The satellites and their instruments
In ten years Europe’s ERS-1/2 spacecraft have provided us with a very different view of planet Earth thanks to a suite of advanced and complementary instruments designed and developed by ESA.
The ERS-2 satellite circles the Earth at a height of 800 km and completes an orbit every 100 minutes crossing each pole in the process. The special orbit – known as a polar orbit – means that the Earth is gradually rotating beneath the spacecraft and so on each different orbit a slightly different part of the ground is seen. Using this type of orbit the ERS-2 can therefore cover the entire globe in just three days.
Both ERS satellites were built with a core payload of two specialised radars and an infrared imaging sensor. The two spacecraft were designed as identical twins with one important difference – ERS-2 included an extra instrument designed to monitor ozone levels in the atmosphere.
Active Microwave Instrument (AMI)
The Active Microwave Instrument is the largest onboard system and combines the functions of a Synthetic Aperture Radar (SAR) and a wind scatterometer (SCATT). The AMI has three modes of operation: image mode and wave mode (performed by the SAR); and wind mode (by the SCATT). In image mode, the SAR produces highly detailed images of a 100 km wide strip of the Earth’s surface day and night and in all weather conditions. In its wind and wave modes, the instrument continuously measures global ocean surface wind speeds and directions, and provides information on the direction and shape of ocean wave patterns.
Radar Altimeter (RA)
This measures variations in the satellite’s height above sea level and ice with an accuracy of a few centimetres and helps provide data for knowing the satellite’s exact orbital position. As well as contributing data on the position of ice flows below, the instrument produces ocean surface wave height and wind speed information for climatologists.
Along-Track Scanning Radiometer (ATSR)
The ATSR consists of two instruments, an Imaging Infrared Radiometer (IIR) and a passive Microwave Sounder (MS). The infrared sensor provides detailed maps of the temperature at the surface of the seas and oceans, accurate to better than 0.5 degrees Celsius. It also measures cloud top temperatures, cloud cover and land surface temperatures (useful in monitoring forest fires). For ERS-2, the infrared capability was enhanced with the addition of visible channels which enable the estimation of vegetation cover. The Microwave Sounder is a passive radiometer providing measurements of the total water content of the atmosphere within a 20 km footprint.
Global Ozone Monitoring Experiment (GOME)
In the light of the increasing concern about atmospheric ozone levels, the GOME instrument was added to the ERS-2 payload. This ultraviolet and visible light spectrometer provides information on ozone, ChloroFluoroCarbons (CFCs) and trace gas levels. A more advanced version of GOME will be carried on the Metop spacecraft series – three polar orbiting satellites currently under development. These will produce high-resolution images, detailed vertical temperature and humidity profiles and temperatures of the land and ocean surface on a global basis.
Additional to the core payload on both ERS-1/2 are:
Precise Range and Range Rate Equipment (PRARE)
An all-weather microwave ranging system designed to provide measurements used for highly precise orbit determination and geodetic applications, such as movements of the Earth’s crust.
This optical device operates in the infrared and is used as a target by ground-based laser ranging stations to determine the precise altitude of the spacecraft.
Last update: 24 August 2004