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Instrument: AMI : Active Microwave Instrument
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The Active Microwave Instrument (AMI) is one of the instruments
carried on-board the first European Remote Sensing Satellites (ERS-1
and ERS-2) launched by the European Space Agency on 17 July 1991 and
20 April 1995.

ERS-1 and ERS-2 operate in a sun-synchronous orbit, was conceived as
an orbiting platform that would be capable of measuring the Earth's
atmospheric and surface properties with a high degree of accuracy on a
global scale.

The AMI incorporates two separate radars, a Synthetic-Aperture Radar
(SAR) operating in image or wave mode, and a Wind Scatterometer. The
Earth's surface is illuminated by four antennas and backscattered
energy is received either to derive data on wind fields and wave
spectra, or to produce high resolution images. The operational
requirements are such that each mode needs to be operated exclusively,
but the Wind and Wave Modes are also capable of interleaved operation,
in so-called 'Wind/Wave Mode'.

The engineering calibration of AMI is performed by making use of
ground-based transponders. The validation of AMI Wave and Wind
products is carried out through dedicated campaigns with surface
measurements from buoys and flights by instrumented aircrafts.


SAR obtains strips of high-resolution imagery 100 km in width to the
right of the satellite track. The 10 m long antenna, aligned parallel
to the flight track, directs a narrow radar beam (at a frequency of
5.3 GHz) onto the Earth's surface over the swath.

On-board and on-ground signal processing is used to build up an image
from the backscattered energy, which depends primarily on the
roughness and dielectric properties of the illuminated area.

The power demands during the Image Mode are such that operating time
has to be limited to a maximum of 12 minutes during each orbit of
which 4 in eclipse. The data rate of 100 Mbits/s is far too high for
onboard storage, so images can only be acquired within the reception
zone of a suitably equipped ground station. Major advantage over
optical sensors is the capability of microwaves to penetrate clouds,
therefore providing all-weather imagery capabilities.

SAR Image-Mode characteristics:

Frequency: 5.3 GHz (C-band)
Peak power: 4.8 kW
Antenna size: 10 m x 1 m
Polarisation: Linear Vertical (LV)
Incidence angle: 23 degrees at mid-swath
Data rate: less than or equal to 105 Mbit/s
Spatial resolution: along track less than or equal to 30 m;
across track less than or equal to 26.3 m
Radiometric resolution: 2.5 dB at -18 dB
Radiometric stability: less than or equal to 0.95 dB
Swath width: 100 km
Localisation accuracy: along track less than or equal to 1
km; across track less than or equal to 0.9 km

Main applications of SAR Image Mode Data:

- Ice mapping and monitoring
- Ocean and coastal areas imaging
- Land imaging


Wave Mode operation of the SAR provides 5 km x 6 km images at
intervals of 200 km along track, which can then be interpreted to
provide wave spectra. Because the data rate is relatively low,
onboard data storage is possible, and there is a global sampling of
wave spectra.

SAR Wave Mode characteristics:

Wave direction: 0-180 degrees
Wave length: 100-1000 m
Accuracy: direction 20 degrees; length 25 degrees;
Spatial sampling: 5 km x 6 km every 200-300 km,
programmable anywhere within the SAR swath
Frequency: 5.3 GHz (C-band)
Polarisation: Linear Vertical (LV)
Incidence angle: 23 degrees nominal
Spatial resolution: along track less than or equal to or
equal to 30 m across track less than or equal to 26.3 m

Applications of SAR Wave Mode Data:

The capability of ERS-1 and ERS-2 to acquire global data sets and
imaging of ocean and ice phenomena, where previously scientists have
had to rely on sporadic measurements from ships or buoys, and in
cloud-covered regions, are important in such disciplines as:

- Oceanography (internal waves, small-scale variations in
wind and
modulations due to surface currents, etc.)
- Glaciology
- Climatology
- Meteorology (forecasts of sea conditions, etc.)
- Geodesy


The Wind Mode uses three antennas to generate radar beams looking 45
degrees forward, sideways, and 45 degrees backwards with respect to
the satellite's flight direction. These beams illuminate a 500 km-wide
swath as the satellite moves along its orbit, and each provide
measurements of radar backscatter from the sea surface on a 25 km
grid. The result is three independent backscatter measurements for
each grid point, obtained using the three different viewing directions
and separated by a short time delay. As the backscatter depends on the
sea surface capillar roughness as a function of the wind speed and
direction at the ocean surface, it is possible to calculate the
surface wind speed and direction by using these 'triplets' within a
mathematical model.

AMI Wind Mode characteristics:

Wind direction range: 0-360 degrees
Accuracy: 20 degrees
Wind speed range: 4-24 m/s
Accuracy: 2 m/s or 10%
Spatial resolution: 50 km
Grid spacing: 25 km
Swath stand-off: 200 km to side of orbital track
Swath width: 500 km
Frequency: 5.3 GHz
Polarisation: Linear vertical (LV)
Peak power: 4.8 kW

Main applications of Wind Mode data:

- Weather and sea state forecasts
- Commercial and scientific uses (offshore exploration,
ship routing, fish resource management, etc.)

Related URL:

Reference online documentation:

For any query, please refer to:

ESA/ESRIN Earth Observation Help Desk


Phone: +39 06 9410777

Fax: +39 06 9418292



Via G.Galilei

00044 Frascati