GMES: Sentinel-3 - Global Sea/Land Monitoring Mission including Altimetry

The Sentinel-3 mission of ESA and the EC is one of the elements of the GMES (Global Monitoring for Environment and Security) program, which responds to the requirements for operational and near-real-time monitoring of ocean, land and ice surfaces over a period of 15 to 20 years. The topography element of this mission will serve primarily the marine operational users but will also allow the monitoring of sea ice and land ice, as well as inland water surfaces, using novel observation techniques. ^{1) 2) 3) 4) 5) 6)}

Table 1: The name of the game ^{7) 8)}

The Sentinel-3 program represents a series of operational spacecraft over the envisioned service period to guarantee access to an uninterrupted flow of robust global data products.

The main observation objectives of the mission are summarized in the following list:

• Ocean and land color observation data, free from sun-glint, shall have a revisit time of 4 days (2 days goal) and a quality at least equivalent to that of Meris instrument on Envisat. The actual revisit obtained over ocean at the equator (worst case) is less than 3.8 days with a single satellite and drops below 1.9 days with 2 satellites, phased 180° on the same orbital plane.

• Ocean and land surface temperature shall be acquired with at least the level of quality of AATSR on Envisat, and shall have a maximum revisit time of 4 days with dual view (high accuracy) observations and 1 day with single view. Achieved performance is shown to be significantly better, even with a single satellite (dual view: 3.5 days max, 1.8 days average).

• Surface topography observations shall primarily cover the global ocean and provide sea surface height (SSH) and significant wave height (SWH) to an accuracy and precision at least equivalent to that of RA-2 on Envisat. Additionally, Sentinel-3 shall provide surface elevation measurements -in continuity to CryoSat-2 - over ice regions covered by the selected orbit, as well as measurements of in-land water surfaces (rivers and lakes).

In addition, Sentinel-3 will provide surface vegetation products derived from synergistic and co-located measurements of optical instruments, similar to those obtained from the Vegetation instrument on SPOT, and with complete Earth coverage in 1 to 2 days.

The EU Marine Core Service (MCS) and the Land Monitoring Core Service (LMCS), together with the ESA GMES Service Element (GSE), have been consolidating those services where continuity and success depends on operational data flowing from the Sentinels.

Spacecraft:

The Sentinel-3 spacecraft is being built by Thales Alenia Space, France. A contract to this effect was signed on April 14, 2008. The spacecraft is 3-axis stabilized, with nominal pointing towards the local normal and yaw steering to compensate for the Earth rotation affecting the optical observations. The spacecraft has a launch mass of about 1200 kg (including margin), the height dimension is about 3.9 m. The overall power consumption is 1100 W. The design life is 7 years, with ~100 kg of hydrazine propellant for 12 years of operations, including deorbiting at the end.

Table 2: Overview of Sentinel-3 spacecraft parameters

RF communications: The S-band is used for TT&C transmissions. The X-band provide the payload data downlink at a rate of 450 Mbit/s.

Four categories of data products will be delivered: ocean color, surface topography, surface temperature (land and sea) and land. The surface topography products will be delivered with three timeliness levels: NRT (Near-Real Time, 3 hours), STC (Standard Time Critical, 1-2 days) and NTC (Non-Time Critical, 1 month). Slower products allow more accurate processing and better quality. NRT products are ingested into numerical weather prediction and seastate prediction models for quick, short term forecasts. STC products are ingested into ocean models for accurate present state estimates and forecasts. NTC products are used in all high-precision climatological applications, such as sealevel estimates.

The resulting analysis and forecast products and predictions from ocean and atmosphere adding data from other missions and in situ observations, are the key products delivered to users. They provide a robust basis for downstream value-added products and specialized user services.

Introduction of new technology: A newly developed a MEMS rate sensor (gyroscope), under the name of SiREUS, will be demonstrated on the AOCS of Sentinel-3. The gyros will be used for identifying satellite motion and also to place it into a preset attitude in association with optical sensors after its separation from the launcher, for Sun and Earth acquisition. Three of the devices will fly inside an integrated gyro unit, each measuring a different axis of motion, with a backup unit ensuring system redundancy. Each unit measures 11 cm x 11 cm x 7 cm, with an overall mass of 750 grams. ⁹⁾

The SiREUS device is of SiRRS-01 heritage, a single-axis rate sensor built by AIS (Atlantic Inertial Systems Ltd., UK), which is using a ’vibrating structure gyro’, with a silicon ring fixed to a silicon structure and set vibrating by a small electric current. The SiRRS-01 MEMS gyro has been used in the automobile industry. These devices are embedded throughout modern cars: MEMS accelerometers trigger airbags, MEMS pressure sensors check tires and MEMS gyros help to prevent brakes locking and maintain traction during skids. - In a special project, ESA selected the silicon-based SiRRS-01 to have it modified for space use (and under the new name of SiREUS).

Figure 1: Photo of the MEMS rate sensor (image credit: ESA)

Figure 2: Artist's rendition of the Sentinel-3 spacecraft (image credit: ESA)

Launch: A launch of the Sentinel-3 spacecraft is planned for late 2012 on a Vega launcher (compatibility with Dnepr launcher).

Orbit: Frozen sun-synchronous orbit (14 +7/27 rev./day), mean altitude = 814 km, inclination = 98.6º, LTDN (Local Time on Descending Node) is at 10:00 hours. The revisit time is 27 days providing a global coverage of topography data at mesoscale (inter-track distance at equator: 104 km).

For the altimetry mission, simulations show that this orbit provides an optimal compromise between spatial and temporal sampling for capturing mesoscale ocean structures, offering an improvement on SSH mapping error of up to 44% over Jason - due to improved spatial sampling (Figure )- and 8% over the Envisat 35-day orbit - due to better temporal sampling. After a complete cycle, the track spacing at the equator is approximately 100 km.

The Sentinel-3 mission poses the most demanding POD (Precise Orbit Determination) requirements, specially in the radial component, not only in post-processing on-ground, but also in real-time. This level of accuracy requires dual-frequency receivers. The main objective of the mission is the observation with a radar altimeter of sea surface topography and sea ice measurements (see columns 3, 4, 5 in Table 3).

Table 3: Error budget requirements in Sentinel-3 as a function of time wrt measurement ¹⁰⁾

Figure 3: Sentinel-3 spacecraft with payload layout (image credit: ESA)

Sensor compliment (optical payload, topographic payload)

In the context of GMES (Global Monitoring for Environment and Security), the objectives of the Sentinel-3 mission, driven by ESA and the user community, encompass the commitment to consistent, long-term collection of remotely sensed data of uniform quality in the areas of sea / land topography and ocean color. Measurements over oceans will be provided jointly with other operational missions, such as the Jason series, to contribute to the realization of a permanent Global Ocean Observing System (GOOS). Regarding ice, it is foreseen to monitor land ice (also denoted as ice sheet) including ice margins and sea ice. At last, measurements over rivers and lakes will help in the water level monitoring of spots of interest throughout the world.

Sentinel-3 will support primarily services related to the marine environment, such as maritime safety services that need ocean surface-wave information, ocean-current forecasting services that need surface-temperature information, and sea-water quality and pollution monitoring services that require advanced ocean color products from both the open ocean and coastal areas. Sentinel-3 will also serve numerous land, atmospheric and cryospheric application areas such as land-use change monitoring, forest cover mapping and fire detection.

Optical payload (OLCI, SLSTR)

The optical payload consists of the OLCI and SLSTR instruments. They provide a common quasi-simultaneous view of the Earth to help develop synergistic products. ¹¹⁾

OLCI (Ocean and Land Color Instrument):

OCLI is a medium resolution pushbroom imaging spectrometer of MERIS heritage, flown on Envisat, but with a slightly modified observation geometry: the FOV (Field of View) is tilted towards the west (~ 12º away from the sun), minimizing the sun-glint effect over the ocean and offering a wider effective swath (~ 1300 km, overall FOV of 68.5º). The sampling distance is 1.2 km over the open ocean and 0.3 km for coastal zone and land observations. The instrument mass is ~ 150 kg; it is being designed and developed at Thales Alenia Space España.

The FOV of OLCI is divided between five cameras on a common structure with the calibration assembly. Each camera has an optical grating to provide the minimum baseline of 16 spectral bands required by the mission together with the potential for optional bands for improved atmospheric corrections.

Figure 4: Schematic view of the OCLI instrument (image credit: ESA)

Each camera is constituted of a Scrambling Window Element to comply with the polarization requirement, a COS (Camera Optical Sub-assembly) for the spectral splitting of the different wavelengths, a FPA (Focal Plane Assembly) with a CCD for the signal detection and a VAM (Video Acquisition Module) for the monitoring of the analog signal. The optical sub-assembly of each camera includes its own grating and provides the 21 spectral bands required by the mission. ¹²⁾

The control of the instrument assembly is realized by a CEU (Common Electronic Unit), which assumes the function of instrument control, power distribution and digital processing.

A calibration assembly, including a rotation wheel with five different functions for normal viewing, dark current, spectral and radiometric calibrations insures the calibration of the instrument.

Figure 5: Schematic view of the OCLI observation geometry with the 5 camera assembly (image credit: ESA)

SLSTR (Sea and Land Surface Temperature Radiometer):

SLSTR is an upgraded version of the AATSR instrument on Envisat, offering a wider swath which completely overlaps the OLCI swath, as required to produce accurate vegetation products. The SLSTR is designed for ocean and land-surface temperature observations. Unlike AATSR, SLSTR has a double-scanning mechanism, yielding a much wider swath stretching almost from horizon to horizon. The OLCI and SLSTR swaths are overlapping broadly, yielding extra information. SLSTR has a wide nadir view and a narrow oblique view.

The 90 kg visible/infrared SLSTR measures sea- and land-surface temperatures, following the AATSR concept. Its rotary scan mirror mechanism produces the wide swath of 750 km. It features ~1 km resolution at nadir for thermal-infrared channels and 500 m for visible and shortwave infrared channels. Like AATSR, SLSTR offers dual views - inclined forward and near-vertical nadir - to provide robust atmospheric correction over the swath. The nadir and forward views are generated by separate scanners, allowing a wider swath than possible with the single conical scan of the original ATSR design.

SLSTR observations are in the following bands (9): VIS (0.55, 0.66 and 0.85 µm), SWIR (1.6 and 3.7 µm), TIR (10.95 and 12 µm); they include the Envisat/AATSR and ERS/ATSR-2 channels for continuity. Additional channels at 1.378 and 2.25 µm improve cloud detection, besides being used for new products. The SSD (Spatial Sampling Distance) is 500 m in the spectral range of 0.55-2.2 µm (solar reflection) and 1000 m in the spectral region of 3.7, 10.95, and 12 µm (MWIR, TIR).

The instrument includes onboard radiometric sources for accurate and stable inflight calibration. The infrared detectors are cooled to 80 K by active coolers.

Figure 6: View of OCLI and SLSTR swath coverage and sampling geometry (image credit: ESA)

Figure 7: Artist's view of the OLCI (left) and SLSTR instruments (right), image credit: ESA

Topography payload: (SRAL, MWR, GNSS receiver, LRR)

The objective of the topography mission is to provide measurements over the open Ocean, coastal zones, ice sheets, rivers and lakes. Measurements over open oceans will contribute jointly with other operational missions to the realization of a permanent Global Ocean Observing System (GOOS). The main parameters measured over the open sea are Sea Surface Height (SSH) and Significant Wave Height (SWH) allowing to retrieve sea surface wind speed.

SRAL (SAR Radar Altimeter):

SRAL is a redundant dual-frequency (C-band + Ku-band) nadir-looking altimeter instrument, and the core instrument of the topographic payload. The overall objectives are to provide altimetric data (basic measurements of surface heights, sea wave heights and sea wind speed) relative to a precise reference frame. SRAL has a strong heritage of the instrument techniques implemented for the Poseidon-3 altimeter and for SIRAL (SAR Interferometer Radar Altimeter) on CryoSat-2. ^{13) 14) 15) 16)}

The SRAL radar uses a linearly frequency-modulated pulse (chirp) and the pulse compression is carried out on-board by means of the deramp technique. The main frequency used for surface height measurements is the Ku-band (13.575 GHz, bandwidth=350 MHz), whereas the C-band frequency (5.41 GHz, bandwidth=320 MHz) is used for the ionospheric corrections. The frequency plan is compliant with the ITU (International Telecommunication Union) regulations. A 50 ms pulse duration for both frequencies has been sized as a trade-off result between a high BT product and the timing constraints of the burst pattern of the SAR mode.

Figure 8: Comparison between Jason (black) and Sentinel-3 (purple) ground tracks for a complete cycle (image credit; ESA)

Figure 9: The measurement principle of the topography payload (image credit: ESA)

Figure 10: A diagram of the corrections applicable to the altimeter range measurement and the contributions to the height of the instantaneous sea surface above a reference earth ellipsoid (image credit: Gary M. Mineart) ¹⁷⁾

The SRAL altimeter instrument is made of one nadir looking antenna subsystem which is externally mounted on the satellite +Zs panel and central electronic chains composed each of a DPU (Digital Processing Unit) and a RFU (Radio Frequency Unit). The central electronic chains are mounted inside the satellite on the -YS panel and are treated according to a cold redundancy scheme.

Figure 11: SRAL accommodation on the Sentinel -3 spacecraft (image credit: ESA)

SRAL modes of operation:

The SRAL instrument includes measurement modes, calibration modes and support modes. The measurement modes are composed of two radar modes associated to two tracking modes. The two radar modes are the following:

• LRM (Low Resolution Mode). It refers to the conventional altimeter pulse-limited resolution mode (so far, the LRM mode is being used on all altimetry missions). It consists of regular emission/reception sequences at a fixed PRF (Pulse Repetition Frequency) of around 1920 Hz leading to an ambiguity rank of 10.

• SAR mode: This is a high along-track resolution mode composed of bursts of Ku-band pulses.

These modes are associated to two tracking modes which consist of the following:

- Closed-loop mode: refers autonomous positioning of the range window (ensures autonomous tracking of the range and gain by means of tracking loop devices implemented in the instrument).

- Open-loop mode: refers to the positioning of the range window based on a-priori knowledge of the terrain height from existing high-resolution global digital elevation models.

The open-loop is intended to be used instead of the more conventional closed-loop tracking over some surfaces, to improve the acquisitions over inhomogeneous or rough topography. While in open-loop, the setting of the tracking window of the altimeter is driven by predetermined commands, stored on board, combined with real-time navigation information available from the GNSS receiver. The main advantage is that the measurements are continuous, avoiding the data gaps typical of closed-loop tracking, which has problems in tracking the rapid topographic changes at coastal margins and in mountainous regions.

Table 4: Summary of SRAL support modes

The SRAL instrument generates either C-band or Ku-band pulses in order to simplify the hardware design. However, the periodic emission of elementary patterns (1 C-band pulse is surrounded by 6 Ku-band pulses, denoted by 3Ku/1C/3Ku) ensures a sufficient correction of the ionosphere bias (Figure 12).

Figure 12: The LRM transmit/receive pattern scheme (image credit: ESA)

After de-ramping and digital processing, the echo received from each pulse is sampled on 128 points corresponding to a 60 m range window. The C- and Ku-band echoes are submitted each to a FFT (Fast Fourier Transform) to return to the time domain after deramp. Then, C- and Ku-band echoes are accumulated separately over a 50 ms cycle corresponding to an accumulation of 84 Ku-band pulses and 14 C-band pulses over that cycle.

SAR mode: The implementation of a nadir SAR mode provides an enhanced along-track (azimuth) resolution (~ 300 m) w.r.t. the LRM mode. This feature allows to acquire height measurements over along-track sliced areas sampled at the 300 m resolution. It is of prime interest to discriminate finely sea/ice transitions, sea/land transitions in a coastal area or inland water areas.

The SAR mode consists of periodical emissions/receptions of bursts composed of 64 Ku-band pulses surrounded by 2 C-band pulses (Figure 13) again for ionosphere delay correction. The 64 Ku-band pulses are generated coherently within a burst to carry out azimuth resolution enhancement on a burst basis by means of Doppler filtering. The burst emission / reception cycle is completed before the next burst cycle. The burst cycle duration is about 12.5 ms in such a way that a 4-burst cycle is equal to the LRM cycle of 50 ms. The PRF within a burst is around 18 kHz.

Figure 13: The SAR burst pattern scheme (image credit: ESA)

Figure 14: Final shape of resolution cells in SAR mode (image credit: ESA)

Calibration mode: Two specific calibration modes have been designed to refresh the calibration parameters required for ground processing and to monitor the good health of the instrument in flight configuration. The CAL-1 mode allows to calibrate the internal impulse responses (range and azimuth impulse responses in C- and Ku-band) whereas the CAL-2 mode allows to calibrate the gain profile of the range window by averaging thermal noise measured at each C- and Ku-band antenna port.

Table 5: Some SRAL instrument parameters

The “dual-like” features of the SRAL instrument (dual frequency, dual radar mode, dual tracking mode) make it possible to acquire very accurate topography data over all types of surfaces covered by the Sentinel-3 mission. And the “dual” central electronic chain ensures a high degree of reliability.

SRAL antenna: The antenna is made up of a 1.20 m parabolic reflector with a C/Ku dual frequency feed horn placed in a centered configuration at a focal length of about 430 mm. The feed is supported by 3 struts separated by a 120º angle: Two of them are doubled to improve the sidelobe ratio performance and the third one supports the Ku-band waveguide. It must be pointed out that the position of the strut ends on the reflector to match the reflector brackets position in order to improve the mechanical robustness of the antenna.

The antenna provides a minimum gain of 41.5 dBi in Ku-Band and 31.6 dBi in C-band at bore sight in the signal bandwidths. The side-lobe level is lower than –18 dB in Ku-band in order to minimize the Range Ambiguity Ratio.

Figure 15: Illustration of the SRAL antenna design (image credit: ESA)

The RFU (Radio Frequency Unit) equipment (Figure 16) is made up of slices which are stacked together except the C- and Ku-band duplexers that will be fixed independently on the satellite panel. The RFU up-converts chirp signals from 50 MHz to C- and Ku-band and provides an output power of 38 dBm in Ku-band and 43 dBm in C-band. The up -conversion stage also includes an expansion of the chirp bandwidth by a factor of 16. Received echoes in C- and Ku-band are deramped down to 100 MHz. The deramp output produces a useful signal bandwidth of 2.86 MHz which is then processed by the DPU.

Figure 16: Conceptual view of the RFU device (image credit: ESA)

MWR (Microwave Radiometer):

MWR is a nadir looking sounder, operating at 23.8 and 36.5 GHz (K/Ka-band) covering a bandwidth of 200 MHz in each channel. The objective is to provide water vapor and cloud water contents in the field of view of the altimeter, necessary to compensate for the propagation delay induced by these atmospheric components and affecting the radar measurements. Such corrections are only possible over the ocean, where the background noise is stable and can be quantified either by the 3rd (optional) radiometer channel, or derived from the altimeter measurements of the backscattered power. Alternatively, over ice and land surfaces where MWR data cannot be used, wet troposphere corrections will be derived based on global meteorological data and dedicated models.

MWR measures the thermal radiation emitted by Earth (brightness temperature). The received signal is proportional to the abundance of the atmospheric component emitted at the observed frequency and the sea-surface reflectivity. This information reveals the delay added to the altimeter pulses by moisture in the troposphere. The MWR instrument has a mass of 26.5 kg.

Figure 17: Conceptual view of the MWR instrument (image credit: ESA)

Table 6: Main characteristics of the MWR instrument

Conceptually, the MWR is a balanced Dicke radiometer for brightness temperatures below the Dicke load temperature. The balancing is achieved by means of a noise injection circuit. For brightness temperatures higher that the Dicke load temperature a conventional Dicke mode is used. The radiometer employs a single offset reflector of 60 cm in diameter and two separate feeds for the two channels. Calibration is achieved through a dedicated horn antenna pointing at the cold sky. The radiometer electronics unit consists of the radiometer processing module that provides the interface to the satellite’s main computer and the radio-frequency front end that contains the amplifiers, filters and the calibration/redundancy switch assembly.

Figure 18: Block diagram of the MWR instrument (image credit: ESA)

The REU (Radiometer Electronics Unit) consists of the RFFE (Radio Frequency Front End) and the RPM (Radiometer Processing Module). The RFFE is located as close as possible to the measurement feeds to optimize the length of the waveguides and thus the radiometric performance. It contains the amplifiers, switches (calibration and redundancy) and other performance determining elements. The RPM contains the thermal control, the RFFE control, the noise injection loop, the power supplies (also for the RFFE) and provides the electronic interface to the platform. The REU includes a mode to blank the receiver inputs when the radar altimeter emits its pulses to avoid potential disturbances. This mode is accessible by ground commands.

Antenna assembly: The antenna assembly consists of the main reflector that has a diameter of 60 cm, the two measurement feeds and the sky horn. The antenna assembly receives the noise temperature emitted by the objects within the antenna field of view. Discrimination between the different measurement frequencies is done by using different feed horns, each covering a separate frequency band. A separate sky measurement is provided by means of a dedicated sky horn. In this way, the satellite can continue the regular nadir measurements without the need of any maneuvers to turn over the satellite to look at the cold sky. The different frequencies received in the sky horn, are separated by a wave guide diplexer. The signals received by the feeds are guided towards the receiver electronics by means of waveguides. The physical temperatures of the different sections of the antenna assembly are measured and sent to the RPM (Radiometer Processing Module) of the REU.

The RFFE (Radio Frequency Front End) is manufactured by Thales Alenia Space Italia (TAS-I) and is implemented in a very compact design of 200 mm x 290 mm x 120 mm, a total mass of 5 kg, and 10 W power consumption.. ¹⁸⁾

Figure 19: Illustration of the RFFE instrument (image credit: TAS-I)

GNSS receiver:

A dual-frequency instrument based on GPS constellation - and optionally on Galileo. The objective is to provide data for precise orbit determination (POD), established after ground processing. In addition, the GNSS receiver will provide real-time navigation bulletins periodically, as required by the open loop tracking mode of the altimeter, with an accuracy of about 3 m rms. This information is used to control SRAL's open-loop tracking and for Sentinel-3 navigation. Ground processing yields the altitude to an accuracy of < 8 cm within 3 hours for operational applications, and 2 cm after some days of refinement.

The 11 kg GNSS receiver can track up to 12 satellites at the same time. The signals transmitted by the navigation satellites are also disturbed by the ionosphere. The effect is corrected by comparing two signals at different frequencies within 1160-1590 MHz.

LRR (Laser Retroreflector):

LRR is a passive device, composed of a set of corner cubes (mass of 1 kg). The LRR is mounted on the Earth panel of the spacecraft. Its purpose is to enable the accurate localization of the satellite from the ground, through laser ranging techniques. A network of laser ground stations (SLR) will be used for this purpose and their measurements will contribute to refining and validating the POD solutions derived from GNSS data.

Figure 20: Mission context showing the data distribution for an operational ocean forecasting system (image credit: ESA)

Sen3Exp (Sentinel-3 Experiment) campaign:

ESA conducted the Sen3Exp airborne campaign in June and July 2009. The campaign started in Barrax, La Mancha, Spain. An aircraft operated by the Spanish National Institute for Aerospace Technology (INTA), equipped with three hyperspectral imaging spectrometers, made two flights over the area. Meanwhile, satellite data were acquired by Envisat’s MERIS and AATSR instrument and by the CHRIS (Compact High Resolution Imaging Spectrometer) instrument aboard ESA’s PROBA-1 satellite. At the same time, ground teams, under the direction of Prof. Jose Moreno from the University of Valencia, made atmospheric radiometric and biophysical measurements. ¹⁹⁾

Figure 21: Photo of the CASA aircraft instrumentation (image credit: ESA)

Legend to Figure 21: Hyperspectral imaging spectrometers were installed on board INTA’s CASA-212-200 aircraft in support of the Sen3Exp airborne campaign. The AHS (Airborne Hyperspectral System) occupies the left-hand port; the CASI-1500i (Compact Airborne Spectrographic Imager) on the left and the SASI-600 (Shortwave Infrared Airborne Spectrographic Imager) occupy the right-hand port.

The campaign then moved to Pisa in Italy, from where a pine forest at San Rossore could be reached. At San Rossore, Prof. Federico Magnani from the University of Bologna oversaw the week-long ground measurement program. The dataset was again complemented with MERIS, AATSR and CHRIS satellite data.

In July, activities focused on the marine environment where measurements were taken at two oceanic sites: the Boussole monitoring buoy in the Ligurian Sea and the Aqua Alta Oceanographic Tower (AAOT) in the Adriatic Sea, close to Venice. Both sites have played an important role in supporting ocean color algorithm development and product validation for many years.

Figure 22: Photo of the AAOT (Aqua Alta Oceanographic Tower) in the northern Adriatic Sea (image credit: ESA)

Boussole typifies the global ocean, where the measured signal is determined solely by the absorption of phytoplankton. AAOT is in an area where there is both open ocean water and also water that is optically complex because phytoplankton, suspended sediments and colored dissolved organic matter also affect the measured signal. Such water can be found in all coastal regions and represents a challenge to analyze and interpret the data from spaceborne measurements.

A unique, comprehensive and valuable dataset has been created that will significantly support the development of the Sentinel-3 mission.

2) M. Aguirre, B. Berruti, J.-L. Bezy, M. Drinkwater, F. Heliere, U. Klein, C. Mavrocordatos, P. Silvestrin, B. Greco, J. Benveniste, “Sentinel-3: The Ocean and Medium-Resolution Land Mission for GMES Operational Services,” ESA Bulletin, No 131, Aug. 2007, pp. 24-29, URL: http://www.esa.int/esapub/bulletin/bulletin131/bul131c_aguirre.pdf

3) Y. Baillion, J. J Juillet, F. Paoli, M. Aguirre, B. Berruti, “GMES Sentinel-3: A long-term monitoring of ocean and land to support sustainable development,” 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07-B.1.2.04

4) http://www.ioccg.org/sensors/Regner_Sentinel.pdf

5) Mark R. Drinkwater, Helge Rebhan, “Sentinel-3: Mission Requirements Document,” ESA, Feb. 17, 2007, URL: http://esamultimedia.esa.int/docs/GMES/GMES_Sentinel3_MRD_V2.0_update.pdf

6) M. Drinkwater, “GMES Sentinel-3,” EUMETSAT SAF (Satellite Application Facility) Network Workshop, Darmstadt, Germany, Jan. 14-18, 2008, URL: http://www.eumetsat.int/Home/Main/What_We_Do/SAFs/The_Network/groups/pps/documents/document/pdf_saf_snw1_14_2.pdf

7) “Kopernikus: Observing our planet for a safer world,” Brussels, 16 September 2008, URL: http://europa.eu/rapid/pressReleasesAction.do?reference=IP/08/1330&format=HTML&aged=0&language=EN&guiLanguage=fr

8) http://www.esa.int/esaCP/SEMBFYP4KKF_index_0.html

9) “ESA preparing ‘sugar-cube’ gyro sensors for future missions,” ESA, Sept. 9, 2009, URL: http://www.esa.int/esaCP/SEMVXYUHYXF_index_0.html

10) J. Roselló Guasch, P. Silvestrin, M. Aguirre, L. Massotti, “Navigation needs for ESA’s Earth Observation missions,” Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-7, 2009, IAA-B7-1401

11) B. Berruti, J. Frerick, C. Mavrocordatos, J. Nieke, H. Rebhan, J. Stroede, and the S3 Team, “Status of the optical payload and processor development of ESA’s Sentinel 3,” 2nd MERIS/AATSR User Workshop, September 22-26, 2008, ESA/ESRIN Frascati, Italy, URL: http://earth.esa.int/workshops/meris_aatsr2008/participants/803/pres_803_Rebhan-Nieke.pdf

12) Henri Laur, “Sentinel-3 Operational Oceanography & Global Land Application,” MERIS US Workshop, Silverspring, MD, USA, July 14, 2008, URL: http://earth.esa.int/workshops/esameris08/presentations/Sentinel_3_OLCI.ppt

13) S. Varchetta, M. L'Abbate, M. Mappini, C. Svara, “From ERS to Sentinel 3 Altimetry Radiometers,” Proceedings of the 58th IAC (International Astronautical Congress), International Space Expo, Hyderabad, India, Sept. 24-28, 2007, IAC-07-B1.3.05

14) Y. Le Roy, M. Deschaux-Beaume, C. Mavrocordatos, M. Aguirre, F. Hélière, “SRAL SAR Radar Altimeter for Sentinel-3 mission,” Proceedings of IGARSS 2007 (International Geoscience and Remote Sensing Symposium), Barcelona, Spain, July 23-27, 2007

15) C. Mavrocordatos, B. Berruti, M. Aguirre, M. Drinkwater, “The Sentinel-3 mission and its Topography element,” Proceedings of IGARSS 2007 (International Geoscience and Remote Sensing Symposium), Barcelona, Spain, July 23-27, 2007

16) U. Klein, B. Berruti, F. Borde, C. Mavrocordatos, and the Sentinel-3 team, “The Sentinel-3 Topography Payload,” Proceedings of the 2nd Workshop on Advanced RF Sensors and Remote Sensing Instruments 2009, Noordwijk, The Netherlands, Nov. 17-18, 2009

17) Gary M. Mineart, “Emerging Space-Based Radar Altimeter Technologies,” 2005, URL: http://www.noblis.org/BusinessAreas/Mineart_Sigma_Paper_2005.pdf

18) F. Barletta, M. Imparato, L. Battaglia, P. V. Giove, P. Colucci, A. Massari, A. Suriani, “Sentinel-3, NIR Radiometer K/Ka-band Radio FrequencyFront End,” Proceedings of the 2nd Workshop on Advanced RF Sensors and Remote Sensing Instruments 2009, Noordwijk, The Netherlands, Nov. 17-18, 2009

19) “ESA campaign reveals glimpse of future Sentinel-3 imagery,” Sept. 9, 2009, URL: http://www.esa.int/esaLP/SEMOW432BZF_LPcampaigns_0.html