-
land water.
- Microwave Water Vapour Radiometer (Fig. 8)
Derived from the similar instrument on Envisat,
with several technological updates and features:
o two frequency operation at 23.8 and 36.5
GHz (a 3-frequency concept with 18.7
Ghz is also under evaluation)
o Dicke-type radiometer with internal
calibration (hot load, sky horn)
o Antenna reflector of 0.6 to 0.8 m diameter,
depending on the concept
- Precise Orbit Determination (POD) including
o GNSS (GPS and Galileo) Receiver with 2
cm radial accuracy
o Laser Retro-Reflector
Figure 7. Radar Altimeter Configuration
Figure 8. Microwave Radiometer Overview
The Optical Payload consists of
- Ocean and Land Colour Instrument (OLCI) with
strong heritage from MERIS (Fig. 9) is a
pushbroom-type imaging instrument featuring 5
cameras, arranged cross-track, with a de-pointing of
19o to minimise the sun-glint effect. It observes in
15 bands in the visible/NIR, very similar to the
MERIS bands. The OLCI uses no mechanisms,
except for calibration purposes. The OLCI spatial
resolution at the sub-satellite point is < 300 m, with
an instantaneous swath of 1120 km.. The required
spatial resolution of < 0.3 km at the sub-satellite
point is met; the swath covers the Earth surface in
less than 3 days for the ocean and near 1 day for
land (based on the two satellite constellation).
The Resolution is optimized for observation with
full resolution over Coastal/Land (300 m ), reduced
for the open ocean (1.2 km).
Figure 9. OLCI Camera Arrangement
The Sea and Land Surface Temperature (SLST)
instrument (with strong heritage from AATSR) is a
dual-conical scanning imager operating in the
Visible and Infrared down to TIR and featuring the
following key characteristics:
Dual-view (along-track) in order to
provide the highest quality atmospheric
correction 3 bands;
Continuous day-night operation;
Accurate two-point radiometric calibration
to maintain the long-term accuracy of
AATSR;
One IR channel used for co-registration
with OLCI
- The baseline SLST design (see Fig. 10) is based on
a dual scanning mechanism with dichroic
separation of the beams and a common focal plane
cooled to 65 K. Two blackbodies provide for
accurate calibration.
Figure 10. SLST instrument
The rotation of the two scanners, both with 150 ms
period results in two scan lines defining the dual-
view and single-view swaths, which, for the
selected orbit, are 1675 km and 750 km wide,
respectively.
The OLCI and SLST have overlapping swaths, whose
observations in the common swath are co-registered to
0.1 pixel RMS. This is facilitated by the two
instruments being mounted on a common base plate,
also supporting the star sensors for improved geo-
location (Fig. 11).
Figure 11. Overlapping Swath of OLCI and SLST
Tab. 4 presents the Sentinel-3 Services.
Table 4. Sentinel-3 Services
GMES Initial Service Sentinel-3 Features
Marine and Coastal
Environment
Sea-surface topography
Mesoscale circulation
Water quality
Sea-surface temperature
Wave height and wind
Sediment load and transport
Eutrophication
Polar Environment
monitoring
Sea-ice thickness
Ice surface temperature
Marine Security Ocean-current forecasting
Water transparency
Wind and wave height
Global Change - Ocean Global sea-level rise
Global ocean warming
Ocean CO2 flux
Global Change - Land Forest cover change mapping
Soil degradation mapping
Land cover & Land
use change
Land use mapping
Vegetation indices
Forest Monitoring Forest cover mapping
Food Security early
warning
Regional land-cover mapping
Drought monitoring
Humanitarian Aid Land use mapping
Air Pollution (local to
regional scales)
Aerosol concentration
Risk Management
(flood and fires)
Burned scar mapping
Fire detection
Key features of Sentinel-3:
- Mass 1270 kg
- Power 1100 W
- Data Storage 2TB : lossy compression (wavelet
technology)
- Downlink rate about 300 Mbps
- Near-polar frozen sun synchronous orbit
- 14 +7/27 rev/day (10:00 - 10:30 LTDN)
- Launcher class: Vega
4. STATUS OF SENTINEL-1, -2 AND -3
For all three Sentinels the early development phases A
and B1 are now concluded. Sentinel-1 has finalized the
tender evaluation procedure for the follow-on phases up
to commissioning and has selected the industrial
consortium for the future tasks. The industrial activities
are starting now.
Sentinel-2 and -3 currently are expecting the industrial
proposals for the next phases, to be followed by the
tender evaluation process. The foreseen start of the next
phase for both of them is October 2007.
Major key data for the Sentinel satellites are:
- Preliminary Design Review
o S-1 February 2008
o S-2 October 2008
o S-3 August 2008
- Critical Design Review
o S-1 March 2009
o S-2 Mid 2010
o S-3 February 2010
- Launch
o S-1 November 2011
o S-2 April 2012
o S-3 August 2012
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