GMES Sentinel-1 mission

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Abstract

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 services and 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, mission control, data processing, dissemination and archiving. Free and open data access is provided. Data quality of the Sentinel-1 data products is shown along with uncertainty estimation of retrieved information products confirming specified performance and indicating application growth potential. The unique data availability performance of the Sentinel-1 routine operations makes the mission particularly suitable for emergency response support, marine surveillance, ice monitoring and interferometric applications such as detection of subsidence and landslides.

Highlights

► Sentinel-1 is designed to work in a pre-programmed conflict-free operation mode. ► A consistent data archive is built for applications based on long time series. ► The two-satellite constellation offers six-day exact repeat. ► The Sentinel-1 mission represents an n-fold evolution of capability. ► Access to the Canadian C-band SAR Constellation may improve overall performance.

Introduction

The short-lived but successful US SEASAT satellite provided a first glimpse of the potential of imaging radar from space in 1978. ESA's own programme to develop advanced microwave radar instruments culminated with the launches of ERS-1 (17 July 1991) and ERS-2 (20 April 1995). ERS demonstrated for the first time the feasibility of flying reliable, stable and powerful radar imaging systems in space. The dependability and all-weather capability of the instruments also provided a foundation for developing and exploiting radar images for a wide variety of applications. While the initial objectives for ERS-1 at launch were predominantly oceanographic, other applications were considered during the project's preparation. The ESA Remote Sensing Advisory Group in 1974, for example, emphasised commercial applications such as agriculture, land-use mapping, water resources, overseas aid and mapping of mineral resources in its advice on ERS objectives. The rigorous design of the ERS SAR hardware – emphasising instrument stability in combination with accurate and well-calibrated data products – created new opportunities for scientific discovery, revolutionised many Earth science disciplines and laid the foundations for commercial applications.

For example, ‘SAR interferometry’, which can track land shifts of only a few millimetres, was developed mainly using ERS data and is now commonly used in Earth sciences and commercial applications. The potential of space radars viewing the same scene only a short time apart was demonstrated in 1995 and 1996 during the ERS ‘tandem mission’, when the orbits of ERS-1 and ERS-2 were carefully matched 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 continuation of C-band data and added enhanced capabilities such as wide swaths and dual polarisation, features that have since rapidly been integrated into and exploited by many applications. The archive of radar data since 1991 is extremely valuable for science and applications, providing a consistent set of data spanning 16 years.

The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the European Union (EU) — ESA Global Monitoring for Environment and Security initiative. The GMES Space Component (GSC) programme relies strongly on new complementary developments by ESA in addition to existing and planned space assets from different agencies. As part of the GSC, ESA is currently undertaking the development of 3 Sentinel mission families.1 Each Sentinel is based on a constellation of 2 satellites in the same orbital plane. This configuration allows to fulfil the revisit and coverage requirements, and to provide a robust and affordable operational service. The life time of the individual satellites is specified as 7 years, with consumables on-board each satellite allowing a mission extension up to 12 years. The life cycle of a satellite generation is planned to be in the order of 15–20 years.

Section snippets

The Sentinel-1 mission

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

Sentinel-1 satellite

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

The spacecraft is based on the PRIMA (Piattaforma Italiana Multi Applicativa) Platform with a mission specific payload module, in this case the CSAR Instrument. The design of the satellite leverages experience gained from the Canadian

Modes of operation

Following the Sentinel-1 mission requirements one main operational imaging mode the Interferometric Wide-swath mode (IW) is provided that, in combination with the Wave mode (WV) satisfies most currently known service requirements. Additionally the mutually exclusive Strip Map mode (SM) and the Extra Wide-swath mode (EW) are provided for continuity reasons (with respect to ERS and Envisat) as well as for accommodation of emerging user requirements. Two mutually exclusive dual polarisation modes

PDGS main functions

The Sentinel-1 Payload Data Ground Segment (PDGS) provides reception of CSAR instrument data, systematic processing, archiving and dissemination of operational SAR data for the GMES services, and carries out various planning and monitoring tasks including CSAR instrument activities and X-Band downlink operations, on the basis of a systematic pre-defined observation scenario.

Mission operation concept

The Sentinel-1 mission is designed to provide ‘guaranteed data services’ for which liability can be accepted commensurate with the user needs following the GMES public institutional user model. This is common practice for meteorological data provision. Most acquisitions can be pre-planned and the routine operations are normally not interrupted. Therefore no specific acquisition requests are required and data access is mainly by subscription supporting multi-temporal observations and near real

Sentinel-1 user products

Following the GMES Space Component data, Sentinel-1 data access is based on systematic and free on-line data dissemination at no charge and without the need for an ordering process except in cases of support to emergency situations.

The Sentinel-1 mission will be able to provide a variety of image and/or information products to the users. The Sentinel-1 Core PDGS will generate some of these products operationally, the Sentinel-1 Core PDGS or the Sentinel-1 Collaborative PDGS may support some of

Calibration and validation of Sentinel-1 products

The Synthetic Aperture Radar measures a series of radar echoes that are processed within the ground segment to generate SAR data products. Main contents of these data products are the normalised radar cross-section and the phase of the individual pixels. Both quantities have to be provided with specified stability and accuracy. This is ensured through calibration and validation tasks executed throughout the mission lifetime.

The SAR images are generated on the basis of the individual radar

Overview

Data products from Sentinel-1 consist of imagery representing the radar echo from the Earth surface. Basic calibrated data are provided in the form of both complex (In-phase and Quadrature components) and intensity images. These are referred to as Level-1. Level-1 performance is defined by the parameter values listed in Table 3. Level-2 data products include extracted information from the Level-1 imagery in the form of estimated values of the geophysical variables listed in Davidson et al.

Conclusion

The Sentinel-1 constellation is 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 GMES programme. The mission ensures continuity of C-band SAR data and builds on ESA's heritage and experience with the ERS and Envisat instruments (Fig. 15), notably maintaining key characteristics such as stability and accurate well-calibrated data products. At the same time, mission parameters have been vastly improved to

Acknowledgement

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

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