What are Polar Orbital Weather Satellites


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.


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