gmes sentinel-1 mission

Remote Sensing of Environment 120 (2012) 9–24

Contents lists available at SciVerse ScienceDirect

Remote Sensing of Environment

j ourna l homepage: www.e lsev ie r .com/ locate / rse

GMES Sentinel-1 mission

Ramon Torres a, Paul Snoeij a,⁎, Dirk Geudtner a, David Bibby a, Malcolm Davidson a, Evert Attema a,Pierre Potin a, BjÖrn Rommen a, Nicolas Floury a, Mike Brown a, Ignacio Navas Traver a, Patrick Deghaye a,Berthyl Duesmann a, Betlem Rosich a, Nuno Miranda a, Claudio Bruno b, Michelangelo L'Abbate b,Renato Croci b, Andrea Pietropaolo b, Markus Huchler c, Friedhelm Rostan c

a ESA/ESTEC, The Netherlands and ESA/ESRIN, Italyb Thales Alenia Space Italia, Italyc EADS Astrium GmbH, Germany

⁎ Corresponding author.E-mail address: [email protected] (P. Snoeij).

0034-4257/$ – see front matter © 2012 Elsevier Inc. Alldoi:10.1016/j.rse.2011.05.028

a b s t r a c t
a r t i c l e i n f o
Article history:Received 20 December 2010Received in revised form 14 April 2011Accepted 7 May 2011Available online 18 February 2012

Keywords:CSARSpace borneC-BandSatelliteRemote SensingGMESConstellation

In the frame of the Global Monitoring for Environment and Security (GMES) Space Component programme,the European Space Agency (ESA) undertook the development of a European Radar Observatory (Sentinel-1),a polar orbiting two-satellite constellation for the continuation and improvement of SAR operational servicesand applications. Satellite and payload are being built to provide routine, day-and-night, all-weather medium(typically 10 m) resolution observation capability. Ground infrastructure is provided for planning, missioncontrol, data processing, dissemination and archiving. Free and open data access is provided. Data qualityof the Sentinel-1 data products is shown along with uncertainty estimation of retrieved information productsconfirming specified performance and indicating application growth potential. The unique data availabilityperformance of the Sentinel-1 routine operations makes the mission particularly suitable for emergency re-sponse support, marine surveillance, ice monitoring and interferometric applications such as detection ofsubsidence and landslides.

© 2012 Elsevier Inc. All rights reserved.

1. Introduction

The short-lived but successful US SEASAT satellite provided afirst glimpse of the potential of imaging radar from space in1978. ESA's own programme to develop advanced microwaveradar instruments culminated with the launches of ERS-1 (17July 1991) and ERS-2 (20 April 1995). ERS demonstrated for thefirst time the feasibility of flying reliable, stable and powerfulradar imaging systems in space. The dependability and all-weather capability of the instruments also provided a foundationfor developing and exploiting radar images for a wide variety ofapplications. While the initial objectives for ERS-1 at launchwere predominantly oceanographic, other applications were con-sidered during the project's preparation. The ESA Remote SensingAdvisory Group in 1974, for example, emphasised commercial ap-plications such as agriculture, land-use mapping, water resources,overseas aid and mapping of mineral resources in its advice onERS objectives. The rigorous design of the ERS SAR hardware –

emphasising instrument stability in combination with accurateand well-calibrated data products – created new opportunities

rights reserved.

for scientific discovery, revolutionised many Earth science disci-plines and laid the foundations for commercial applications.

For example, ‘SAR interferometry’, which can track land shifts ofonly a few millimetres, was developed mainly using ERS data and isnow commonly used in Earth sciences and commercial applications.The potential of space radars viewing the same scene only a shorttime apart was demonstrated in 1995 and 1996 during the ERS ‘tan-dem mission’, when the orbits of ERS-1 and ERS-2 were carefullymatched but with a 1-day gap.

An important milestone was the launch of the Advanced SAR(ASAR) on Envisat on 28 February 2002. This ensured the contin-uation of C-band data and added enhanced capabilities such aswide swaths and dual polarisation, features that have since rapid-ly been integrated into and exploited by many applications. Thearchive of radar data since 1991 is extremely valuable for scienceand applications, providing a consistent set of data spanning16 years.

The ESA Sentinels constitute the first series of operational satel-lites responding to the Earth Observation needs of the EuropeanUnion (EU) — ESA Global Monitoring for Environment and Securityinitiative. The GMES Space Component (GSC) programme reliesstrongly on new complementary developments by ESA in additionto existing and planned space assets from different agencies. As partof the GSC, ESA is currently undertaking the development of 3

http://dx.doi.org/10.1016/j.rse.2011.05.028

mailto:[email protected]

http://dx.doi.org/10.1016/j.rse.2011.05.028

http://www.sciencedirect.com/science/journal/00344257

Table 1Sentinel-1 mission requirements.

Data availability1 In order to avoid service provision gaps Sentinel-1 operations shall start be-

fore the end of the operational lifetime of ERS/Envisat and last for a period ofat least 10 years.

2 Unless specified otherwise performance shall satisfy ERS/Envisatrequirements.

3 End-to-end product availability to the end user shall typically be better than95%, based on a main mode of observation and on systematic operations withon-demand observations in exceptional cases.

4 Coverage, revisit and exact repeat observation necessitate the use of non-ESAsatellites. Therefore similarity of data products and observation geometry isrequired as well as ground segment interoperability.

5 All products shall be processed to Level-0 and Level-1 and archived.Note: Level-0 is reformatted, time-ordered satellite data, in computer-compatible format. Level 1-products are geo-located products in which datahas been converted to engineering units, auxiliary data has been separatedfrom measurements, and selected calibrations have been applied to the data.

6 Data shall be delivered to the user within the timeliness specification from thearchive or in near real time (NRT).

Coverage and revisit7 For interferometry global exact repeat coverage shall be achieved bi-weekly

(e.g. with an interval of less or equal 14 days).8 Fast global access on demand shall be provided within the timeliness

specification.9 Daily full coverage shall be achieved north of +45° latitude north and south of

−45°.Note: A constellation of 4 satellites would satisfy the correspondingobservation requirement. As the proposed concept initially includes only twosatellites an average revisit once every two days is achieved. Consequentlycollaboration and interoperability with non-ESA satellites would be required.

Timeliness10 Near real time data shall be delivered to users within 3 h of observation.11 User request for archived data shall be completed within typically 24 h and in

no case within more than 3 days after issuing of a user request.12 Emergency operations shall be completed typically within 24 h and in no case

within more than 48 h after issuing of a user request.

Characteristics of data products13 The centre frequency shall be selected within 5250–5570 MHz (C-Band).14 The system shall be designed to support interferometry.15 A 240 km swath width follows from revisit requirements for the main mode of

observation.16 A Wave Mode shall be provided with 20×20 km imagettes every 100 km

along track17 The main mode of observation shall have both VV and VH polarisation.18 Optional additional modes shall be provided including a mode with both HH

and HV polarisation.

10 R. Torres et al. / Remote Sensing of Environment 120 (2012) 9–24

Sentinel mission families.1 Each Sentinel is based on a constellation of2 satellites in the same orbital plane. This configuration allows to fulfilthe revisit and coverage requirements, and to provide a robust and af-fordable operational service. The life time of the individual satellites isspecified as 7 years, with consumables on-board each satellite allow-ing a mission extension up to 12 years. The life cycle of a satellite gen-eration is planned to be in the order of 15–20 years.

2. The Sentinel-1 mission

Sentinel-1 is designed to work in a pre-programmed conflict-freeoperation mode, imaging global landmasses, coastal zones, sea–ice,polar areas, and shipping routes at high resolution, and covering theglobal ocean with imagettes. This ensures a reliability of service re-quired by operational services and a consistent long-term data archivebuilt for applications based on long time series. Sentinel-1 revisit andcoverage are dramatically improved with respect to the ERS-1/2 SAR

1 In addition, two units of Sentinel-4 instruments to be embarked on Meteosat ThirdGeneration (MTG) meteorological satellites and a Sentinel-5 Precursor mission havebeen adopted as part of the GSC Programme.

and ENVISAT ASAR. The two-satellite constellation offers six day exactrepeat and conflict-free operations based on two main operationalmodes allowing exploiting every single data take. In comparison withits predecessors the Sentinel-1 mission represents an n-fold evolutionof capability as expressed by the figure-of-merit developed inSection 9.5. In the framework of international interoperability agree-ments the effective revisit and coverage performance may be furtherimproved by access to the planned Canadian C-band SAR Constellation(RADARSAT Constellation Mission, see for example Flett et al. (2009)).

Observation requirements from GMES services are defined in termsof data availability, coverage and revisit, timeliness and characteristicsof data products. The service requirements for C-band SAR observationswere translated into 18 Sentinel-1 mission requirements shown inTable 1, providing the basis for the mission design, Attema (2005).

To deal with user requirements for both high and medium resolu-tion data conventional SAR system designs include different opera-tional modes that either optimise the spatial resolution (at theexpense of the swath, hence the coverage) or the swath width (atthe expense of the resolution). Taking account of data access throughGMES to complementary national very high-resolution SAR missions(TerraSAR-X by DLR/Astrium GmbH, Cosmo-SkyMed by ASI)Sentinel-1 has been designed to address primarily medium to highresolution applications through a main mode of operation that fea-tures both a wide swath (250 km) and high geometric (5 m×20 m)and radiometric resolution. Over sea–ice and polar zones or certainmaritime areas, an extra-wide swath mode may be used to satisfythe observation requirements of certain service providers (e.g. sea–ice monitoring), in order to ensure in particular wider coverage andbetter revisit time by sacrificing geometric and radiometricresolution.

As the operation follows a pre-programmed conflict-free scenariothere is no need to make data acquisition requests for these modes.The imaging modes can be operated for a maximum of 25 min perorbit. The remaining time the instrument operates over the openocean in the Wave Mode providing sampled images of 20×20 km at100 km along the orbit with low data rate.

It is expected that Sentinel-1A will be launched mid 2013 andSentinel-1B about 18 to 24 months later.

3. Sentinel-1 satellite

Sentinel-1 (see Fig. 1) is being realised by an industrial consortiumlead by Thales Alenia Space Italy as Prime Contractor, with AstriumGermany being responsible for the CSAR payload, incorporating thecentral radar electronics sub-system developed by Astrium UK.

Fig. 1. Artist impression of Sentinel-1 in orbit.

11R. Torres et al. / Remote Sensing of Environment 120 (2012) 9–24

The spacecraft is based on the PRIMA (Piattaforma Italiana MultiApplicativa) Platform with a mission specific payload module, inthis case the CSAR Instrument. The design of the satellite leveragesexperience gained from the Canadian RadarSat-2 and the ItalianCosmo-Skymed programmes for which PRIMA was also used.

3.1. Platform

The spacecraft designed for the Sentinel-1 mission is a 3-axis stabi-lised satellite, directly placed into the Earth orbit by the launcher. It ischaracterised by sun, star, gyro and magnetic field sensors, and a setof 4 reaction wheels, dedicated to orbit control and to attitude controland 3 torque rods as actuators, to provide steering capabilities on eachaxis. The spacecraft (S/C) is equipped with 2 solar array wings capableof producing 5900W (at End of Life) to be stored in a modular battery.

The Platform provides highly accurate pointing knowledge (betterthan 0.004°) on each axis, high pointing accuracy (about 0.01° oneach axis), and real time orbit determination together with a dedicat-ed propulsion system for precise orbit control. The reference orbitwill be maintained within an Earth-fixed orbital tube of a diameterof 100 metre-rms during the nominal mode operation time. The sta-tistical definition of the orbital tube implies the comparison of theperturbed orbit with respect to the reference orbit at the same argu-ment of latitude. Sentinel-1A and Sentinel-1B shall have the same ref-erence orbit with a 180 degree orbital phasing difference.

Computer, avionics and the satellite communication backbone arebased on the MIL-1553 command bus with the ERC32-SC processorproviding high performance computing and processing capabilitieson-board.

The total mass of the spacecraft at launch is around 2300 kg.The Sentinel-1 spacecraft main characteristics are shown in Table 2.The Platform provides features for the management of the Atti-

tude and Orbit Control Systems (AOCS), Data Handling and Commandand Control functions of the Spacecraft and all the necessary support-ing functions.

The main Platform functions are:

● AOCS including:○ Precise attitude measurements○ Stable and precise 3 axis pointing○ Precise orbit determination

● Propulsion:○ Orbit injection○ Orbital tube maintenance

Table 2Sentinel-1 spacecraft characteristics.

Lifetime 7 years (consumables for 12 years)

Orbit Near Polar Sun-Synchronous @ 693 km;12-day repeat cycle; 175 orbits per cycle

Mean local solar time 18:00 at Ascending NodeOrbital period 98.6 minMax eclipse duration 19 minAttitude stabilisation 3 axis stabilisedAttitude accuracy 0.01° (each axis)Nominal flight attitude Right lookingSteering Zero Doppler yaw steering and roll steering

(−0.8 to 0.8°)Attitude profile Geo-centric and geodeticOrbit knowledge 10 m (each axis, 3 sigma) using GPSOperative autonomy 96 hLaunch mass 2300 kg (incl. 130 kg monopropellant fuel)Dimensions (stowed) 3900×2600×2500 mm3

Solar array average power 5900 W (End-of-Life)Battery capacity 324 AhSpacecraft availability 0.998S-Band Telemetry, Tracking &Command (TT&C) data rates

64 kbps Telecommand; 128 kpbs/2 MbpsTelemetry (programmable)

Launcher Soyuz from Kourou

○ Attitude control during Ultimate Safe Mode○ End of life disposal

● Spacecraft Data Handling including:○ Telecommand reception, handling and distribution○ Telemetry collection, packetisation and forwarding to ground

by TT&C communication equipment○ Time and synchronisation signals management also in support

of payloads● Spacecraft autonomy and failure detection identification and

recovery:○ Payload and spacecraft status monitoring, management and

recovery○ S/C Operations Planning support (Mission Time Line and Sub

schedule) housekeeping telemetry and command● Power:

○ Power generation and storage○ Power conditioning, protection, distribution and switching○ Pyrotechnic devices actuation

● Thermal control:○ Temperature monitoring○ Heat generation and removal

● Structure:○ Primary structural support and interface on the launch vehicle○ Appendages tie-down points and release mechanisms

● Communication with Ground:○ Spacecraft telemetry transmission and telecommand reception

via S-Band transponder○ Instrument telemetry transmission via X-Band Payload Data

Handling Terminal

The Platform general architecture is based on three main structuremodules and functionally decoupled to allow the parallel modules in-tegration and testing activities up to the satellite final integration.

The modules are:

● the Service Module carrying only Platform units apart from thepropulsion ones;

● the Propulsion Module carrying all the propulsion items con-nected by propellant lines;

● the Payload Module carrying al the payload equipment includingthe pertinent appendages.

Most of the Propulsion module is enclosed in the Service module,being integrated into the cone section interfacing the S/C to thelauncher adaptor, while the Payload module is mounted onto the Ser-vice module allowing the Payload (P/L) units/appendages allocationthrough four lateral panels and the upper platform.

3.2. Payload Data Handling Terminal

The PDHT comprises all functions necessary for the acquisition,storage and handling of data generated by different sources:

● CSAR data● GPS raw data (or auxiliary data) and S/C telemetry data

In addition to data storage and handling functions the PDHT pro-vides data formatting and transmissions capability of the data to theground stations.

Direct downlink transmission capability is available for all the ac-quisition modes of the CSAR in single or dual polarisation.

The PDHT is build up from the following subsystems: Data Storageand Handling Assembly (DSHA), Transmission Assembly (TXA) andX-band Antenna Assembly (XBAA).

The on-board memory has been sized to 1410 Gb. The memoryhas been designed to enable simultaneous storage of data from thetwo polarisation channels of the CSAR instrument and from


Housekeeping Telemetry (HKTM), and auxiliary data for data ac-quired during at least two orbits.

The PDHT storage equipment is based on solid-state memory de-vices. The number of packet stores has been defined in order toallow prioritising of the stored data during downlink activities withat least 4 levels of priority, and temporarily storing data to be trans-mitted in direct downlink.

The main tasks of the TXA are:

● Provide the necessary performance to meet the communicationlink requirements;

● Provide full redundancy and cross-strapping for each function;● Receive discrete and serial telecommands to allow its configura-

tion in flight and for on-ground testing;● Provide discrete and serial telemetry to allow its control in flight

and for on-ground testing;● Interface the satellite power bus and generate the necessary

power supply interfaces for TXA equipments.

The X-Band Antenna Assembly provides transmission to ground ofthe X-Band modulated signals received by the TXA. One of the maintasks of the XBAA is that of limiting the radiation of RF energy outsidethe nominal beam coverage, to avoid interference with the spacecraftenvironment and especially with the CSAR antenna.

The PDHT is able to support the mission operations (data storageand data downlink) by performing:

● data acquisitions from the CSAR potentially for the full extent ofthe orbit duration and on all orbits;

● a continuous dump equal to 20 min;● an overall dump duration within any 100 min window of up to

30 min.

4. Sentinel-1 CSAR instrument

4.1. Modes of operation

Following the Sentinel-1 mission requirements one main opera-tional imaging mode the Interferometric Wide-swath mode (IW) is

Fig. 2. CSAR antenna folded around the platform.

provided that, in combination with the Wave mode (WV) satisfiesmost currently known service requirements. Additionally the mutual-ly exclusive Strip Map mode (SM) and the Extra Wide-swath mode(EW) are provided for continuity reasons (with respect to ERS andEnvisat) as well as for accommodation of emerging user require-ments. Two mutually exclusive dual polarisation modes are providedfor the imaging modes. A summary of the SAR instrument measure-ment modes and their characteristics is given in Table 3.

Except for the Wave Mode, which is a single polarisation mode(selectable between HH and VV), the CSAR instrument supports oper-ation in dual polarisation (HH+HV, VV+VH) implemented throughone transmit chain (switchable to H or V) and two parallel receivechains for H and V polarisation. The specific needs of the four differentmeasurement modes with respect to antenna pointing require theimplementation of an active phased array antenna.The CSAR antennafolded around the platform is presented in Fig. 2.

To meet the demanding image quality and swath width require-ments, the IW mode is implemented as a ScanSAR mode with pro-gressive azimuth scanning. This requires a fast antenna beamsteering in elevation for ScanSAR operation, i.e. transmitting a burstof pulses towards a sub swath. In addition, fast electronic azimuthscanning has to be performed per sub swath using the Terrain Obser-vation by Progressive Scan (TOPSAR) operation (De Zan and MontiGuarnieri (2006)) in order to harmonise the performance in alongtrack direction and reduce scalloping. Hence, the IW mode willallow combining a large swath width (250 km) with a high geometricresolution (5 m×20 m on ground). Interferometry is ensuredthrough sufficient overlap of the Doppler spectrum (in the azimuthdomain) and the wave number spectrum (in the elevation domain).

The Wave mode data product is composed of single strip map op-erations with an alternating elevation beam (between 23 and 36.5mid incidence angle) and a fixed on/off duty cycle, which results inthe generation of vignettes of 20 km×20 km size with a 5 m×5 mon ground resolution in regular intervals of 100 km.

In Strip Map mode the instrument provides coverage with a highgeometric resolution (5 m×5 m on ground) over a narrower swathwidth of 80 km. Six overlapping swathes cover the access range of375 km. For each swath the antenna has to be configured to generatea beamwith fixed azimuth and elevation pointing. Appropriate eleva-tion beam forming has to be applied for range ambiguity suppression.

Finally the Extra Wide-swath mode covers an ultra-wide swathwidth of 400 km at medium resolution (20 m×40 m on ground),which is achieved through the implementation of a ScanSAR modewith a fast beam scanning capability in elevation. As in the case ofthe IWmode the Extra Wide-swath mode is implemented with a pro-gressive azimuth scanning (TOPSAR operation).

The three imaging modes are high bit rate (HBR) modes, while thewave mode is a low bit rate (LBR) mode. The four operational modesof Sentinel-1 are illustrated in Fig. 3.

4.2. Instrument architecture and key parameters

The CSAR instrument is a synthetic aperture radar operating in theC-Band using horizontal and vertical polarisation for transmit and re-ceive. A functional block diagram of the instrument is given in Fig. 4.The CSAR instrument is composed of two major subsystems

● the SAR Electronics Subsystem (SES)● the SAR Antenna Subsystem (SAS)

The radar signal is generated at base band by the chirp gener-ator and up-converted to C-band within the SES. This signal isdistributed to the High Power Amplifiers inside the ElectronicFront End (EFE) Transmit/Receive Modules (TRMs) via thebeam-forming network of the SAS. Signal radiation and echoreception is realised with the same antenna using slotted

image of Fig.�2

Table 3Main characteristics of the Sentinel-1 nominal measurement modes.

Parameter Interferometric Wide-swath mode (IW) Wave mode (WV) Strip Map mode (SM) Extra Wide-swath mode (EW)

Polarisation Dual (HH+HV, VV+VH) Single (HH, VV) Dual (HH+HV, VV+VH) Dual (HH+HV, VV+VH)Access (incidence angles) 31°–46° 23°–37° (mid incidence angle) 20°–47° 20°–47°Azimuth resolution b20 m b5 m b5 m b40 mGround range resolution b5 m b5 m b5 m b20 mAzimuth and range looks Single Single Single SingleSwath >250 km Vignette 20×20 km >80 km >410 kmMaximum NESZ −22 dB −22 dB −22 dB −22 dBRadiometric stability 0.5 dB (3σ) 0.5 dB (3σ) 0.5 dB (3σ) 0.5 dB (3σ)Radiometric accuracy 1 dB (3σ) 1 dB (3σ) 1 dB (3σ) 1 dB (3σ)Phase error 5° 5° 5° 5°


waveguide radiators. In receive, the echo signal is amplified bythe low noise amplifiers inside the EFE TRMs and summed upusing the same network as for transmit signal distribution. Afterfiltering and down conversion to base band inside the SES, theecho signal is digitised and formatted for recording.

Table 4 provides a brief overview on the instrument keyparameters.

The key design aspects of the CSAR Payload can be summarised asfollows:

● Active phased array antenna providing fast scanning in elevation(to cover the large range of incidence angles and to support Scan-SAR operation) and in azimuth (to allow use of TOPSAR techniqueand meet the image quality requirements)

● Dual channel Transmit and Receive Modules and H/V-polarisedpairs of slotted waveguides (to meet the polarisationrequirements)

● Internal Calibration scheme, where transmit signals are routedinto the receiver to allow monitoring of amplitude/phase varia-tions for later correction in the processing, leading to high radio-metric stability.

● Metallised Carbon Fibre Reinforced Plastic radiating waveguidesto ensure good radiometric stability outside the internal calibra-tion scheme (Fig. 5).

Fig. 3. Sentinel-1 ope

● Digital Chirp Generator and selectable receive filter bandwidths toallow efficient use of on board storage considering the groundrange resolution dependence on incidence angle

● Flexible Dynamic Block Adaptive Quantisation to allow efficientuse of on-board storage and minimise downlink times with negli-gible impact on image noise.

The up-conversion and amplificationmodule of Sentinel-1 is givenin Fig. 6, while a three dimensional view of the SAS Tile, which is theelementary building block of the SAR Antenna Subsystem, is given inFig. 7.

Important to mention is also the roll steering mode of the space-craft. This continuous roll manoeuvre around orbit (similar to yawsteering in azimuth) compensates for the altitude variation suchthat it allows usage of a minimal number of different pulse repetitionfrequencies (PRFs) and sample window lengths (SWLs) around theorbit. Only a single PRF and SWL are necessary per swath/sub-swatharound orbit, with the exception of swath 5 of the Strip Map modewhere two different PRFs have to be used. In addition, the updaterate of the sampling window position around orbit is minimised(b1/2.5 min), leading to significant simplification of instrument oper-ations. Since the instrument can work with a single fixed beam foreach swath/sub-swath over the complete orbit, the number of eleva-tion beams is minimised. The roll steering rate is fixed at 1.6°/27 km

rational modes.

image of Fig.�3

Fig. 4. CSAR instrument functional block diagram.


altitude variation. The roll applied to the sensor attitude depends lin-early on altitude and varies within the interval−0.8° (minimum sen-sor altitude) to 0.8° (maximum sensor altitude).

5. Sentinel-1 PDGS and CSAR processor

5.1. PDGS main functions

The Sentinel-1 Payload Data Ground Segment (PDGS) provides re-ception of CSAR instrument data, systematic processing, archivingand dissemination of operational SAR data for the GMES services,and carries out various planning and monitoring tasks including

Table 4Instrument key parameters.

Parameter Value

Centre frequency 5.405 GHzBandwidth 0 … 100 MHz (depending on

the mode and sub swath)Polarisation HH+HV, VV+VHAntenna size 12.3 m×0.821 mRF Peak Power (sum of allTRMs, at TRM o/p)

4368 W

Pulse width 5–100 μs (programmable)Transmit duty cycleMax 12%Strip map 8.5%Interferometric Wide-swath 9%Extra Wide-swath 5%Wave 0.8%Receiver noise figure at TRM input 3.2 dBPulse repetition frequency 1000–3000 Hz (programmable)ADC sampling frequency 300 MHz (real sampling) (Digital

down-sampling after A/D conversion)Sampling 10 bitsData compression Selectable according to FDBAQ, see

Attema et al. (2010).Instrument operation Up to 25 min per orbit continuously

in any of the imaging modes and forthe rest of the orbit in Wave mode

Instrument mass 945 kgCSAR DC power 3870 W (Interferometric Wide-swath Mode,

two polarisations)CSAR data storage capacity 1410 Gb (End-of-Life)Effective CSAR downlink data rate Two channels of 260 Mbps each

CSAR instrument activities and X-Band downlink operations, on thebasis of a systematic pre-defined observation scenario.

5.1.1. Data reception and ingestionCSAR real-time sensed data as well as data played back from the

on-board PDHT are downlinked directly to ground or via the Europe-an Data Relay Satellite (EDRS), received, down-converted, de-modulated and transferred to the Processing facilities for systematicgeneration and archiving of Level-0 and Level-1/2 data products.This function also includes frame synchronisation and reconstructionof the CSAR data (Instrument Source Packets, ISP) in a ComputerCompatible Format (CCF).

5.1.2. Data processingPayload Level-0 data are processed to produce Level-1 and Level-2

products applying all the necessary processing algorithms and for-matting techniques. The desired products are produced systematical-ly or on request either in near real time (i.e. within 3 h after sensing)for time-critical data and within 24 h from sensing for all acquireddata. Processing management capabilities and re-processing capabil-ities are provided as well as the possibility to re-assemble “partial”

Fig. 5. SAS tile, front view with H/V-polarised pairs of slotted waveguides.

image of Fig.�4

image of Fig.�5

Fig. 6. Up-conversion and amplification module of Sentinel-1.


Level-0 products (i.e. covering only part of an acquisition segment orpart of a dual-polarisation acquisition segment) resulting from frag-mented downlinks over different stations (or over the same stationin different passes).

5.1.3. ArchivingArchiving and long-term preservation of data is ensured for all

Level-0 data and for a set of configurable systematic higher-levelproducts. In addition a configurable mid-term (months-years) stor-age capability is provided for the products systematically generatedto fulfil GMES Service needs. This function includes all operations tobe put in place to store these data and ensure their integrity accordingto the applicable requirements.

5.1.4. Data dissemination and exchangeGMES Sentinel-1 products are provided to the user typically by

electronic server access (e.g. on-line data provision) and data is ex-changed within and amongst the PDGS physical Centre(s) andStation(s). Near real time dissemination is allocated to the receivingStation(s) and less time-critical data dissemination is allocated tothe Assembly, Processing and Archiving Centre(s).

5.1.5. Mission planningDetailed payload sensing and downlink schedules are defined and

exchanged with the GMES Sentinel-1 Flight Operations Segment(FOS). SAR instrument parameters tables required to configure theinstrument operations are also transferred to the FOS for uplink tothe spacecraft. Satellite contact schedules are transmitted to the re-ceiving ground stations.

Fig. 7. Three-dimensional vie

5.1.6. Mission access and user interfaceSeveral different capabilities are provided for permitting and sup-

porting successful handling of submitted user queries. These includemanagement of user access, user information management, docu-mentation and Help Desk.

5.1.7. Mission performance assessment and data quality controlCapabilities are provided for mission performance assessment and

data quality control, ensuring that data quality and instrument per-formance meet the expected requirements. These include verifica-tion, calibration and validation.

5.1.8. Monitoring and controlTools are provided to ensure that all available resources (i.e. hard-

ware, software and network) required to operating the PDGS accord-ing to its functional and performance requirements are operatingnominally.

5.2. Sentinel-1 CSAR Level-1 and Level-2 data products

The Sentinel-1 CSAR Instrument Processing Facility (IPF) can gen-erate the following Level-1 (L1b) products from the four acquisitionmodes:

● Slant Range, Single-Look Complex (SLC)● Ground Range, Multi-Look, Detected (GRD)● Browse (BRW).

The products are further classified according to their resolutioninto:

● Full Resolution (FR) (SM mode)● High Resolution (HR) (SM, IW and EW mode)● Medium Resolution (MR) (SM, IW, EW and WV modes).

Table 5 gives an overview of the characteristics of the GRDproducts.

The IPF supports the processing of dual-polarisation data. The fol-lowing algorithms are implemented in the Sentinel-1 IPF are:

● Pre-Processing algorithms● Doppler Centroid Estimation algorithms● SLC Processing algorithms● GRD Processing algorithms.

The Sentinel-1 CSAR IPF will also be able to generate Level-2 oceanproducts, both from wave mode data and from high bit rate modedata (SM, IW and EW modes). Level-2 ocean products include infor-mation on ocean swell spectra, ocean wind and currents (in theform of radial surface velocity). The exact content of the Level-2ocean products depends on the specific Sentinel-1 operational mode.

w of the SAS tile layout.

image of Fig.�6

image of Fig.�7

Table 5Sentinel-1 CSAR L1 ground range detected products characteristics.

SM IW EW

S1 S2 S3 S4 S5 S6

Swath width (km) 80 80 80 80 80 80 250 400GRD FR Ground range resolution (m) 8.1 8.4 8.8 9.0 9.2 9.2

Azimuth resolution (m) 8.1 8.7 8.8 8.9 8.9 9.1Total number of looks 4.0 4.0 4.0 4.0 4.0 4.0Pixel spacing (m) 4.0 4.0 4.0 4.0 4.0 4.0Size MByte/5 min of acquisition 19,495 19,495 19,495 19,495 19,495 19,495

GRD HR Ground range resolution (m) 21.4 22.2 23.1 23.7 24.2 24.4 20.3 49.3Azimuth resolution (m) 21.3 23.0 23.2 23.6 23.4 24.0 21.8 50.4Total number of looks 36.0 36.0 36.0 36.0 36.0 36.0 5.0 3.0Pixel spacing (m) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 25.0Size MByte/5 min of acquisition 3143 3143 3143 3143 3143 3143 9744 2499

GRD MR Ground range resolution (m) 78.0 80.7 84.1 86.4 87.9 88.7 87.8 91.1Azimuth resolution (m) 77.3 83.8 84.4 85.7 85.0 87.4 89.2 88.3Total number of looks 484.0 484.0 484.0 484.0 484.0 484.0 110.0 12.0pixel spacing (m) 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0Size MByte/5 min of acquisition 128 128 128 128 128 128 392 629


5.3. TOPSAR specific processing

The Sentinel-1 IW and EW modes acquire wide-swath data (com-posed of 3 and 5 sub-swaths respectively) using the TOPSAR imagingtechnique. This requires TOPSAR-specific algorithms to deal with theantenna steering rate and the Doppler Centroid (DC) rate due to thesteering. The required azimuth pre and post-processing of the datainclude de-ramping of the data prior to base band DC estimation, az-imuth ambiguity estimation and GRD azimuth processing.

The Sentinel-1 IW and EW SLC products contain one image persub-swath and one per polarisation channel, for a total of 3 (singlepolarisation) or 6 (dual polarisation) images for IW data and 5 (singlepolarisation) or 10 (dual polarisation) images for EW data.

Each sub-swath image consists of a series of bursts, where eachburst has been processed as a separate SLC image. The individually fo-cused complex burst images are included, in azimuth-time order, intoa single sub-swath image, with black-fill demarcation in between. Dueto the one natural azimuth look inherent in the data, the imaged groundarea of adjacent bursts will only marginally overlap in azimuth — justenough to provide contiguous coverage of the ground (Fig. 8). The im-ages for all bursts in all sub-swaths of an IW and EW SLC products arere-sampled to a common pixel spacing grid in range and azimuth.Burst synchronisation is ensured for both IWand EWproducts. The pro-cessing is phase preserving.

5.4. Dual-polarisation processing

The processing steps required for the dual-polarisation data pro-cessing are essentially the same as those required for the processingof single-polarisation data, and generally speaking each polarisationis processed in the same manner. However, there are a few additionalconsiderations when processing dual-polarisation data:

1. A separate image is generated for each polarisation, andpolarisation-specific correction and calibration factors will be ap-plied during the processing of each polarisation (for example, cal-ibration pulses, antenna patterns, etc.).

2. The same estimated Doppler Centroid (DC) must be used for pro-cessing both polarisations, so that the two resulting images areaccurately co-registered. Therefore the DC estimated from theco-polarisation channel will be used, for DC estimation.

5.5. The slicing concept

As a continuation of ENVISAT ASAR services Sentinel-1 is required togive users systematic access to long product strips similar to the

currently disseminated ASARWide SwathMode and ImageMode prod-ucts today systematically. The product size for L1 imagingmodeswouldhowever be difficult to manage at the user side (Table 6).

Therefore the level-1 image mode products are segmented in“slices” of defined length along track, optimised per mode and producttype. The level-1 slices cover a sub-set of the data take in along-track di-rection and the complete data take area in the across track direction.

● Slices are referred to the start of each acquisition segment.● Slices are in the nominal product type projection (slant-range for

SLC, ground range for GRD).● Slices are stand-alone products and can be handled separately in

terms of archiving and dissemination.● Slices are seamlessly “concatenable” into a continuous product or

“strip” covering up to the complete data take.● Slice concatenation may be performed before dissemination by

the PDGS or after dissemination by the user.

6. Sentinel-1 operation and observation capabilities

6.1. Mission operation concept

The Sentinel-1 mission is designed to provide ‘guaranteed dataservices’ for which liability can be accepted commensurate with theuser needs following the GMES public institutional user model. Thisis common practice for meteorological data provision. Most acquisi-tions can be pre-planned and the routine operations are normallynot interrupted. Therefore no specific acquisition requests are re-quired and data access is mainly by subscription supporting multi-temporal observations and near real time product generation. Thisis particularly important for responding to emergency requests tosupport disaster management in crisis situations as no time is wastedfor satellite commanding and conflict resolution. However, if specialoperational modes are required the system is designed to respondat the expense of slightly increased response time. All data is system-atically processed and available within 24 h. Data can be also re-trieved off-line from the archive.

6.2. Spacecraft operations concept

The following list summarises the main characteristics of theSentinel-1 mission that determine the Operations Concept:

● The Sentinel-1 spacecraft is designed such that its on-board re-sources allow storing the complete instrument schedule coveringa maximum time interval of 4 days.

IW SLC IW SLC de-burst

Fig. 8. IW SLC de-burst azimuth overlap.


● In terms of on-board autonomy, the satellite can operate nominal-ly for at least 96 h without any ground intervention.

● The resulting routine Operations Concept is characterised by thefollowing main aspects:○ With the exception of emergency response acquisitions the

spacecraft will operate according to a pre-planned schedule ofcommands (a predefined mission timeline similar for everycycle, with few updates during the year to cope with seasonalvariations e.g. for sea–ice and iceberg monitoring) uploadedfrom ground.

○ The ground operator will acquire housekeeping data via theTT&C ground station network. The downlinked housekeepingdata from the platform and the payload will be processed toverify the spacecraft health;

○ Real time transmitted SAR data will be received by X-band sta-tions and via the European Data Relay Satellite System (EDRS);

○ The recorded SAR data will be downlinked every orbit using anetwork of X-band stations and via EDRS. The data dump willbe managed via a predefined sequence of scheduled telecom-mands stored on-board.

○ Updates to the mission timeline can be generated regularly toincorporate for instance changes to instrument settings aswell as non-nominal specific requests for instrument sensingor data download.

6.3. Flight operations segment concept

The FOS provides the capability to monitor and control the space-craft during all mission phases. This includes the facilities for the gen-eration and uplink of the mission schedules, and for the systematicreception, processing, archiving and analysis of the acquired satellite

Table 6Sentinel-1 data volume examples.

Data volume in GByte for dual polarisation

Segment length [min] Slice length[sec]

SM IW SM IW

Length 2 10 2 10 25 25SLC 36 182 30 150 8 6GRD HR 3 13 8 39 1 2

housekeeping telemetry. Moreover, the FOS includes a Flight Dynam-ics System facility responsible for orbit determination and prediction,and generation of attitude and orbit control telecommands. Other FOSfunctions comprise the monitoring and control facilities for the Track-ing, Telemetry and Telecommand ground station network.

The FOS provides spacecraft monitoring and control services tosupport the mission exploitation according to specifications and re-quirements previously introduced. More specifically, the FOS imple-mentation shall:

● offer flexibility and scalability capabilities to integrate comple-mentary functions for evolving requirements (e.g. extension toemergency services)

● offer efficient data exchange between FOS and PDGS● for each Sentinel mission, support the progressive build-up and

operations management of the satellite constellation in particularby provision of orbit determination and maintenance services

● guarantee a high degree of reliability and availability in order toensure the spacecraft safety under nominal and failure scenarios.

6.4. Sentinel-1 observation scenario

Following Sentinel-1 mission requirements, as outlined inSection 2, into account a Sentinel-1 observation scenario has been de-veloped based on systematic global coverage to secure the referenceimage archive and with sufficient resources to accommodate specificemergency acquisitions. The main output of the Sentinel-1 mission isa dependable SAR image data flow and following a systematic, pre-programmed and conflict-free operational scenario with time-critical data dissemination in near real time and all data availablewithin 24 h. Special efforts are made to support rapid mapping foremergency response services. Global average response time is largelydefined by orbit configuration and swath width and amounts in theworst case to about 5 days for a single satellite and 2.5 days for the 2-satellite constellation. The actual performance depends on the locationof the satellite(s)with respect to the emergency site and on latitude andis worst at the equator and best at high latitude.

In its main operational IW mode operating up to 25 min per orbit,it potentially provides complete global coverage of all relevant landsurfaces, sea ice, coastal zones and North Atlantic shipping routes atleast once per 12 days for each of the two satellites. For high priorityareas within Europe, Canada and the Northern Atlantic coverage will

image of Fig.�8

Fig. 9. Sentinel-1 coverage after 12 days for one satellite with the IW mode.


be more frequent ranging from 4 to 2 days per satellite depending onlatitude (see Fig. 9).

Over the open ocean, except the North Atlantic, data are normallycollected in the wave mode operating all the time. This provides im-ages of 20 by 20 km separated by 100 km for use in data assimilationin global wave models.

In order to satisfy the service requirements for Sentinel-1 geo-graphic coverage and temporal re-visit, 2 satellites (Sentinel-1A and1B) are required.

Table 8Sentinel-1 user products — processing concept and timeliness.

Processingconcept

Producttype

Timeliness(availabilityafter sensing)

Comments

Systematic SAR L0 and b3 h For regional areas defined through

6.5. Long-term continuity of observations

The capacity of the GMES Space Component (GSC) is being builtup progressively, starting with the launch of the A-units of Sentinels1–3 in the 2013–2014 timeframe. This will be augmented by thelaunch of B-units (2015–2017).

Table 7Sentinel-1 User Products from Core and Collaborative PDGS.

Core PDGS (committedquality and timeliness)

Collaborative PDGS(non-exhaustive list)

L0 SAR L0 see Table 8L1 SAR L1 SLC see Table 8

SAR L1 GRD SAR L1 OrthorectifiedL2 and higher SAR L2 OCN (wind, wave and currents) Soil Moisture

Swell propagationPS Cal products (*)Sigma0 mosaic (*)

Products with a (*) are generated by the Core PDGS for calibration purposes only. Seealso Table 8 for information about the processing concept and timeliness.

The role of GMES Contributing Missions (GCMs) is essential, par-ticularly in the time frame until all B-units are deployed. The GCMsare important in completing the observations provided by the Senti-nels for GMES in time and space. As such, the GCMs and the Sentinelsprovide the full observational capabilities, necessary, to sustain theGMES services.

A long-term continuity of observations from the GMES SpaceComponent beyond the period of the Sentinel A- and B-units is fore-seen and will be necessary to sustain these services in the future.

Global SAR L1GRD

HLOP.b24 h All acquired data.

SAR L2OCN

b3 h For WV mode data acquired over theocean and SM, IW and EW mode dataover regional areas defined throughHLOP.

Systematicregional

SAR L1 SLC b3 h Data available from the core PDGS forregional areas defined through HLOP.Faster availability from sensing relies oncollaborative PDGS

b24 h Data available from the core PDGS forregional areas defined through HLOP.

Systematiclocal

SAR L0 b10 min Only for data over ocean acquired indirect downlink over a S-1 Core GroundStation. For areas outside the visibility ofS-1 core stations, 10 min availabilityfrom sensing relies on the collaborativePDGS.

Radiometric resolution in dB of a 150 x 150m spatial resolution product

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-30 -25 -20 -15 -10 -5 0Radar Backscatter Coefficient σ0 [dB]

Sentinel-1Interferometric WideSwath 20*5 1 look 250 km

Sentinel-1 Stripmap 5*51 look 80km

Sentinel-1 Extra-wideSwath 40*20 1 look 400 km

ASAR Wide Swath 11.5 looks 400km

Fig. 10. Radiometric resolution in dB as a function of the radar backscatter coefficient.


7. Sentinel-1 user products

Following the GMES Space Component data, Sentinel-1 data ac-cess is based on systematic and free on-line data dissemination atno charge and without the need for an ordering process except incases of support to emergency situations.

The Sentinel-1 mission will be able to provide a variety of imageand/or information products to the users. The Sentinel-1 Core PDGSwill generate some of these products operationally, the Sentinel-1Core PDGS or the Sentinel-1 Collaborative PDGS may support someof them and the GMES Core and Downstream services are expectedto generate the final information products (e.g. ice charts, forestmaps, ground-surface deformation maps). The Sentinel-1 productsoperationally generated by the Core PDGS and available to users in-clude SAR “raw” Level-0 data and “SAR processed” Level-1/2 data.

Level-1 data include two main categories: complex imagery (SLC)for interferometric applications and Ground Range Detected Geo-referenced imagery (GRD) for most intensity based applications. Forthe SLC and GDR categories image stacks (time series of repeat cover-age) can be provided.

As a baseline Sentinel-1 provides ocean Level-2 products (wind,wave and currents).

The Sentinel-1 user products generated by the Core PDGS aresummarised in Table 7. The table introduces as well the concept ofproducts generated by the Collaborative PDGS, with some examplesof candidate collaborative products.

Table 9C-band SAR comparison.

Mission/mode Azimuth resolution (m) Ground range resolution (m)

Sentinel-1/InterferometricWide-swath

20 5

Sentinel-1/Strip Map 5 5Sentinel-1/Extra Wide-swath 40 20Envisat/ASAR Wide Swath 150 150Envisat/Image 5 20ERS-2/Image 5 20

7.1. Processing concepts and timeliness

The processing concept for the Sentinel-1 Core PDGS User prod-ucts is summarised in Table 8. All Sentinel-1 SAR data acquired is sys-tematically processed to a predefined product type and availablewithin a defined timeliness. Systematically generated products canbe classified in:

● Global productsGlobal products are those systematically generated for all acquireddata. They include Level-0, detected Level-1 and Level-2 oceanproducts. These products are made available within 3 h from ob-servation over near real time areas defined in the High Level Oper-ations Plan (HLOP) and in any case, within 24 h from observation.These products are made available on-line, accessed through sub-scription and archived for long-term access.

● Regional productsRegional products are those systematically generated for a sub-setof the total acquired data, over well-defined regional areas in theHLOP. These products are made available within 3 h from observa-tion over specific near real time areas in line with the HLOP and inany case, within 24 h from observation. Regional products includeLevel-1 SLC products. These products are made available on-line,accessed through subscription and archived for long-term accesslike the global products

● Local productsFor critical GMES services, notably maritime surveillance, requir-ing fast delivery, Level-0 products will be made available within10 min from observation over local areas defined in the HLOP.This timeliness is foreseen for data acquired over European watersand only possible when Sentinel-1 is inside the coverage zone ofone of the Core ground stations. Local products are systematicallygenerated, made available on-line, accessed through subscriptionand archived for long-term access like the global products.

All systematically generated products (as shown in Table 8) arearchived and available for download from archive within 24 h fromobservation. Products different from those available in the archiveover a given area can be generated from the archived Level-0 data.

On-request generation of new products, different from those sys-tematically generated, is supported but restricted to a limited numberof authorised users (currently restricted to GMES emergency and se-curity services).

8. Calibration and validation of Sentinel-1 products

The Synthetic Aperture Radar measures a series of radar echoesthat are processed within the ground segment to generate SAR dataproducts. Main contents of these data products are the normalisedradar cross-section and the phase of the individual pixels. Both quan-tities have to be provided with specified stability and accuracy. This isensured through calibration and validation tasks executed through-out the mission lifetime.

The SAR images are generated on the basis of the individual radarechoes that will contain instrument noise. Transmit power, receiver

No. of looks (N) NESZ (dB) Swath width (km) PI Abs. PI relative to ERS

1 −22 250 2300 4.4

1 −22 80 1472 2.81 −22 400 1301 2.5

11.5 −23.5 400 937 1.81 −22 80 736 1.41 −20 80 526 1.0

image of Fig.�10

Table 10Intensity contrasts between major ice classes in Sentinel-1 IW images and classification accuracy.

Location Pairs of dominant ice types IC [dB] and A [%] for types 1 and 2,VV-polarisation

IC [dB] and A [%] for types 1 and 2,HH-polarisation

IC [dB] and A [%] for types 1 and 2,cross-polarisation

Storfjord 1: young level ice 1.46 (62.9) 1.67 (64.7) 1.02 (59.1)2: broken+brash ice1: new ice 3.58 (78.9) 7.93 (96.0) 0.21 (51.9)2: young level ice

Barents Sea 1: young/first-year level ice 2.96 (74.7) 3.26 (76.8) 1.16 (60.3)2: brash+rubble ice1: new ice 2.92 (74.4) 2.40 (70.5) 0.02 (50.2)2: young/first-year level ice

Fram Strait 1: first-year level ice 7.28 (94.7) 6.42 (92.3) 11.47 (99.3)2: ridges+brash ice1: new+young ice 5.62 (89.5) 6.37 (92.2) 0.57 (55.1)2: first-year level ice

IC = intensity contrast, A = classification accuracy (in parentheses).


gain and to a lesser extent antenna gain are subject to variations intime due mainly to temperature changes but also due to ageing orother effects. One main task of the internal calibration is to providecorrections for changes of the transmit power and the electronicsgain. The Sentinel-1 Antenna Model is a software tool that can accu-rately estimate the Sentinel-1 SAR antenna radiation patterns basedon the instrument configuration, pre-launch measurements and re-sults derived from in-orbit measurements. The antenna model nomi-nally covers the antenna gain and it is a task of the internal calibrationto ensure the validity of the antenna model. Internal calibration alsocovers the signal phase. The overall phase of the echo signal dependson two major elements, the measurement geometry and on instru-ment internal phase stability. As the hardware can in general not pro-vide the required phase stability, it is a task of the internal calibrationscheme to cover the internal phase variations by adequate measure-ments. All internal calibration measurements, either for gain or forphase, are used within the ground processing to correct the dataproducts in order to achieve the required stability.

Main task of external calibration is to derive the calibration con-stant by measurements of targets with exactly known backscatter

Fig. 11. Example of a

coefficients. This is necessary as in general it will not be possible toknow all parameters with sufficient accuracy prior to the in flightmeasurements.

The key outputs of the calibration and validation activities are

● Deriving and maintaining the system calibration parameters;● Quality Assessment of the Level-1, Level-2, and Level-3 products,

in the form of reports, diagrams, statistics;● Feedback into the Sentinel-1 Space and/or Ground Segment by

tuning the instrument, the Level-1 or the Level-2 and Level-3 pro-cessors e.g. through calibration parameter updates.

9. Sentinel-1 product and application performance

9.1. Overview

Data products from Sentinel-1 consist of imagery representing theradar echo from the Earth surface. Basic calibrated data are providedin the form of both complex (In-phase and Quadrature components)and intensity images. These are referred to as Level-1. Level-1

sea–ice product.

Fig. 12. Snow Cover Algorithm (SCA) classification result. White colour represents wetsnow, grey represents dry snow, dark red represents bare-soil and red represents areascovered by radar shadow. Further, masked out lakes and forested areas are cyan andgreen, respectively.

Table 11Requirements for Snow Covered Area (SCA) products.

Level Measurementaccuracy

Spatialresolution

Temporalresolution

Driver

Threshold 10% 0.5 km 1 daya HydrometTarget 5% 0.1 km 12 ha

a Temporal requirements are relaxed for climate research (5–7 days) and for hydrologyof mountain areas (3–5 days).


performance is defined by the parameter values listed in Table 3.Level-2 data products include extracted information from theLevel-1 imagery in the form of estimated values of the geophysical vari-ables listed in Davidson et al. (2009). This type of Level-2 productrequires a retrieval algorithm and the Level-2 performance dependstherefore on both the Sentinel-1 system performance and the retrievalalgorithm. In the following subsections the link between the instrumentspecifications in Table 3 and Level-1 and Level-2 image quality arebriefly discussed.

9.2. System noise

The Sentinel-1 specification of −22 dB for noise equivalentsigma zero (NESZ) encompasses all sources of system noise includ-ing thermal noise and quantisation noise sources. The impact of sys-tem noise on Level-1 quality will depend on the actual signalstrength of the backscatter, and will generally broaden the distribu-tions of amplitude and phase, decreasing the accuracy of the Level-2product.

Phase noise effects interferometric performance and in particular theerror budget associated with subsidence monitoring using PermanentScatterers.

9.3. Radiometric accuracy and stability

Radiometric accuracy and stability are specified for Sentinel-1 forall measurement modes to be within 1 dB (3σ) and 0.5 dB (3σ) re-spectively. The 3σ specification is based on the confidence intervalsof a Gaussian (normal) distribution and refers to the probability ofthe instrument exceeding the specified interval.

The accuracy and stability numbers refer to measurement uncer-tainty of the radar backscattering coefficient using the image intensityexcluding the effect of speckle and noise. These effects are defined bythe radiometric resolution that is only implicitly part of the systemspecification and can be derived from the noise equivalent sigmazero and the spatial resolution.

9.4. Radiometric resolution

Radiometric resolution refers to measurement uncertainty of theradar backscattering coefficient using the image intensity arisingfrom speckle and noise for an otherwise perfect system. Radiometricresolution is expressed as the standard deviation of the measurementuncertainty normalised with respect to the mean.

For an imaging systemwith noise bias subtraction Eq. (1) (Ulaby et al.,1982) gives the expression for radiometric resolution or accuracy. The

Table 12Specifications for Snow Cover products derived from Sentinel-1 IW mode data.

Mode Pixel size ofSCA product

Azimuthlooks

Rangelooks

Total number of looksfor SCA product

IW 100×100 m2 5 20 100

expression applies if the noise bias is derived from a relatively large num-ber of system noise estimates.

σ st

I0¼ 1þ 1

SNRffiffiffiffi

Lp ð1Þ

where,

σst is the standard deviation of the image intensity in linearpower

I0 is the mean image intensity in linear powerSNR is the signal-to-thermal noise ratioL is the effective number of looks and is equal to (mean in-

tensity)2/variance

The radiometric resolution has been calculated using the equa-tions given above for a radar backscatter coefficient σ0 varying from−26 dB m2/m2 up to 0 dB m2/m2 for the different imaging modesof Sentinel-1 with a noise equivalent sigma zero (NESZ) of −22 dB.For comparison the radiometric resolution of the Envisat ASAR WideSwath is also given. The results for a 150×150 m spatial resolutionproduct are shown in Fig. 10.

Fig. 13.Minimum detectable ship length for Sentinel-1 HH (blue) and Envisat ASAR HH(green).

image of Fig.�12

image of Fig.�13


From Fig. 10 it is clear that the IWmode of Sentinel-1 combines itsexcellent coverage with a high radiometric resolution capability.

9.5. ERS, ENVISAT and SENTINEL-1 performance indicator

In order to put the performance of the Sentinel-1 Level-1 dataproducts in perspective with a view to demonstrate the expected ap-plication potential with reference to its predecessors a performanceindicator (PI) has been defined that takes not only account of sensi-tivity, spatial resolution and radiometric resolution but also coverage(and revisit) in terms of swath width. PI is derived by dividing theswath width (in km) by the radiometric resolution of a 150×150 mproduct with a backscatter level of −20 dB. A large swath width incombination with a good radiometric resolution will yield a highvalue for the PI. The results for ERS, ENVISAT ASAR and Sentinel-1CSAR are summarised in Table 9 and underline the emphasis duringthe Sentinel-1 design put on high qualitymulti-temporal data products.

9.6. System errors related to Level-2 retrieval performance

Retrieval algorithms for e.g. soil moisture and wind speed estima-tion rely on the absolute level of the received radar signal while re-trieval algorithms for land cover classification are in general basedon simple generic algorithms:

● application of a threshold to a single intensity channel;● maximum likelihood as a discriminator between two surfaces;● estimation of the ratio between two intensity channels (prior to

comparison with a threshold).

For a number of algorithms currently in use and expected to beused for Sentinel-1 Level-2 product generation, the performances re-lated to the measurement uncertainties of the Level-1 products havebeen studied.

9.6.1. Sentinel-1 subsidence monitoring performanceThe subsidence rate error budget is calculated by inserting the

total phase noise coming from the individual terms in the model asderived in De Zan and Monti Guarnieri (2006).

The calculated subsidence rate (mm/year), taking into account thesystem geometry, parameters and performances of Sentinel-1, as wellas other items affecting the estimate, e.g. temporal changes, atmo-spheric disturbances, orbital tubes, S/C attitude and assuming linearmotion, and the standard deviation of the overall phase noise forSentinel-1. The resulting subsidence rate error is in the order of1.3 mm/year.

9.6.2. Sentinel-1 sea–ice classification accuracyAmongst potential users of Sentinel-1 SAR image products are

operational ice services that provide information on ice conditions(extent, concentration, thickness, deformation) to ships and marineplatforms in ice-covered ocean regions. Sea–ice mapping includesthe visual or automatic separation of different types and structuresof ice. In Dierking (2009) it was shown that the radar intensity con-trast between different ice types depends on the ice regime and onenvironmental conditions. A list of mean intensity contrasts betweendominant ice classes has been derived for three different ice regimesin the region of Svalbard. The notation “dominant class” indicates thatan operator who analyses a SAR image can clearly recognise the re-spective ice type or feature and distinguish it from other classes.Table 10 is based on Fig. 8 of Dierking (2009). The data are valid foran incidence angle range of 30 to 45°. They are based on the simula-tions of images with the characteristics of the Sentinel-1 Interfero-metric Wide-swath mode, using a pixel size of 20×20 m, whichimplies approximately four looks.

At like-polarisation, the classification accuracy using an imagewith number of looks=4 (20 m×20 m) is moderate for the Storfjord

and the Barents Sea ice regime. With pixels of double size and a cor-responding increase of the number of looks, the accuracy improves tovalues >80%. The use of even larger pixel sizes, however, will have aconsiderable impact on the perceptibility of narrow deformationstructures such as ice ridges and brash ice between ice floes becauseof the related decrease of spatial resolution. It has to be taken into ac-count that the effective radar intensity contrast of a structure relativeto the adjacent ice is decreased if the structure does not fill the entirearea of a given pixel. For the Fram Strait ice regime, the classificationaccuracy is already ≥90% for 20×20 m pixels.

At cross-polarisation, the intensity contrast between the dominantice classes is low, with the exception of the pair level — deformed icein the Fram Strait ice regime. Hence, the classification accuracy for anautomated separation of ice types is not sufficient. Only if the neigh-bouring pixels are averaged such that the number of looks is in-creased to values close to L=100, the accuracy would be >80%. Butin this case, the theoretical gain may be counter-balanced by theloss of spatial resolution as explained above.

Increasing the coverage by using the EW mode would reduce theclassification accuracy to unacceptable low levels due to the decreasein radiometric resolution of the EW mode compared to the IW mode.

Fig. 11 shows an example of a sea–ice product.

9.6.3. Sentinel-1 snow cover monitoring performanceC-band SAR can be applied for mapping the extent of snow area

(melting snow only). The requirements on spatial resolution and re-peat observations of the Snow Covered Area (SCA) product are spec-ified in the Integrated Global Observing Strategy (IGOS) CryosphereTheme Report (Table 11).

Spatial resolution of the SCA product is not a primary issue withSentinel-1, allowing for considerable multi-looking. The temporal re-quirement of 1 day is challenging. This number refers to hydrometeorol-ogy of lowlands, where snow cover may be quite volatile. Obviously, thecontemporaneous operation of 2 Sentinel-1 satellites will enable opera-tional products with a temporal resolution of less than 5 days. For practi-cal applications, the integration of SCA products in hydro-meteorologicalmodels enables to bridge gaps in the temporal sequence.

It has to be pointed out that C-band SAR is not able to supply thefull SCA product, because it is not sensitive to dry snow. There are var-ious possibilities to infer the extent of dry snow from other sources,briefly addressed below. For hydrological applications, the timelyand accurate observation of melting snow areas, as possible bymeans of Sentinel-1, is very important.

For the spatial resolution of the snowproduct the target requirementof 5% measurement accuracy from IGOS for 100 m×100 m products isassumed (a 10% figure is assumed for the threshold requirement).Only the speckle-related uncertainty for classification of wet snow istaken into account. The Interferometric Wide-swath Mode of Sentinel-1 has been used as the baseline for the SCA retrieval (Table 12).

The probability of error for the discrimination of two classes is 5%for a 2 dB difference between the classes using the Sentinel-1 IWmode. Using the average of several images for the reference and byreducing the spatial resolution of the product the error can be re-duced. An example of the Snow Cover Algorithm (SCA) classificationresult is given in Fig. 12.

9.6.4. Sentinel-1 rice field mapping performanceRice is a vital world food crop. In many countries, it forms the basis

of the economy and provides the staple food of the people. To controland maintain a close balance between rice production and foodneeds, an effective rice-monitoring programme is required at region-al, national, and international scale.

For ricefieldmapping C-band SAR intensity data are particularlywellsuited to rice monitoring, because of a) the all weather capability of theSAR systems to acquire data over rice growing regions which are underfrequent cloud cover, b) the remarkable increase in backscatter intensity

Fig. 14.Minimum detectable ship length for Sentinel-1 HV (blue) and Envisat ASAR HV(green).

Table 14Sentinel-1 Interferometric Wide-swath Mode Level-2 product performance.

S-1 Level-2 product Resolution Performance Units

Subsidence rate 5×20 m2 1.3 Rate uncertainty inmm/year

Land cover (2 dBcontrast)

30×30 m2 90.0% % correct classification

Forest non-forestclassification

30×30 m2 74.3% % correct classification100×100 m2 100%

Soil moisture 100×100 m2 3.9 (3σ) Moisture contentuncertainty in volume%

Flood mapping 30×30 m2 79.4% % correct classificationSnow coverclassification(3 dB contrast)


Ship detection HH+HV polarisation

5×20 m2 25–34 (90%)(Swathdependent)

Minimum ship length inm, (probability ofdetection)

Sea surface windspeed

100×100 m2 2.4 (3σ) Speed uncertainty in m/s

Sea surface currents 5 Hz 30 Speed uncertainty incm/s


throughout the growth cycle as the rice plants grow above the watersurface and interact with the incident radar signal.

Quantitative performance requirements in terms of the thresholdor target accuracies required by individual applications are generallynot available.

The traditional method for cultivating rice is flooding the fieldswhile, or after, setting the young seedlings. From a previous assess-ment of the use of ENVISAT ASAR data for rice mapping in China, itis shown that for Sentinel-1 the temporal behaviour of co-polar sig-nals can be used as a classifier over areas where flooding practicesare used for rice crops.

Using Sentinel-1 for rice crops mapping the following conclusionscan be drawn:

● There is a limited impact of the NESZ on the quality of the seg-mented maps based on the multi-temporal co-polar images. Thiswas to be expected as the values of σ0 for these natural surfacesare relatively high, at least for co-polar. A sensitivity analysisshows that there is no much gain in performance when the NESZis lowered below −22 dB/−23 dB.

Table 13Sentinel-1 Strip-Map mode Level-2 product performance.

S-1 Level-2product

Resolution Performance Units

Subsidence rate 10×10 m2 1.3 Rate uncertainty inmm/year

Land cover (2 dBcontrast)


Forest Non-ForestClassification

30×30 m2 98.3% % correct classification100×100 m2 100%

Soil Moisture 100×100 m2 3.2 (3σ) Moisture contentuncertainty in volume%

Flood Mapping 30×30 m2 93.0% % correct classificationSnow CoverClassification(3 dB contrast)


Ship Detection HH+HVpolarisation

5×5 m2 12–23 (90%)(Swath dependent)

Minimum ship length inm, (probability ofdetection)

Sea Surface WindSpeed

100×100 m2 1.3 (3σ) Speed uncertainty inm/s

Sea SurfaceCurrents

5 Hz 30 Speed uncertainty incm/s

● The increased number of acquisitions by S-1 during the growing sea-son enables to properly sample the growth cycle and to limit the im-pact of geophysical noise (for example by identifying a date with nowind where the water surface is very smooth). With one Sentinel-1acquisition every six days, it is expected that the performance of themulti-temporal algorithm drastically improves compared to whatwas obtained with ASAR. In addition, in regions where multiple agri-cultural practices are common (e.g. different sowing dates throughoutthe region), a higher temporal sampling will be very beneficial.

The better range resolution of S-1 will allow for more efficient spa-tial multi-looking, which is important for this application that deals insome regions with very small paddy fields.

9.6.5. Sentinel-1 ship detection performanceThe minimum detectable ship lengths for Sentinel-1 are shown in

Figs. 13 and 14. The fixed parameters used in each case are shownacross the title of each plot. The plots use the actual ground range res-olution and NESZ specific to each position in the swath. The plots arecolour coded by sensor and include Sentinel-1 (blue), and EnvisatASAR (green). The beams are plotted alternately with solid anddashed lines for each sensor and the beam mode labels are placedat the centre of each swath (some of the plots are quite busy). Thecurves for the HH case tend to be smooth and co-linear due to theclutter-limited imaging. The curves for the HV case follow the shapeof the noise floor due to the noise limited imaging.

By inspection the following can be noted:

1. The minimum detectable ship length for the groups of single beammodes decreases with increasing incidence angle, which illustratesthe importance of incidence angle in reducing the backgroundocean clutter level.

2. The minimum detectable ship length decreases for modes withincreasing (i.e., smaller) resolution.

Table 15Sentinel-1 Wave Mode Level-2 product performance.

S-1 Level-2 product Resolution Performance Units

Significant waveheight

20×20 km2 0.3 Height uncertainty in m

Dominant wavelength 20×20 km2 40 Wavelength uncertainty in mDominant direction ofpropagation

20×20 km2 12 Direction uncertainty indegrees

image of Fig.�14

Fig. 15. Multi-temporal ASAR image for land use.


3. For co-polarisation, the ship RCS is larger, but the clutter level ishigher, especially for smaller incidence angles. The co-polarisationcase is generally clutter limited.

4. For cross-polarisation the ship RCS is smaller, but the clutter isbelow the noise floor; the cross-polarisation case is noise limited.There are significant benefits to using cross-polarisation, especiallyfor acquisitions at smaller incidence angles. Furthermore, cross-polarisation provides more uniform ship detection performanceacross the image swath since the noise floor is somewhat indepen-dent of incidence angle.

Considering Sentinel-1, there is a penalty in using EWS for itsbroader swath due to its coarser resolution than IWS.

9.7. Sentinel-1 Level-2 product performance summary

A key element in the design of the Sentinel-1 mission is the rela-tion between the technical implementation of the mission and theaccuracy of the geophysical information products that the mission isrequired to generate and support for operational applications andservice provision. Analysis is required to demonstrate the compliancebetween the technical choices of the mission and system and userrequirements. It also provides data users with quantitative estimatesof the system contribution to the overall performance (i.e. accuracy)of geophysical products along with the methodological frameworkused to estimate such system contributions. The complete analysisis given in Davidson et al. (2009). The expected Sentinel-1 Level-2product performances are summarised in Tables 13 to 15 for a givenspatial resolution adapted to a product type. It is noted that theresults given are based on ideal retrieval cases, with no other errorsthan the ones related to the Sentinel-1 performance. The retrievalalgorithm is assumed to be error free.

10. Conclusion

The Sentinel-1 constellation is a completely new approach to SARmission design by ESA in direct response to the operational needs forSAR data expressed under the EU-ESA GMES programme. The missionensures continuity of C-band SAR data and builds on ESA's heritageand experience with the ERS and Envisat instruments (Fig. 15),

notably maintaining key characteristics such as stability and accuratewell-calibrated data products. At the same time, mission parametershave been vastly improved to meet major user requirements collect-ed and analysed through EU Fast Track and ESA GMES Service Ele-ment activities, especially in areas such as reliability, revisit time,geographical coverage and rapid data dissemination. As a result, theSentinel-1 pair is expected to provide near daily coverage over Europeand Canada, independent of weather with delivery of radar data withinan hour of acquisition — vast improvements over the existing SARsystems.

In addition to responding directly to the current needs of GMES,the design of the Sentinel-1 mission is expected to enable the devel-opment of new applications and meet the evolving needs of GMES,such as in the area of climate change and associated monitoring

Acknowledgement

The content and figures in this paper are based on the inputs fromthe ESA Sentinel-1 team members, the industrial consortium lead byThales Alenia Space Italy as Prime Contractor and the members offormer The SAR Advisory Group (TSAG).

References

Attema, E. (2005). GMES Sentinel-1 mission requirements document ES-RS-ESA-SY-0007,issue 4, revision 4, 29/9/2005.

Attema, E., Cafforio, C., Gottwald, M., Guccione, P., Monti Guarnieri, A., Rocca, F., et al.(2010, Oct.). Flexible dynamic block adaptive quantization for Sentinel-1 SARmissions.IEEE Geoscience and Remote Sensing Letters, 7(4), 766–770.

Davidson, M., Attema, E., Snoeij, P., Rommen, B., & Floury, N. (2009). Sentinel-1 systemerror budget. S1-TN-ESA-SY-0046.

De Zan, F., & Monti Guarnieri, A. M. (2006, Sept.). TOPSAR: Terrain observation byprogressive scans. IEEE Transactions on Geoscience and Remote Sensing, 44(9),2352–2360.

Dierking, W. (2009). Mapping of different sea ice regimes using images from Sentinel-1and ALOS synthetic aperture radar. IEEE Transactions on Geoscience and RemoteSensing, 48(3), 1045–1058 March 2010.

Flett, D., Crevier, Y., & Girard, R. (2009, July 12–17). The RADARSAT constellation mission:Meeting the government of Canada's needs and requirements. Proceedings of IGARSS(IEEE International Geoscience and Remote Sensing Symposium) 2009 Cape Town,South Africa.

Ulaby, F. T., Moore, R. K., & Fung, A. K. (1982).Microwave remote sensing, Vol. 2, Reading,MA: Addison-Wesley.

image of Fig.�15

gmes sentinel-1 mission

Documents