Envisat Project

Launched in March 2002 is the most ambitious project by ESA for Environmental monitoring. With its ten on-board instruments the Envisat keeps track the of the ensemble of parameters that measure our planet's health (temperature, ice, atmospherical gas species, etc ...)

The Satellite on a polar orbit at about 800 km from the surface, covers the whole Earth. Each orbit is completed in 100 minutes and the cycle is of 35 days (the satellite scans the same point on earth every 35 days). Allowing not only to monitor every place in the globe, but also to perform measurements of the evolution of the monitored physical quantities.

Envisat Mission Objectives

Global mission objectives

Continuous and coherent global data sets are needed by the scientific and application community in order to understand better climatic processes and to improve climate models.

Some global applications require near real time data delivery (from a few hours to one day from sensing). Specific examples include:

Some of the global objectives require products available in off-line mode (days to weeks from sensing). Specific examples include quantitative monitoring of:

Regional mission objectives

Continuous and coherent regional data sets are needed by the scientific and application user community for a variety of objectives such as:

Some of the regional objectives (e.g., sea ice applications, marine pollution, maritime traffic, hazard monitoring, etc.) require near real time data products (within a few hours from sensing) generated according to user requests. Some other of these objectives (e.g., agriculture, soil moisture, etc.) require fast turnaround data services (a few days). The remainder would be satisfied with off-line (few weeks) data delivery.

On-board instruments

AATSR (Advanced Along Track Scanning Radiometer)

The Advanced Along Track Scanning Radiometer (AATSR) is one of the Announcement of Opportunity (AO) instruments on board ENVISAT. It is the most recent in a series of instruments designed to measure Sea Surface Temperature (SST) to the high levels of accuracy and precision required for the monitoring and detection of climate change. In order to achieve this, the (A)ATSR instruments include several innovative features in their design:

ASAR (Advanced Synthetic Aperture Radar)

ASAR is a high-resolution, wide-swath imaging radar instrument that can be used for site-specific investigations as well as land, sea, ice, and ocean monitoring and surveillance. Important contributions of ASAR to the global mission include:

ASAR will make a major contribution to the regional mission by providing continuous and reliable data sets for applications such as:

MERIS (Medium Resolution Imaging Spectrometer)

MERIS is a 68.5° field-of-view push-broom imaging spectrometer that measures the solar radiation reflected by the Earth, at a ground spatial resolution of 300 m, in 15 spectral bands, programmable in width and position, in the visible and near infrared. MERIS allows global coverage of the Earth in 3 days. The primary mission of MERIS is the measurement of sea colour in the oceans and in coastal areas. Knowledge of the sea colour can be converted into a measurement of chlorophyll pigment concentration, suspended sediment concentration and of atmosheric aerosol loads over the marine domain. MERIS is also capable of retrieving:

RA2 (Radar Altimeter 2)

The RA-2 measures the transit time and radar backscatter power of individual transmitted pulses. The transit time is proportional to the satellite's altitude above the ocean, land, or ice surface. Over ocean surfaces the measured range is accurate to better than 2.5 cm. The magnitude and shape of the returned echoes also contain information about the characteristics of the reflecting surface, from which it is possible to retrieve geophysical parameters such as (when over the ocean) significant wave height, wind speed, and sea ice edge location. The RA-2 instrument has several new features offering a significant advance upon the performance of the RA-1 flown on ERS. The RA-2 has a second radar channel (S-band, 3.2 GHz) allowing in-situ correction of the range delay due to the ionosphere. The S-band should als benefit new applications including ice type classification and rain-cell detection. The RA-2 uses a robust Model Free Tracker and surface tracking logic which switches autonomously between 3 different resolution modes to provide greater coverage in areas of difficult terrain.

MWR (Microwave Radiometer)

The MWR is a nadir-viewing, two channel, passive microwave radiometer operating at 23.8 and 36.5 GHz. At these two frequencies, it receives and measures microwave radiation generated and reflected by the Earth. The signals received can be related to surface temperature but, most importantly, combined together they provide an estimate of the total water content in the atmosphere, which will be used to correct for the altimeter measurements path delay. The MWR has a 20 km diameter field of view

DORIS (Doppler Orbitography Radiopositioning Integrated by Satellite)

The Doppler Orbitography and Radio-positioning Integrated by Satellite instrument is a microwave tracking system that can be utilized to determine the precise location of the ENVISAT satellite. Versions of the DORIS instrument are currently flying on the SPOT-2 and Topex-Poseidon missions. DORIS operates by measuring the Doppler frequency shift of a radio signal transmitted from ground stations and received on-board the satellite. The reference frequency for the measurement is generated by identical ultra-stable oscillators on the ground and on-board the spacecraft. Currently there are about 50 ground beacons placed around the globe which cover about 75% of the ENVISAT orbit. On the ground, DORIS data is used to create precise orbit reconstruction models which are then used for all satellite instruments requiring precise orbit position information.

GOMOS (Global Ozone Monitoring by Occultation of Stars)

GOMOS is a medium resolution spectrometer designed to measure the concentrations of, and monitor the trends in, stratospheric ozone with very high accuracy and to observe other atmospheric trace gases. Using the star occultation technique, GOMOS combines the features of self-calibration, high vertical resolution and good global coverage. Due to its high sensitivity down to 250 nm, which is one of its main design drivers, GOMOS can measure ozone profiles from 15 km to 90 km. Accessible altitude ranges, accuracy and global coverage are optimum on the night side. In addition, it can measure atmospheric turbulence, which is of interest for understanding the vertical exchange of energy in the Earth's atmosphere.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding)

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:

SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography)

The Scanning Imaging Absorption Spectrometer for Atmospheric Cartography instrument provides spectra measured from light transmitted, back scattered or reflected by trace gases in the atmosphere. The instrument is designed for the global measurement of trace gases in the troposphere and stratosphere by means of a spectrometer scanning the atmosphere either at nadir or in limb. It records radiation in the range 0.24 micronm to 2.4 µm with 8 detector modules. The goal is to allow small optical absorptions (as small as 2E-4 in some regions of the spectrum) to be detected. SCIAMACHY is designed to measure the global distribution of trace gases, aerosols and clouds in both the troposphere and the stratosphere. The abundances of a number of atmospheric constituents which are targeted species include:

LRR (Laser Retroflector)

Not a real sensing instrument, but a tracking device. A laser retroreflector is mounted on a pillar attached to the nadir panel close to the RA-2 antenna. It has two functions:

The LRR is a passive device which is used as a reflector by ground-based SLR stations using high-power pulsed lasers.