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.
Geostationary
In the late 70's it became obvious that a satellite transmitting
24 hours a day would be desirable. A satellite 40,000km away
will orbit the earth once every 24 hours, exactly the same
rate as the earth rotates, so from a fixed point on the earth
the satellite appears stationary. A geostationary satellite
is much further away from the earth than a polar orbiting type
and often the resolution is not as good. However because geostationary
satellites appear fixed, they can send images of exactly the
same area as frequently as every 30 minutes in some cases.
These frequent images can be processed by software to loop
or animate, that is so that you see moving clouds from, say,
the last 8 hours. This gives valuable information on the type,
direction, and magnitude of the cloud and hence leads to very
easy forecasting. There are several geostationary satellites
such as Meteosat 7 over Europe, GOES 8 over Eastern America,
GOES 10 over Western America, GMS over Australia/Japan and
INSAT/Meteosat 5 over Russia/India.
Meteosat and GOES re-transmit images from other geostationary
satellites so you can see the weather from Australia in London
or Chicago for example.
Polar and Geostationary, can you explain further ?
Yes, if you look at the image below you can see 2 satellites.
This is a snapshot in time. GOES 10 (Geostationary) has a big
circle round it, and this is the area that it covers, like
the whole of north and south America. Meteosat covers the whole
of Europe and Africa and GMS the whole of Australia, Japan
etc. To put these extra satellites on the map would confuse
the issue and so we have tried to make it simple ! NOAA 15
(Polar Orbiting) is over New Guinea and is receivable in the
smaller circle as shown. Remember that this is a snap shot
in time and over the next 100 minutes it would travel right
around the world. The circles of coverage that are shown do
not appear as true circles; this is because of the map projection.
If they were displayed onto a true 3D spherical globe, then
they would be circular. And remember, there are a lot of satellites
out there, more like a dozen instead of the 2 shown for simplicity.
What
will weather satellites do for me ?
A lot ! Everyone needs to know what the weather is doing.
Receiving live weather satellites gives you a unique real time
view of the earth. If you plan to cut your grass, or sail round
the world, you need to know what is happening. The level of
detail can be quite dramatic, moving fronts can tell you not
to go to sea, or a long clear period can change your plans
easily. Astronomers can check for cloud cover in other continents
before calling a friend to confirm a new sighting for example.
Cloud temperature can be measured giving an indication of height,
and sea surface temperature can be monitored for fishing or
pollution control.
There are at least two types of image you can receive. Visible
light is one, and this shows what you may expect to see if
you looked out of the satellite. Visible sensors are good for
low cloud and land detail but do not work in the night time
though ! Infrared detects heat and as high cloud is very cold
it appears white; space is even colder and appears even whiter.
You can see cities as darker spots on the land as they generate
more heat than the surrounding land. In fact the overall resolution
in general is less than the visible sensor, but because it
is a heat sensor it does not matter if it is day or night.
Satellite technology is fun too. The interaction of satellite,
receiver and computer is one which is challenging and can stretch
the mind. Teachers will value the ability to monitor the weather
almost anywhere in the world from their classroom.
Remind me of the history; and, what about the future ?
Weather satellites started in 1960 and to this day the
standard is exactly the same. As with everything electronic,
nothing stays still, so some time the standards may change.
However we have been assured that current systems will work well
for at least the next 10 years ! Even then, in this far distant
future, antennas and other parts will still be useable. Just
like over the last 10 years we have progressed from using 8088
PCs to Quad Cores, in the next 10 years we will all slowly
upgrade our weather satellite systems, and who knows where
computers will be in 10 years time?
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 crossed dipole
antenna can be used, or for marine use a stainless steel Quadrifilar
helix antenna. These simple omni-directional antennas do not
need moving or tracking to receive the satellite. The satellite
is so strong that when is about 20 degrees above the horizon
perfect results will be obtained. As this is the lowest cost
entry into weather satellites, a lot of people have asked if
they could use their "scanner" that they use for
monitoring other transmissions. 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 brochures. Our PROscan receiver
has been specially designed to provide optimum performance.
MSG Geostationary
Meteosat Second Generation (MSG) has totally revolutionised
weather satellite technology in Europe. Using simple reception
systems, achieving 1km resolution and new images every 15
minutes gives the user unparalleled imagery. Animation has never
been so detailed or so smooth before. It’s like the difference
between WEFAX and PDUS but all over again, totally surpassing
PDUS is a dream come true using the HRIT data stream. And now
that the last WEFAX image has been transmitted, you must
consider buying our MSG system!
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. The system is, though, both complex and expensive and
is very definitely not for the beginner. 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. There are only a few HRPT system manufacturers in
the world, no commercially available receiver is capable of
receiving HRPT and so, unless you are good at receiver design
and home construction, you need to purchase a complete system
from us.
WEFAX Geostationary (no longer
transmitted)
WEFAX satellites give anything up to 400 images a day and
can provide relayed images of the other side of the world.
The resolution varies between 2.5 and 10km. There are 3 sensors,
visible, infrared and upper atmosphere water vapour. The main
frequency is 1691MHz and we provide a special receiver for
this purpose. This signal from a 90cm (3 foot) dish and is
amplified by a low noise preamplifier mounted directly on the
dish, the 1691MHz signal is run down special high quality cable
into our WEFAX receiver. The input to the receiver is 1691MHz
and the output is an audio signal that you can hear. There
are people who convert the 1691MHz signal down to 137MHz to
use their existing APT receiver, the philosophy is fraught
with problems and customers who have tried this and then upgraded
to our WEFAX receiver have reported a huge increase in image
quality and resolution. Our WEFAX receiver has been designed
to provide optimum performance.
PDUS (no longer transmitted)
PDUS is available from Meteosat and gives images of Europe
in 2.5km resolution every 30 minutes. A 1.8m (6 foot) dish
is required and it is fixed in one direction. The rest of the
world is transmitted frequently in varying resolutions. The
2.5km resolution gives considerable detail to clouds and the
image is not cut up at all like WEFAX. You will never find
that you are on a join, like Greece is! We find the main use
of PDUS is to provide a continuous animating sequence of all
of the Atlantic Ocean right through to the Middle East, real
time, dynamic pan and zoom together with good colour provides
a huge increase in forecasting accuracy. The data is a digital
stream on 1694.5 MHz and there are less than a handful of manufacturers,
so you need to buy a complete system from us.
