Polar orbiting weather satellites were the first satellites launched
to look at the earth's weather.
Polar orbital satellites are different than the geosynchronous satellites
which are stationed 22,000 miles above the equator and give a view of the total
earth at a much lower resolution than polar orbital satellites. Geosynchronous
satellites appear to never move and maintain their position relative to earth.
Polar orbital satellites rotate arround the earth at a low altitude usually measured
in hundreds of miles not thousands, in nearly polar orbits and pass over the same
portion of the earth twice in one day, usually in the morning and then again at night.
USA satellites are sunsynchronous, that is, they appear to pass over the same area of
eath at the same time of day each day. Most other polar orbital satellites are not
sunsynchronous.
The first in the USA series was called TRIOS 1, it was launched in 1960. It carried
a Television camera pointed at the earth and was able to take pictures of the
clouds and store them for re-transmission to earth when requested. The
problem with this technique was the fact that a ground station was required
to tell the satellite when to take and send images.
Since the satellite was orbiting the earth another technique was
developed which required very little attention by controllers
on earth. This technique is called:
APT for Automatic Picture Transmission.
The first satellite to use APT was TIROS 8 in 1963.
The APT system uses a scanner rotating perpendicular to the satellites
orbital direction to make a one line scan of a section of earth.
Since the satellite is rotating the next scan will cover a different
section of earth very similar to how television scans each picture one
line at a time.
The reception of a picture consists of receiving a multitude
of these scans over time and painting each line onto film or a
computer terminal. Because this system worked so well and was
inexpensive, The USA NOAA (National Oceanic and Atmospheric
Administaration) supplied information to the world so everyone
could receive weather pictures. The NOAA satelites signals are available
night and day, 365 days a year.
The USA satellite system was designed to send two pictures side
by side, the first picture is a visible spectrum scan during day passes, and
an infrared scan at night. The second picture is always an infrared
scan. With the proper software quite a bit of information can
be calculated from these two pictures, such a cloud height and
temperatures. Even false color images can be produced.
Russian satellites offer the same type of information but only provide
one picture at a time. This result in higher resolution pictures for
Russian satellites. The only flaw in their system is that it is not quite as
good at resolving land from water as the USA satellites are, so pictures
of land surfaces may appear washed out.
All of the APT satellites transmit an analog stream of data
continually amplitude modulated on an frequency modulated carrier,
in the range of 137 MHz. The signal is circular polarized for
the USA satellites and linearly polarized for the Russian satellites.
Several changes are expected in the future affecting reception of these
signals such as digital signals instead of the analog ones this will change
how you receive weather satellite pictures.
See Radio Equipment Needs for details
on the specifics.