- Meteorologists fear they are losing one of their essential
forecasting tools - microwave frequencies uniquely able to "see"
through clouds from satellites.
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- They say commercial applications, for example mobile
phones and collision avoidance systems, are ruining them.
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- The use of the bands in this way causes interference
and contaminates the data from the satellites, making it useless.
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- Not only weather forecasting is put at risk, but also
a better understanding of how climate change is developing.
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- Progress in both forecasting and climate studies depends
on observations from space of the Earth's surface and atmosphere.
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- Unequal competition
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- Many of these observations depend in turn on using microwave
frequency bands, which are increasingly in demand for terrestrial use.
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- Examples include mobiles, wireless networking, other
long-distance radio communications, and remote triggering devices. New
military communications technologies are reported to pose another threat.
Dr Stephen English is manager of the satellite radiance assimilation group
at the UK Met Office.
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- He told the BBC: "Microwave observations are vital
because they see through cloud - this is not possible in any other frequency
band.
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- "We only need a few narrow-frequency bands for Earth
remote-sensing, but most of these are unique, so there is no alternative.
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- Gone already
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- "These bands are primarily used for temperature,
water vapour, sea ice, clouds (ice and liquid), and rainfall and snowfall
estimation.
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- "We use them as well for monitoring surface snowpack,
soil moisture and sea surface temperature."
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- A meteorologists' working group on frequency management
says protecting key regions of the microwave spectrum for passive remote-sensing
is "a dramatic challenge", because of "the huge pressure
of the commercial and military telecoms".
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- Two important bands (6.8 GHz and 10.7 GHz) have been
lost already for use over land, but in the next few years the threat is
likely to spread to other bands.
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- There is particular concern about protecting the 23.6-24
GHz band, which has the unique property of being sensitive to water vapour
but not to liquid water.
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- Dr English said: "There is no other frequency where
this occurs. But car 'radars' will now be allowed to broadcast in this
frequency band."
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- Data lost
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- An instrument called the advanced microwave scanning
radiometer, carried on Nasa's Aqua satellite, monitors rainfall as it "sees
through" the cloud above the rain.
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- Land and sea look very different at this frequency (in
the image, right, taken in mid-October, the ocean appears black where it
is not raining, and magenta or blue where it is).
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- Blobs of red and yellow over the main urban areas show
radio frequency interference (RFI), which is much hotter than actual surface
or atmospheric temperatures over the UK in October.
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- Dr English said: "The 'hot spots' are easy to spot,
but more worrying is the fact that smaller variations may be RFI, or they
may be due to rain. "The truth is we can't tell. Therefore the channel
is rendered useless not only in the hot spots but everywhere, because we
can no longer uniquely interpret the variations in terms of rainfall.
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- "Of course, over the ocean man-made signals are
limited, so we still regard this channel as useful over the ocean, but
it's no longer useful over land."
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- Experts say this band should not be jeopardised under
any circumstances, and all emissions able to cause interference should
be prohibited.
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- The UN body which is the final arbiter on frequency use
is the International Telecommunication Union.
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- Dr Steve Foreman of the Met Office told the BBC: "We're
in a David and Goliath situation, arguing to the ITU for the safety and
humanitarian uses of frequencies against some applications with very strong
financial backing."
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- © BBC MMIV
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- http://news.bbc.co.uk/2/hi/science/nature/4104355.stm
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