ENVISAT PAYLOAD

ASAR (Advanced Synthetic Aperture Radar) AATSR (Advanced Along Track Scanning Radiometer) DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) GOMOS (Global Ozone Monitoring by Occultation of Stars) MERIS (MEdium Resolution Imaging Spectrometer) MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) MWR (MicroWave Radiometer ) RA-2 (Radar Altimeter 2) SCHIAMACHY

An Advanced Synthetic Aperture Radar (ASAR), operating at C-band, ensures continuity with the Image Mode (SAR) and the Wave Mode of the ERS-1/2 AMI. It features enhanced capability in terms of coverage, range of incidence angles, polarisation and modes of operation. This enhanced capability is provided by significant differences in the instrument design: a full active array antenna equipped with distributed transmit/receive modules which provides distinct transmit and receive beams, a digital waveform generation for pulse "chirp" generation, a block adaptive quantisation scheme and a ScanSAR mode of operation by beam scanning in elevation.

The prime scientific objective of the Advanced Along Track Scanning Radiometer (AATSR) is to establish continuity of the ATSR-1 and ATSR-2 data sets of precise Sea Surface Temperature (SST), thereby ensuring the production of a unique 10 year near-continuous data set at the levels of accuracy required (0.3K or better) for climate research and for the community of operational as well as scientific users who will have been developed through the ERS-1 and ERS-2 missions.

Doppler Orbitography and Radiopositioning Integrated by Satellite
DORIS is a tracking system providing range-rate measurements of signals from a dense network of ground-based beacons. These data are precission processed on ground providing the satellite orbit with an accuracy of the order of centimetres. They are also processed on board to provide real-time satellite positions with an accuracy of some tens of centimeters.

In addition to enabling orbit determination, data are provided to

• help in the understanding of the dynamics of the solid Earth;
• monitor glaciers, landslides and volcanoes;
• improve the modelling of the Earth gravity field and the ionosphere

Global Ozone Monitoring by Occultation of Stars
During the last decades it has become increasingly apparent that the chemical composition of the atmosphere is changing on a global scale and that human activity is partly responsible for this change. Ozone plays a central role in the atmospheric chemistry. It is largely responsible for stratospheric heating through absorption of harmful UV radiation, it determines to a large extent the oxidative capacity of the troposphere and is an important "greenhouse" gas. The discovery of the "Ozone hole" over the Antarctic has also drawn attention to the global ozone budget.

In order to understand the processes which determine the physical and the photochemical behavior of the atmosphere, detailed global measurements of the amount and of the horizontal and vertical distribution of ozone and of the other gases is necessary. Using measurements from ground-based and satellite-based instruments, Ozone trends have been analyzed from 1964 to the present. These measurements agree within their range of errors of several percent but leave a large band of uncertainty (Ozone depletion rates have been on the order of fractions of a percent per year, which is much below the measurement accuracy of most of the existing space borne instruments). A comparison of Ozone trend measurements with simulation models still shows large discrepancies.

• Consequently, there is a clear need to increase our understanding of the central processes involved in atmospheric chemistry, and it is vitally important to monitor and investigate global budgets of Ozone and other chemically important gases. In order to fulfill this need, accurate altitude-resolved data are necessary.

  With GOME (Global Ozone Measurement Experiment), the first European Ozone monitoring instrument onboard the ERS-1 satellite, the European Space Agency (ESA) has joined successfully the worldwide effort of Ozone measurement and of understanding its chemistry. The primary measurement objective of GOME is the measurement of total column amounts and profiles of Ozone and of other gases involved in Ozone photo chemistry.

  The Global Ozone Monitoring by Occultation of Stars (GOMOS) Instrument onboard ENVISAT-1 is the newest ESA instrument aiming at Ozone monitoring. It is a tool to provide altitude resolved global Ozone mapping and trend monitoring with very high accuracy, as needed for the understanding of Ozone chemistry and for model validation.

  The primary GOMOS mission objectives are:

  • Measurement of profiles of Ozone, NO2, NO3, OClO, Temperature and water vapor.
  • Day and night side measurement capability
  • Global coverage with typically over 600 profile measurements per day
  • Altitude measurement capability between the Tropopause and 100 km
  • Altitude resolution of better than 1.7 km

The MEdium Resolution Imaging Spectrometer (MERIS) addresses the needs of three disciplines, primarily oceanographic and secondarily atmospheric and land observations. The main applications are listed below. MERIS, complemented by the ASAR, RA-2 and AATSR provides a unique synergistic mission for bio/geophysical characterization of the oceans and coastal zones and thus for climate and global environment study and monitoring.

MERIS is a push-broom instrument and measures the solar reflected radiation from the Earth's surface and from clouds through the atmosphere in the visible and near infrared range during daytime.

The 1150km wide swath is divided into 5 segments covered by 5 identical cameras having corresponding fields of view with a slight overlap between adjacent cameras. Each camera images an across-track stripe of the Earth's surface onto the entrance slit of an imaging optical grating spectrometer. This entrance slit is imaged through the spectrometer onto a two-dimensional CCD array, thus providing spatial and spectral information simultaneously.

The spatial information along-track is determined by the push-broom principle via successive read-outs of the CCD-array. Full spatial resolution data, i.e. 300m at nadir, will be transmitted over coastal zones and land surfaces. Reduced spatial resolution data, achieved by on-board combination of 4x4 adjacent pixels across-track and along-track resulting in a resolution of approximately 1200m at nadir, will be generated continuously.

The calibration is performed at the orbital south pole, were the calibration diffusers is illuminated by the sun by rotating a calibration mechanism. During calibration, the earth-view port is closed and the sun-view port opened to provide, in the case of radiometric calibration, a uniform (spectrally and spatially) radiance source, and in the case of spectrometric calibration, a radiance source with a spectral signature

The data will be pre-processed in flight and further on ground to provide images of the Earth in 15 spectral bands. These top of atmosphere radiance images, radiometrically corrected, geolocated and annotated with auxiliary data, constitute the MERIS Level 1b data product. The MERIS Level 2 data products are derived from the Level 1b and consist of atmospherically corrected reflectances which are then used for the generation of large scale maps, e.g. for

• ocean pigment concentrations, suspended sediments and disolved organic matter;
• clouds, aerosols and water vapour content;
• vegetation status and distribution.

The instrument is developed under the leadership of Aerospatiale (Cannes). The data product algorithms are developed under the leadership of ACRI.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a Fourier transform spectrometer for the measurement of high resolution gaseous emission spectra at the Earth's limb. It operates in the near to mid infrared where many of the atmospheric trace-gases playing a major role in atmospheric chemistry have important emission features.

The objectives of MIPAS are:

• simultaneous and global measurements of geophysical parameters in the middle atmosphere
  • Stratosperic chemistry: O₃, H₂O, CH₄, N₂O and HNO₃
  • Climatology: Temperature, CH₄, N₂O, O₃
• Study of chemical composition, dynamics and radiation budget of the middle atmosphere
• Monitoring of stratospheric O₃ and CFC's.

The main objective of the MicroWave Radiometer (MWR) is the measurement of the integrated atmospheric water vapour column and cloud liquid water content, as correction terms for the Radar Altimeter signal. In addition, MWR measurement data are useful for the determination of surface emissivity and soil moisture over land, for surface energy budget investigations to support atmospheric studies and for ice characterization.

The Radar Altimeter 2 (RA-2) is an instrument for determining the two-way delay of the radar echo from the Earth's surface to a very high precision: less than a nanosecond. It also measures the power and the shape of the reflected radar pulses.

The RA-2 is derived from the ERS-1 and 2 Radar Altimeters, providing improved measurement performance and new capabilities.

Operating over oceans, its measurements are used to determine the ocean topography, thus supporting the research of ocean circulation, bathymetry and marine geoid characteristics. Furthermore, the RA-2 is able to map and monitor sea ice, polar ice sheets, and most land surfaces.

Measurement of the radar echo power and shape enables the determination of wind speed and significant wave height at sea, thus supporting weather and sea state forecasting.

SCIAMACHY is an imaging spectrometer whose primarly mission objective is to perform global measurements of trace gazes in the troposphere and in the stratosphere.