High Resolution Weather Pictures (HRPT)
While waiting for the
weather to improve for microwave work, its worth keeping an eye
on it too!
I've had under construction for some time now a system to take
images from the orbiting NOAA series of satellites. These orbit
the earth at roughly 90 minute intervals at a height of approx.
700kms and continually transmit images of clouds and land to
earth bound observers. Low resolution APT pictures are
transmitted on 137Mhz and can be received on an omni-directional
antenna but in addition much higher resolution pictures are
transmitted at 1700Mhz. These transmissions require the use of a
tracking antenna, in my case a helix, to maintain the signal
level into the converter throughout the pass. For a full 20
minute pass the data transmitted can be up to 40Mb at a data rate
of 646kb/s!
The equipment in use is a home built helix antenna, plus an
antenna mounted low noise amplifier on an old motorised pan and
tilt head. This receives azimuth and elevation tracking commands
via a PC using the Fodtrack tracking software. The downconverter
uses an old home built Meteosat converter revamped for the two
HRPT frequencies. The signals are converted from 1700 to 137MHz
and then down to 10.7MHz where they are decoded in an ISA PC card
which also applies Doppler correction to the 10.7Mhz. The picture
is then processed in David Taylor's HRPT software and Paint Shop
Pro, and false colour added. The results are shown below as part
of a much larger image taken on the 11th September 2002. This
picture does not do justice to the resolution since to display it
here I have had to use .JPG compression which reduces the
resolution and introduces compression artifacts.
Unfortunately the pan and tilt head developed a terminal fault in December of 2007 so the system has now been dismantled.
The antenna and tracking system is shown below in its permanent position on the house wall, this situation, in my case, minimises cable runs. The downconverter is shown strapped to the bottom of the antenna pole while the large grey enclosure houses the tracking PCB and power supplies.
The helix antenna is a design by Arne van Belle and details can be found on this web site:-