When you watch the news and see pictures of weather from around the world, you are
more than likely seeing data from NOAA’s environmental satellites. NOAA’s
environmental satellites provide data from space to monitor the Earth to analyse
the coastal waters, relay life-saving emergency beacons, and track tropical
storms and cyclones.
Img 2: Raw APT Picture Sample
Img 3: Coloured Map Overlay with APT Data
There are many amateur radio operators and scanner fanatics tuning into the NOAA
weather satellites as they circumnavigate the globe. As the satellite
passes in close proximity from horizon to horizon, it transmits a signal
to Earth in the form of a coloured fax. The fax is better known as Automatic
Picture Transmission (APT) and is transmitted at 120 lines per minute. It
takes approximately 10 – 15 minutes for a NOAA Satellite to complete a
pass.
The signal is generally recorded as a wave sound file and programs like WXtoImg are
able to decode the wave file into a coloured picture of the earth’s
surface. Currently, there are approximately 8 daytime passes from four
different satellites in the NOAA family:
* NOAA-15 -
Launched in May 1998, It is the oldest NOAA Satellite still functioning. It is orbiting the globe at 807km above the Earth and transmits on 137.620MHz
* NOAA-17 -
Launched in June 2002, and is orbiting the globe at 810km above the Earth. Transmitting at 137.500MHz
(Scanning mirror motor failed September 2010 - images no longer usable)
* NOAA-18 -
Launched in May 2005 and is in orbit at 854km above the Earth. Transmitting at 137.9125MHz
* NOAA-19 -
launched in February 2009 and transmitting high quality images at 137.1000MHz
The received pictures form only
part of the passion for this hobby. The radio operator gets more enjoyment building antennas
and setting up receivers with the ultimate goal of receiving a perfect picture stretching from horizon to horizon. Many pictures have some sort
of interference band – particularly near the horizon where the signal is weakest - and the objective is to fine tune all the equipment in order
to eliminate the interference "bands". The example shows the effect of bands on the final picture.
Antenna design, cabling, topography, elevation, all play important roles in receiving the perfect picture.
Here is a small audio sample of the APT
signal received from NOAA 19. The signal has a distinctive tick-tock sound with a bell indicating a carriage return for the start of a new line. The
signal was transmitted at 5 Watts and from a distance of 1,700 km from the base station. When a Satellite begins a pass at the horizon, It can be
anywhere up to 3,000 km away. It is interesting to note that 5 Watts signal output is considered fairly low power output for the distance the signal has to carry
which is why antennae design is critical for receiving good quality pictures. 5 Watts power output is equivalent to the output power of a conventional CB Radio
which is expected to transmit up to a maximum of 20 kms.
And here is the actual raw picture generated from the small audio sample when processed by WXtoIMG.
NOAA Channel 2 (Visible)............................................................................................................
NOAA Channel 4 (Infrared)
The raw picture then undergoes further processing so that the visible and infrared data can be superimposed on a coloured map overlay, thus providing a coloured image from the signal. The entire audio file is processed to produce a picture similar to this.
This particular picture was recorded on 27 May 2009 and shows an example of a jet-stream of cloud over the Eastern
side of Australia. Also unique about this picture is that it spans from 1 degree South above the horizon to 1
degree North above the horizon without any interference bands and presents a good example of the quality of picture
hobbyists are trying to achieve.
Img 4: Lindenblad Antennae
Lindenblad Antennae:
In the past, I have been using a 4 element, home made Lindenblad (Img 4.) antennae and
received reasonable results. The Lindenblad does have inherent problems when the Satellite is
positioned towards one of the corners of the antennae. A momentary drop in the signal strength
causes some interference bands to appear in the picture.
Quadrifilla Antennae: Further experimenting with a home made Quadrifilla antennae (QFA) has produced
far superior results and all my Satellite passes are now received via the QFA (Img 5.). As this is a circular antenna, it is less
prone to signal drop out as the Satellite moves across the horizon. I found that positioning the antennae on the roof was made much
easier when I ran an audio cable from my stereo amplifier to the roof so that I could listen to the Satellite signals via some headphones.
By simultaneously listening to the signal and rotating the antennae during a Satellite pass, I was able to find three "sweet spots" where
the signal strength remained constant from horizon to horizon and this gave the added advantage of being able to rotate the antennae
away from a TV station's frequency which had previously given me some grief on NOAA 15 Satellite passes.
Img 5: Quadrifilla Antennae ..............................
Img 6: Reception Area .............................................
Img 7: Satellite Transmission
Reception Area:
Each satellite pass is received when the satellite’s elevation is above 0 degrees to the horizon from the receiver’s position.
Based on this info, it is possible to receive pictures with far extremities of 4,500km away. This is possible when the satellite is positioned on the
very extreme of the reception area - as it broadcasts pictures from 1,500km either side of the satellite’s path. In Img 7. the brown circle shows the
extreme edge of the satellite reception area from Brisbane.
The distance from Brisbane to the edge of the circle is approximately 3,000km and the
dotted blue line represents the scanning width of the satellite, i.e. 1,500km either side of the satellite. The Blue circle represents the area that a
receiver should be able to pick up the signal from the satellite. While this may seem impressive, the signal reception at this elevation
(approx 1-2 degrees) is relatively poor and the signal lasts for less than 5 minutes. By comparison, the recording from the pass shown in Img 7.
lasted 4 minutes whereas the pass shown in Img 3. lasted 15 minutes and the image from the Img 7. pass was mostly unusable.
Another interesting fact is that a receiver in Brisbane should be able to pick up three passes from the same satellite in a 4 hour period. The brown lines south to north represent the projected path the satellite will travel during the 4 hour time frame.
In this example, NOAA 18 was in the Brisbane reception area 3 times on this particular day. WXtoIMG has a built in feature that joins the pictures from multiple passes and
produces a composite picture spanning up to 4,000km of the earth's surface.
