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Instrument: RA : ERS Radar Altimeter
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The Radar Altimeter (RA) is one of the instruments carried on-board of
the 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, operating in a sun-synchronous orbit, were 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 RA is a nadir-pointing active microwave sensor designed to make
precise measurements of the echoes from ocean and ice surfaces. It
provides information on:

- significant wave height
- surface
wind speed
- sea surface elevation, which relates to ocean currents, the
geoid and tides
- various parameters over sea ice and
ice sheets

The RA is relatively simple in concept, but depends on electronic
precision and a sophisticated data processor to achieve its
performance. The RA operates by timing the two-way delay for a short
duration radio frequency (RF) pulse, transmitted vertically
downwards. The required level of accuracy in range measurements
(better than 10 cm) calls for a pulse length of a few nsec, therefore,
in order to reduce the RF power requirements a pulse compression
(chirp) technique is used.

Instrument characteristics:

Frequency: 13.8 GHz
Bandwidth: 330 MHz (ocean mode); 82.5 MHz (ice mode)
Beamwidth: 1.3 degrees at 3 dB
Footprint: up to 20 km (depending on sea state)
Mass: 96 kg
Antenna diameter: 1.2 m
Pulse length: 20 ms chirp
RF transmit power: 50 W
Pulse repetition frequency: 1020 Hz

The RA operates in three modes:

- acquisition mode, during which the radar finds the
approximate distance to the surface and then switches to
one of the tracking modes
- ocean tracking mode
- ice tracking mode where an increased dynamic range is
used, obtained by reducing the chirp bandwidth by a factor
of four to 82.5 MHz, resulting in a coarser

Echo characteristics are analysed with respect to:

- time delay of return pulse, providing altitude
- slope of the echo leading edge, relating to wave height
- power level of return signal, affected by small scale
surface roughness giving an indication of surface wind
field parameters

Over the ocean the waveform profile is sufficiently well understood to
permit real-time estimates of ocean parameters to be carried out
on-board the satellite. For other surfaces the waveform shape does not
always conform to a simple model and further data analysis is
necessary. The return echo from sea ice appears more specular than
from the ocean and has a peaked trace.

The information provided by the RA, in accordance with the objective
of the ERS programme, is of significant importance to the commercial
and scientific user communities, with marked benefits for ocean
related activities, including ship routing and the design of offshore
facilities. Infact the RA was designed to permit the following:

- Ice mapping and monitoring
- Weather and sea state forecast
- Sea surface topography and ocean currents
- Experimental altimetry over land

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Reference online documentation:

For any query, please refer to:

ESA/ESRIN Earth Observation Help Desk


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Fax: +39 06 94180292



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