Weather satellites were first launched in 1960 by America
to look at weather in real time. The National Geographic Magazine
in August 1960 reported on the first image of the earth taken
from space. Since then there has been steady progress, and
more satellites have been launched on a regular basis. Remarkably,
since the early 60's the format has not changed and the rest
of the world has adopted the same standards.
Polar Orbiting
The first satellite was a polar orbiting type. Usually known
as NOAA (National Oceanographic and Atmospheric Administration),
these satellites orbit at about 850km above the earth. This
means that they have a circular orbit that takes them almost
directly over the North pole and South pole, and as the Earth
rotates a different strip is imaged. The width of the image
is 3000km and as the satellite travels above the earth's surface
it scans a line twice a second. It scans continuously and therefore
never sends a complete image, rather a continuous, never ending
strip. The satellites use VHF radio to transmit their information
directly into your home or office. Using frequencies just above
137MHz means that the satellite is receivable when it comes
above your horizon, which can be as far away as 3,000km. In
England for example this is from the middle of Africa to way
over Iceland. The whole pass from horizon to horizon will take
about 15 minutes and the area where you are situated will probably
take the middle 5 minutes. Each satellite overlaps each pass
and you can expect 2 good passes from each satellite twice
a day, so 4 passes per satellite per day. There are two series
of satellites, the afternoon NOAAs like NOAA 14 that pass over
at about 2 o'clock after midnight and after midday. The morning
NOAAs such as NOAA 12 and 15 pass over at about 7:30 in the
morning and early evening. So with 3 operational satellites
there are 12 good images a day. The Soviet Union also have
a series of satellites and these can be received on any of
our multi-channel receivers (not 2 channel). Soviet activity
is usually fairly constant but can be unreliable and difficult
to predict.
The NOAA satellites have two separate transmissions, APT (automatic
picture transmission) is on 137MHz and is very easy to receive.
APT is so strong that a fixed antenna will receive nearly all
of the pass and certainly much more than you would ever want.
So APT is easy and you do not need to move or track the small
antenna at all. The other transmission is HRPT (high resolution
picture transmission) on 1707MHz and is not so easy to receive.
HRPT requires a small dish and you need to track it across
the sky following the satellite.
The different systems
APT Polar
APT satellites give about 12 good images a day, wherever you
are in the world. The resolution is 4km per pixel and there
are visible and infrared sensors. This is a simple system where
the satellite is in low earth orbit and very easily received
at 137MHz. The two frequencies in main use are 136.50 for NOAA
12 and 15 and 137.62MHz for NOAA 14. A simple stainless steel Quadrifilar
helix antenna is used. This simple omni-directional antenna does not
need moving or tracking to receive the satellite. The satellite
is so strong that when is about 10 degrees above the horizon
perfect results will be obtained. There are two problems with
using receivers not specifically designed for weather satellites.
The first and most important is bandwidth, an APT signal needs
about 40kHz of bandwidth, between 30 and 50 is OK. Most scanners
have 15kHz which is far too narrow and 180kHz or more which
is far too wide. Only a few scanners have the correct bandwidth
and these are currently the AOR5000 and Icom IC-PCR1000. The
second problem with all scanners, including the ones mentioned,
is that a weather satellite system needs a very good performance
receiver, one that provides high sensitivity, good signal to
noise and high immunity to other adjacent transmissions. So,
you may get your scanner to work, but it will not provide the
results you see in our colour images. Our PROscan receiver
has been specially designed to provide optimum performance.
HRPT
HRPT satellites
give 1.1km resolution in 5 spectral bands. Two are visible, and
3 infrared. There are about 12 good images a day and this system
will provide the very highest resolution possible from weather
satellites. Because there are 5 sensors they can be mixed
together to provide stunning colour images showing an incredible
amount of detail. Remember that APT gives a pixel size of 4km
and therefore an area of 16 square km per pixel. HRPT gives a
pixel size of 1.1km and therefore an area of 1.21 square km per
pixel, an amazing increase of 13 times resolution. But there is
more, remember that there are 5 bands and that this is also a
digital system that gives 10 bit data, 1024 grey or colour
levels per band, giving a total of 50 bit data. A 90cm (3 foot)
dish has to be tracked across the sky as the satellite orbits.
This is all taken care of automatically and works really well in
practice.
