GMES SENTINEL-1 MISSION AND SYSTEM

Evert Attema(1), Paul Snoeij(1), Berthyl Duesmann(1), Malcolm Davidson(1), Nicolas Floury(1), Betlem Rosich(2), Bjorn Rommen(1) and Guido Levrini(1)

(1) ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk, Netherlands (2) ESA/ESRIN, Via Galileo Galilei, 00044 Frascati, Italy

ABSTRACT

The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. This paper describes the Sentinel-1 mission, an imaging synthetic aperture radar (SAR) satellite constellation at C-band. It provides an overview of the mission requirements, its applications and the technical concept for the system. 1. INTRODUCTION

‘Global Monitoring for Environment and Security (GMES)’ is a joint initiative of the European Commission (EC) and the European Space Agency (ESA), designed to support Europe’s goals regarding sustainable development and global governance of the environment by providing timely and quality data, information, services and knowledge. In the frame of the GMES programme, ESA is undertaking the development of the European Radar Observatory Sentinel-1, a European polar orbiting satellite system for the continuation of SAR operational applications. Sentinel-1 is an imaging radar mission in C-band, aimed at providing continuity of data for user services, in particular with respect to the ESA ERS and Envisat missions. The Sentinel-1 space segment will be designed and built by an industrial consortium with Thales Alenia Space Italia as prime contractor and EADS Astrium GmbH as C-SAR instrument responsible. 2. ESA’S SAR MISSION HERITAGE

A first glimpse of the potential of imaging radar from space was provided by the short-lived but ultimately successful Seasat mission launched in 1978 by the US. Subsequently the European Space Agency then initiated its own program to develop and launch advanced microwave radar instruments culminating with the launch of the European Remote Sensing Satellites ERS-1 (launched 17 July 1991) and ERS-2 (launched 20 April 1995). The ERS missions demonstrated for the first time not only the feasibility of developing reliable, stable and

powerful radar imaging systems for space, but, the dependability and all-weather capability of the instruments also provided a foundation for the development and exploitation of radar images in a wide variety of applications. While the initial objectives for ERS-1 at launch were predominantly oceanographic, other applications were also considered during the initial phases of the mission preparation. Interestingly the ESA Remote Sensing Advisory Group in 1974, as an example, emphasized commercial applications such as agriculture, land use mapping, water resources, overseas aid and mapping of mineral resources when formulating their advice on the ERS mission objectives. As the ERS mission showed, the rigorous design of the SAR instruments - which emphasized instrument stability combined with accurate well-calibrated data products – has created new opportunities for scientific discovery, revolutionized many Earth science disciplines and laid the foundations for the development of commercial applications exploiting spaceborne SAR data. An example of note can be found in the field of SAR interferometry that was developed mainly on the basis of ERS SAR data and is now commonly used in Earth sciences and a number of commercial applications. In a further step, the potential of spaceborne radar with short time-intervals between successive acquisitions in applications was demonstrated in 1996 and 1997 during the ERS tandem mission, when ERS-1 and ERS-2 flew in a two satellite constellation with one day interval between acquisitions. A further important milestone came with the launch of the Advanced SAR (ASAR) onboard ENVISAT that was launched on 28 February 2002. This not only ensured the continuation of C-Band data for applications, but provided new enhanced capabilities such as wide swaths and dual polarisation which have since rapidly been integrated into and exploited by many radar-based applications. The archive of radar data spanning from 1991 to the present has also proven highly valuable for science and applications providing consistent time-series of data over 16 years. 3. GMES USER REQUIREMENTS

Data products from the Agency‘s successful ERS-1, ERS-2 and Envisat missions form the basis for many of the pilot GMES services. Consequently Sentinel-1 data

_____________________________________________________ Proc. of ‘4th Int. Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry – PolInSAR 2009’, 26–30 January 2009, Frascati, Italy (ESA SP-668, April 2009)

products need to maintain data quality levels of the Agency‘s previous SAR missions in terms of spatial resolution, sensitivity, accuracy, polarisation and wavelength. Feedback from users following the consolidation phase of the GMES services indicates unambiguously that the crucial requirements for operational sustainable services are continuity of data supply, frequent revisit, geographical coverage and timeliness. Compared to the current satellites in orbit substantial improvements of data provision in terms of revisit frequency, coverage, timeliness and reliability of service would be required. As an example, services related to ocean and sea ice encompassing ship and oil spill detection, wind speed measurements and sea ice monitoring require frequent revisit (once per day, mostly at northern latitudes) and timeliness (near-real-time delivery of data within one hour of acquisition). Services related to land encompassing interferometry and land cover classification mostly require global coverage every two weeks at most and consistent data sets that are suitable for interferometry [1]. 4. SENTINEL-1 SYSTEM

Sentinel-1 is a constellation of two imaging Synthetic Aperture Radar missions at C-band. The design of its system has been driven by the need for continuity of ERS/Envisat class data provision with improved revisit, coverage, timeliness and reliability of service. The main technical characteristics are listed below. Figure 1 shows an artist impression of Sentinel-1A in orbit.

Figure 1 Artist impression of Sentinel-1A 4.1. Spacecraft

Sentinel-1 is being realized by an industrial consortium lead by Thales Alenia Space Italy as Prime Contractor, with Astrium Germany being responsible for the C-SAR payload, incorporating the central radar electronics sub-system developed by Astrium UK. The spacecraft is based on the PRIMA (Piattaforma Italiana Multi Applicativa) bus, with a mission specific payload

module as per the PRIMA concept. Experience gained from the RadarSat-2 and from the Cosmo-Skymed programs, in which PRIMA also was selected as the spacecraft baseline, is a benefit for the Sentinel-1 implementation. The Sentinel-1 main characteristics are shown in Table 1. Table 1 Sentinel-1 Spacecraft Characteristics

Lifetime 7 years (consumables for 12 years) Orbit Near-Polar Sun-Synchronous @

693km; 12-day repeat cycle; 175 orbits per cycle

Mean Local Solar Time

18:00 at Ascending Node

Orbital Period 98.6 minutes Max Eclipse Duration 19 minutes Attitude Stabilisation 3 axis stabilised Attitude Accuracy 0.01 deg (each axis) Nominal flight Attitude

Right Looking

Attitude Profile Geo-centric and Geodetic Orbit Knowledge 10 m (each axis, 3 sigma) using

GPS Operative Autonomy 96 hrs Launch Mass 2300 kg (incl. 130 kg

monopropellant fuel) Dimensions (stowed) 3900 x 2600 x 2500 mm3 Solar Array Average Power

4800 W (End-of-Life)

Battery Capacity >300 Ah Spacecraft Availability

0.998

Science Data Storage Capacity

1400 Gigabits (End-of-Life)

S-Band TT&C Data Rates

4 kbps TC; 16/128/512 kbps TM (programmable)

X-Band Science TM Data Rate

600 Mbps

Launcher Soyuz from Kourou (baseline), Zenith II (backup)

4.2. Instrument

The Sentinel-1 satellite carries a Synthetic Aperture Radar (SAR) instrument. The SAR design capitalises on Envisat ASAR technologies and consists of an electronically steered array antenna. 4.3. Data Delivery

Due to consistent and conflict-free mission operations Sentinel-1 provides a high level of service reliability with near-real time delivery of data within one hour after reception by the ground station and with data delivery from archive within 24 hours.

4.4. Launch Date

It is expected that Sentinel-1A be launched in 2011. 4.5. Mission Operations

Once in orbit Sentinel-1 will be operated from two centres on the ground, the Agency‘s facilities in Darmstadt, Germany for commanding the satellite and ensuring its proper functioning along the orbit. The mission exploitation will be managed at the Agency’s facilities in Frascati, Italy, including the planning of the acquisitions by the SAR instrument according to the mission requirements, the processing of the acquired data and the provision of the resulting products to the users. 4.6. Ground Segment

The ground segment design and operations concept however allow for operations to be handed over – partially or fully - to other operating entities in the future. Compared to previous missions the Sentinel-1 satellite operations and its mission exploitation present new challenges; the spacecraft needs to operate within a tight orbital tube of only 100 meters and needs to comply with GMES security requirements for command and control of the spacecraft. Its operations will be largely automated for nominal operations but emergency requests in support of disaster management applications have to be accommodated on short notice. Following mission requirements mission exploitation plans need to facilitate systematic acquisition, reception, processing, archiving and provision of large amounts of data to the users. The SAR instrument will be operated - to the maximum extent possible - in a pre-programmed conflict-free sensing mode. Payload data handling will be driven by the data as received from the two satellite constellation by a large system able to handle data flows from the satellite exceeding 2 Terabyte per day and to provide large data volumes within one hour after reception on the ground. The operations concept allows the satellite to operate autonomously and in a cost-effective manner with a mission plan on-board covering a period of 4 days, thus allowing automated operations over weekends. At the same time it is possible to insert individual emergency requests up to three hours prior to the nominally planned update of the mission plan to the satellite. This allows considerably shortening the response time of Sentinel-1 compared to its predecessors. An extensive ground segment is required with several ground stations receiving instrument data from the satellite at a rate of 600Mbit/sec, with cumulative processing capacities above 500 GHz, with archiving requirements exceeding 10000 Terabytes, and with a data dissemination exceeding current systems by one order of magnitude. In order to fully satisfy the GMES service requirements the Sentinel-1 ground segment is required to facilitate

coordinated mission planning and data exchange with other GMES contributing missions. The ground segment needs to guarantee a Quality of Service to the user in line with the operational nature of GMES, ensuring that the data products are accurate, complete and provided on time. 5. C-SAR INSTRUMENT DESIGN

5.1. Introduction

The Sentinel-1 mission requirements ask for data products with different image characteristics, which require the implementation of four different measurement modes:

• Stripmap Mode • Interferometric Wide-swath Mode • Extra-Wide-swath Mode • Wave Mode

Except for the Wave Mode, which is a single polarisation mode (HH or VV), the SAR instrument has to support operation in dual polarisation (HH-HV, VV-VH), which requires the implementation of one transmit chain (selectable to H or V) and two parallel receive chains for H and V polarisation. The specific needs of the four different measurement modes with respect to antenna agility require the implementation of an active phased array antenna. In Stripmap mode the instrument has to provide an uninterrupted coverage with a high geometric resolution (5 m x 5 m) at a medium swath width of 80 km. Six overlapping swathes cover the required access range of 375 km. For each swath the antenna has to be configured to generate a beam with fixed azimuth and elevation pointing. Appropriate elevation beam forming has to be applied for range ambiguity suppression. To meet the ambitious image requirements, the Interferometric Wide-swath mode has to be implemented as a ScanSAR mode with progressive azimuth scanning. This requires a fast antenna beam steering in elevation for ScanSAR operation, i.e. transmitting a burst of pulses towards a sub swath. In addition, fast electronic azimuth scanning has to be performed per sub swath (TOPS operation [2]) in order to average the performance in along track direction (reduction of scalloping). Hence, the Interferometric Wide-swath mode will allow combining a large swath width (250 km) with a moderate geometric resolution (5 m x 20 m). Interferometry has to be ensured by sufficient overlap of the Doppler spectrum (in the azimuth domain) and the wave number spectrum (in the elevation domain). Extra-Wide-swath mode and Wave mode complement the SAR data products. In Extra-Wide-swath mode a huge swath width of 400 km has to be covered with low resolution (20 m x 40 m), which can be met by the implementation of a ScanSAR mode with a fast beam

scanning capability in elevation. As the Interferometric Wide-swath mode also the Extra-Wide-swath mode is implemented with a progressive azimuth scanning (TOPS operation). Finally, the Wave mode data product is composed of single strip map operations with an alternating elevation beam (between 23 and 36.5 mid incidence angle) and a fixed on/off duty cycle, which results in the generation of vignettes of 20 km x 20 km size in regular intervals of 100 km. A summary of the SAR instrument measurement modes and their characteristics is given in Table 2. Table 2 Instrument Mode Requirements Mode Access

Angle Deg.

Single Look Resolution Range x Azimuth

Swath Width

Polarisation

Strip Map 20-45 5 x 5 m > 80 km HH+HV or VV+VH

Interferometric Wide Swath

> 25 5 x 20 m > 250 km HH+HV or VV+VH

Extra Wide Swath

> 20 20 x 40 m > 400 km HH+HV or VV+VH

Wave mode 23 and 36.5

20 x 5 m > 20 x 20 km Vignettes at 100 km intervals

HH or VV

For All Modes

Radiometric accuracy (3 σ) 1 dB

Noise Equivalent Sigma Zero -22 dB

Point Target Ambiguity Ratio -25 dB

Distributed Target Ambiguity Ratio -22 dB

5.2. Instrument Architecture and Functionality

The C-SAR instrument is a synthetic aperture radar operating in the C-Band with horizontal and vertical polarisation. It is composed of two major subsystems

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

The radar signal is generated at baseband by the chirp generator and up-converted to C-band within the SES. This signal is distributed to the High Power Amplifiers inside the EFE Transmit/Receive Modules via the beam-forming network of the SAS. Signal radiation and echo reception is realized with the same antenna using slotted waveguide radiators. In receive, the echo signal is amplified by the low noise amplifiers inside the EFE Transmit/Receive Modules and summed up using the same network as for transmit signal distribution. After filtering and down conversion to base band inside the

SES, the echo signal is digitised and formatted for recording. Table 3 provides a brief overview on the instrument key parameters. Table 3 Instrument Key Parameter Parameter Value Centre Frequency 5.405 GHz Instrument Mass 945 kg DC-Power Consumption 4075 Watt

(Interferometric Wide-swath Mode, two polarisations)

Bandwidth 0 … 100 MHz (programmable)

Polarisation HH-HV, VV-VH Antenna Size 12.3 m x 0.821 m RF Peak Power (sum of all TRM, at TRM o/p)

4368 W

Pulse Width 5-100 us (programmable) Transmit Duty cycle

• max 12%

• Stripmap 8.5 %

• Interferometric Wideswath 9 %

• Extra Wide swath 5 %

• Wave 0.8%

Receiver Noise Figure at Module input 3 dB Pulse Repetition Frequency 1000- 3000 Hz

(programmable) ADC Sampling Frequency 260 MHz (real sampling)

(Digital down-sampling after A/D conversion)

Sampling 10 bits Data Compression According to FDBAQ

Figure 2 C-SAR Instrument Functional Block Diagram

The key design aspects of the C-SAR Payload can be summarized as follows:

• Active phased array antenna providing fast scanning in elevation (to cover the large range of incidence angle and to support ScanSAR operation) and in azimuth (to allow use of TOPS technique to meet the required image performance)

• Dual channel Transmit & Receive Modules and H/V-polarised pairs of slotted waveguides (to meet the polarisation requirements)

• Internal Calibration scheme, where transmit signals are routed into the receiver to allow monitoring of amplitude/phase to facilitate high radiometric stability

• Metallised Carbon Fibre Reinforced Plastic radiating waveguides to ensure good radiometric stability even though these elements are not covered by the internal calibration scheme

• Digital Chirp Generator and selectable receive filter bandwidths to allow efficient use of on board storage considering the ground range resolution dependence on incidence angle

• Flexible Dynamic Block Adaptive Quantisation to allow efficient use of on-board storage and minimise downlink times with negligible impact on image noise

Figure 3 Three-Dimensional View of the SAS Tile Layout A three dimensional view of the SAS Tile, which is the elementary building block of the SAR Antenna Subsystem, is given in Figure 3 Important to mention is also the roll steering mode of the spacecraft. This continuous roll manoeuvre around orbit (similar to yaw steering in azimuth) compensates for the altitude variation such that it allows usage of a minimal number of different pulse repetition frequency’s (PRFs) and sample window lengths (SWLs) round the orbit. So, only one single PRF and SWL are necessary per swath/sub-swath around orbit, with the single exception of Stripmap swath 5 where two different PRFs and SWLs have to be used. In addition, the update rate of the sampling window position around orbit is minimised (< 1 /2.5 min), which simplifies instrument operations significantly. Since the instrument can work with a single fixed beam for each swath/sub-swath over the complete orbit, also the number of elevation beams is minimised. The roll steering rate has been fixed to 1.6 deg/ 27 Km altitude variation. The roll applied to the sensor attitude depends linearly on altitude and varies within the interval -0.8 deg

(minimum sensor altitude) to 0.8 deg (maximum sensor altitude). 5.3. Radiometric Resolution

For the end-user of SAR imagery the radiometric resolution is a critical parameter as it defines the typical image noise in radar images caused by thermal noise and speckle. Image noise defines how well different surfaces (e.g. ice types, agricultural crops, soil moisture levels) can be classified. To illustrate the improvement to be expected from Sentinel-1 compared to existing systems in orbit Figure 4 shows the radiometric resolution of a 500 x 500 metre image pixel for the ASAR wide swath mode, the Sentinel-1 extra-wide swath mode and the Sentinel-1 interferometric wide swath mode. In its extra-wide swath Sentinel-1 offers 30% performance improvement with respect to Envisat while the interferometric wide swath of Sentinel-1 offers a further improvement by a factor of three.

0%

5%

10%

15%

20%

25%

-25 -20 -15 -10 -5 0

Sentinel-1 InterferometricWide SwathSentinel-1 Extra-wide Swath

ASAR Wide Swath

Figure 4 Radiometric Resolution of Radar Images: percentage image fluctuation in radar imagery as a function of radar backscattering coefficient for a pixel size of 500 x 500 metres. 6. FDBAQ

Raw SAR data compression has been applied for the first time in the NASA Magellan mission to Venus from 1989 to 1994 [3] with a view to reduce the instrument data rate to fit the available data downlink. For the same reason the ASAR data from the ENVISAT satellite are also transmitted in a raw compressed format. The type of compression applied in these cases is Block Adaptive Quantisation (BAQ). It selects a limited number of bits (typically 4) out of the available number of bits (typically 8) thus reducing data rates by typically a factor of two. Bit allocation depends on the signal levels in the radar receiver averaged over a block of typically a few hundreds of pixels. BAQ raw SAR data compression is not lossless. The digitisation and coding process introduces additional noise effects on the SAR images to be processed. A novel algorithm for SAR compression is presented below, that is able to meet in particular the interferometric requirements for the Sentinel-1 mission by taking the limitations of the traditional compression

schemes related to a poor response of weak targets in the presence of strong permanent scatterers in consideration: Flexible Dynamic Block Adaptive Quantisation (FDBAQ). Unlike conventional BAQ coding schemes that operate with a fixed output bit rate FDBAQ is designed to provide a variable bit rate coding that increases the number of bits to be allocated to the bright target areas whereby the actual Clutter level of the raw data determines the number of allocated bits. 6.1. FDBAQ Principle

The quantisation of raw SAR data introduces a quantisation noise power Q that dependents upon the input signal, clutter and thermal noise, and the quantiser bit rate as follows:

Q =T + Cg(R)

(1)

where C is the clutter power at the receiver, T the thermal noise power and g(R) the quantiser gain, a function of the bit rate R. It follows that at the output of the quantiser the total (quantisation and thermal) noise N equals: N = T + Q = T +

T + Cg R( )

(2)

For an entropy constrained quantiser the following expression has been derived in [4]:

g R( )=10−0.156+0.0595R (3)

FDBAQ selects the rate R based on both the average clutter level and the average thermal noise level in a data block to achieve the specified radar sensitivity at the lowest possible data rate. Contrary to intuitive expectation it follows from (2) and (3) that in order to keep the total noise power below a specified level the number of allocated bits need to be increased with increasing echo signal strength. 6.2. Bit Rate Selection

The sensitivity requirement for a SAR is commonly specified in terms of noise equivalent sigma zero (NESZ) representing a value and the radar backscattering coefficient that produces the same signal levels in the radar receiver as the noise. The NESZ depends on observation geometry and system parameters (transmitted power, antenna gain, noise figure, etc), as modelled by the RADAR equation. In particular, there is a systematic variation of NESZ with range (r) that is mainly due to the antenna pattern, with minor contribution from spreading loss and incidence angle variation.

If n(r) is defined as the calibration function that represents the ratio of receiver noise and NESZ in the processed radar image it follows from (3) that:

NESZ r,R( )= n r( )T + n r( )T + Cg R( )

(4)

The first term in this expression represents the thermal noise equivalent sigma zero (TNESZ) and the second term represents the quantisation noise equivalent sigma zero (QNESZ). For Sentinel-1 the sensitivity requirements is defined by:

NESZ ≤10−2.2 (5)

The bit rate requirement follows from (4) and (5):

g R( )≥n r( ) T + C[ ]

10−2.2 − TNESZ r( ) (6)

and for an entropy constrained quantiser it follows from (3) that:

R ≥

LOG10n r( ) T + C[ ]

10−2.2 − TNESZ r( )⎡

⎣ ⎢

⎤

⎦ ⎥ + 0.156

0.0595 (7)

This expression defines the bit rate requirement as a function of parameters derived from system characterisation, n(r) and TNESZ(r) as well as on-board measurement of [T+C]. The implementation of the quantiser is given in the block diagram shown in Figure 5.

Figure 5 Block Diagram of FDBAQ It is noted that the FDBAQ quantiser measures averages of clutter power + noise of raw data in blocks of a few hundred samples, like any other BAQ. The principal difference between the FDBAQ and the conventional BAQ is that the number of bits per sample, R, is dynamically adjusted to provide sufficient sensitivity at the lowest allowable data rate. For practical implementation reasons for Sentinel-1 the bit rate selection is limited to R = 3.0, 3.4, 3.8, 4.2 or 5.0. The

upper bound avoids the indefinite increase in R as TNESZ(r) approaches 10−2.2 . 6.3. FDBAQ Advantages

The main driver for applying FDBAQ is data rate reduction. As the simulation below indicates the average data rate for the Sentinel-1 interferometric wide swath mode is reduced from 301 Mbit/s to 245 Mbit/s per polarization channel, a reduction of about 20%. Furthermore the signal-to-noise ratio is significantly improved for regions of radar echo signals such as urban environments and large portions of Greenland. This is illustrated in Figure 6 showing on a logarithmic scale total power (NESZ) as a function of the backscattering coefficient of the area being imaged.

Sentinel-1 Data Compression

-25

-22

-19

-16

-13

-10

-25 -20 -15 -10 -5 0 5 10

Backscatter Coefficient (dB)

FDBAQ ECBAQ

Figure 6 Sentinel-1 Thermal and Quantisation Noise It is demonstrated that the system reduces the bit rate below a backscattering coefficient of -7 dB compared to ECBAQ at the expense of a small permissible reduction of sensitivity. Above -7 dB the system uses more bits to significantly improve the sensitivity. 6.4. Simulation Results

The simulation described in this section calculates performances in terms of CNR and bit rate by using synthetic Sentinel-1 acquisitions along the orbit.

Figure 7 ENVISAT GM Mosaic

The simulation assumes the nominal Sentinel-1 orbit and swaths, and estimates the ground backscatter by using a Global Monitoring ENVISAT/ASAR Mosaic covering almost the entire world (Figure 7). Such mosaic has been normalized after processing, therefore a renormalisation and calibration is needed in order to use this data for simulation purposes. The calibration and validation of the mosaic are beyond the scope of this paper. The aim of this simulation is to estimate a range and azimuth varying bit-rate and assign, to each portion of illuminated ground, a figure of merit related to the total noise power (sum of the thermal noise power and the quantisation noise power).The FDBAQ operates block wise in the range direction, as usual for BAQ SAR compression. The block size should be dimensioned in order to have enough samples to estimate the clutter power and, at the same time, to exploit an homogeneous set of samples, from a statistical point of view. In the frame of the GS1 mission the block size is not critical, as all the samples within the duration of a chirp are correlated. Typical chirp duration corresponds to several km on the ground, therefore several thousands of samples. Therefore the 1 km quantisation of the backscatter mosaic is sufficient. Likewise, the azimuth resolution of the unfocused SAR, due to the real antenna pattern, is in the order of a few km, here again we observe that the mosaic sampling of 1 km is more than sufficient. A Sentinel-1 Interferometric Wide (IW) swath mode simulation has been performed over a Greenland test site. Eight different strips have been considered, each one comprising 400 [s] of acquisition time. A net download capacity of 260 Mbit/s for one channel has been assumed and a threshold level of -22 dB was chosen. Figure 8 shows the ENVISAT/ASAR GM calibrated mosaic of all the considered strips.

Figure 8 ENVISAT ASAR GM calibrated mosaic of eight GMES Sentinel-1 acquisitions over Greenland Figure 9 presents the estimated quantisation bits. Discrete quantisation levels haven't been applied, so that

estimated values can change continuously from a minimum of 3 to a maximum of 5 bit/sample.

Figure 9 Estimated number of quantisation bits over 8 simulated GMES Sentinel-1 IW strips Figure 10 shows the Total Estimated Noise Power [dB], for the GMES Sentinel-1 IW strips over Greenland.

Figure 10 Estimated Total Noise Power in dB It can be seen, that the IW1 (the right hand swath) is the swath with the highest value of Total Noise Power. In the next simulation five different quantisation levels have been used: 3.0; 3.4; 3.8; 4.2; and 5.0. Figure 11 shows the mean bit rate for all IW beams and acquired strips. As it can be seen, the IW1 mean data rate is higher than the required 260 Mbit/s, but the overall average is well within the limits.

Figure 11 Mean bit rate over all Greenland strips

7. Conclusions

The Sentinel-1 synthetic aperture radar (SAR) constellation represents a completely new approach to SAR mission design by ESA in direct response to the operational needs for SAR data expressed under the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The mission ensures continuity of C-Band SAR data to applications and builds on ESA’s heritage and experience with the ERS and ENVISAT SAR instruments, notably in maintaining key instrument characteristics such as stability and accurate well-calibrated data products. At the same time a number of mission design parameters have been vastly improved to meet major user requirements collected and analysed through EU Fast Track and ESA GSE activities, especially in areas such as reliability, revisit time, geographical coverage and rapid data dissemination. As a result, the Sentinel-1 constellation is expected to provide near daily coverage over Europe and Canada, global coverage all independent of weather with delivery of radar data within 1 hour of acquisition – all vast improvements with respect to the existing SAR systems. In addition to responding directly to current needs of the GMES program, the design of the Sentinel-1 satellite mission with its focus on stability, reliability, global coverage, consistent operations and quick data delivery is expected to enable the development of new applications and meet the evolving needs of GMES, for instance in the area of climate change and associated monitoring needs. The FDBAQ quantiser has been introduced and a method for the evaluation of FDBAQ performances has been presented. The FDBAQ theoretical performances have been evaluated and different TOPSAR IW simulations using Sentinel-1 orbits and parameters have been performed. FDBAQ satisfies the requirement of an average bit rate of less than 260 Mbit/s. 8. References

[1] E. Attema, “Mission Requirements Document for the European Radar Observatory Sentinel-1”, ES-RS-ESA-SY-0007, Issue 1.4, 11 July 2005.

[2] F. De Zan, A.M. Monti Guarnieri, “TOPSAR: Terrain Observation by Progressive Scans”, IEEE Transactions on Geoscience and Remote Sensing, Volume 44, Issue 9, Sept. 2006 Page(s): 2352 - 2360.

[3] Kwok, R., W.T.K. Johnson, “Block Adaptive Quantisation of Magellan SAR Data”, IEEE Trans. On Geoscience and Remote Sensing, Vol. 27, No. 4, pp. 375-383, July 1989.

[4] Algra, T., “Data compression for operational SAR missions using Entropy-Constrained Block Adaptive Quantisation”, Proc. IGARSS 2002, Toronto, Canada, 24-28 June 2002.