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Article Images
First GRAS bending angle shows instrument to be in excellent working order
 
17 November 2006

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The animation shows GRAS's (Global Navigation Satellite System Receiver for Atmospheric Sounding) zenith-pointing antenna tracking a number of GPS satellites in range. This is indicated by the blue lines in the animation. The track-line changes to yellow once the signal goes into occultation. As it is in range of the GAVA antenna this is an example of a set event. From these signals, profiles can be derived for the lowermost 80 kilometres of the Earth's atmosphere. The red line indicates where a rise event begins and the signal is received by the GVA antenna.

The nominal GPS constellation consists of 24 satellites at an altitude of about 20,000 kilometres and distributed in six orbital planes around the Earth. An occultation occurs whenever a GPS satellite rises or sets on the Earth's limb as seen by the GRAS instrument. The Doppler shift of the received signals is directly affected by the refraction of the signal as a result of gradients of atmospheric temperature and humidity. Therefore, profiles of temperature, humidity and pressure can be derived from the refraction angles through the occultation phase. In the stratosphere and upper troposphere, where water vapour density is low, refraction is dominated by vertical temperature gradients. In the lower troposphere, the water vapour effects dominate refraction.

Credits: ESA - AOES Medialab

 
 
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The first bending angle measured by GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) carried on MetOp-A. The graph clearly shows how the signal begins to bends at around 30 km above the surface of the Earth due to the humidity in that atmosphere. Occultation has been tracked up to 500metres above the surface of the Earth where atmospheric conditions bend the angle up to 2°.

Credits: ESA
 
 
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Carried on MetOp, GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) uses radio occultation to measure vertical profiles of atmospheric temperature and humidity by tracking signals received from a constellation of GPS navigation satellites while they are setting or rising behind the Earth's atmosphere. By processing the navigation signals received by GRAS, 500 highly accurate profiles of atmospheric temperature, humidity and pressure can be derived every day. These profiles are distributed fairly uniformly over the globe.

Credits: ESA - AOES Medialab
 
 
MetOp
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The MetOp series of three satellites is Europe's first polar-orbiting missions dedicated to operational meteorology and represents the European commitment to a new venture with the United States. They are launched sequentially, providing data until at least 2020.

MetOp-A was launched on 19 October 2006 from the Baikonur Cosmodrome in Kazakhstan, on a Soyuz ST rocket with a Fregat upper stage.

Credits: ESA /AOES Medialab
 
 
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