Tuesday, May 15, 2012

Noise on NOAA images

Figure 1:
A tall, narrow QHA with mast-
mounted GaAsFET preamplifier
below it.
Click on the image for a larger version.
Several years ago (in November 2008) I put together a weather satellite receiver from scratch - that is, I started out with things in my parts bins, parts in the junk box and a few ideas.  I'd never actually built a synthesized receiver from the ground up before and, having no definite plan in mind, I was looking forward to see what I ended up with!

The result is described for all to see on my Weather Satellite Page including schematic diagrams, pictures and descriptions.

Since then I've had the receiver online for pretty much the entire time, originally pressing an old 333 MHz W98SE laptop into service but then switching to a small, low-power desktop computer (an Aopen M945D) that had far more computational horsepower and enough capacity to be used for several other things at the same time - like monitoring my GPS receivers and providing a network time source.

Years before I'd built the receiver I'd constructed a "Tall, Narrow Quadrifilar Helix Antenna (QHA)", a funny-looking corkscrew affair that is purported to have a bit more gain on the horizon than a typical "fat" QHA where the satellite's signals are weak by sacrificing some overhead gain - where the signals are strong.  For a while, I would occasionally mess with this antenna, receiving images using either a service monitor (its IF bandwidth being a bit too wide to work really well) or my trusty FT-817 (its bandwidth being too narrow to work properly.)  Once I'd put together my purpose-built weather satellite receiver, it was a natural fit!

A good idea, in theory.

From some unknown source there's some power line related "buzz" on frequency that seems to be finding its way into the pictures.  This buzz isn't there all of the time, but when it is, I can't seem to readily "hear" it on the audio - or on the nearby 2-meter amateur band.

For decoding the NOAA satellite images I use "WXtoIMG", a shareware program that, for a modest fee, offers even more features than the "free" version.  This program will automatically download the orbital parameters of the satellites, switch the receiver to the proper frequency, receive and decode the pictures from the satellite, place the images atop a map and apply a number of different transformations to the data to better-show various types of meteorological phenomenon and then upload these to a web page - such as the one that contains the images from my receiver.

Figure 2:
A typical image (with noise).
Click on the image for a larger version.
This noise shows up as wavy, diagonal lines in the image - particularly near the north/south extremes where the satellite's signal is a bit weaker.  The picture to the right also has another type of noise evidenced by horizontal bars:  These are due to nulls (e.g. fade-outs) where the signal was momentarily blocked by some other object (such as a tree) or reduced by some aspect of the antenna's pattern and are not due to man-made interference.

Fortunately, WXtoIMG contains a number of filters that can greatly reduce the effects of various types of noise and interference, but these can only do so much!

So, what's the solution?  Several things spring to mind:
  • Seek and destroy!  Find the source of noise and do something about it.  So far, this has eluded me.  As noted before, the noise is not only intermittent, but it seems to defy detection on a portable radio (such as the FT-817) when I attempt to determine its source.  (I'm pretty sure that the noise isn't coming from my own house...)
  • Relocate the antenna.  It's possible that the noise is emanating from something in a neighbor's house or garage.  In this case, moving the antenna from its present location in the back yard, atop a mast to somewhere away and (maybe) more in the clear may reduce/eliminate it - or, if it doesn't, provide additional clues as to its source.
  • Change to a different type of antenna.  Perhaps an antenna with less gain at low elevation angles may reduce the susceptibility to noise as it is presumed that any terrestrial noise source will also arrive at low angles!  A "normal" QHA might work, as might a "Turnstile" antenna.  The ultimate solution would be to construct a circularly-polarized beam antenna (crossed-Yagi or a Helix) and have a pair of rotators automatically track the satellite as it moved across the sky, but this strikes me as overkill!  (But it would be a fun project!)
 Anyway, it's something that's on my huge "to-do" list, falling under a category that I call "YAFP" - which stands for Yet Another Fun Project - or something very close to that!

2 comments:

  1. Hello, I am a fan in Italy receiving satellites noaa, I solved the problem of interference by building a simple resonant filter in two cells centered on 137.5 MHz. It 'very easy to do and is Publicato on a forum in Italy.
    http://www.rogerk.net/forum/index.php?topic=57475.270

    ReplyDelete
    Replies
    1. In the U.S. - unlike many places in Europe - there are no other terrestrial services (such as paging) on the frequencies near the 137 MHz weather satellite frequencies: The closest users are the Aviation users in the 108-136 MHz range.

      As mentioned in the article, the noise source was powerline-related, hence the frequency relationship between the horizontal scan rate of the images as evidenced by the spacing of the wavy lines on the images in the horizontal plane.

      As it turns out, not too long after the article was posted the local power company completely replaced a section of power lines about 2 blocks east of me and the noise source largely disappeared, saving me the trouble of tracking down the source!

      As it turns out the mast-mounted preamplifier has a two-stage bandpass filter in it not unlike the one in your link, but I'm sure that the article/link that you provided will be helpful to others!

      Delete





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