Home-brewing your own NOAA weather satellite receiving station.

Introduction:

There are many excellent publications which describe how to construct one's own weather satellite receiving station, and with the different choices of receiver, antenna, computer and software, there are many ways in which it can be assembled.  As a guide to an aspiring builder, the following notes describe how we put together our particular system (i.e. the one which produces the weather images displayed on our main web page).  Although I regard the assembly of a NOAA satellite receiving station as a fairly straight forward task for a technically competent adult or enterprising schoolkid, it's perhaps like riding a bicycle; easy when you already know how.  Consequently, the following notes will explain things in straight forward not-too-technical language.  Personally I found the assembly of our receiving system was really good fun, and I think the process makes an excellent educational project for students of any age.  When a NOAA satellite receiving system is working properly, simply observing its routine day-to-day operation can be interesting and educational.  Once a receiving station is displaying cloud imagery reliably and routinely, it can also be configured to automatically publish its up-to-the-minute imagery onto the internet, thereby also becoming a useful public service.

Thinking about it.

Before committing any funds to buying hardware, one can start to explore the world of Earth orbiting satellites by reading, browsing the internet, and also simply using one's eyes to watch satellites pass across the night sky.  It's surprising what spacecraft one can see flying, if one knows exactly when and where to look.  There are many bright easy-to-spot satellites such as the International Space Station  the Hubble Space Telescope, the Space Shuttle (SST), Iridium satellites, NOAA satellites... to name a few... and of course numerous large luminous hunks of space junk.  One excellent resource to start visually observing satellites can be found at Heavens Above.  After registering at this website and logging in, one can obtain highly accurate forecasts for visible satellite passes, tailor-made for one's particular geographic location.  Particularly impressive are overflights of the ISS, and the spectacular flashes of sunlight reflecting off Iridium satellite antennas (so bright that they can occassionally be observed during daylight hours).  The Visual Satellite Observer's website also specialises in observing naked-eye-visible satellites, and is very good.


You don't need to ride the Space Shuttle to see the earth from space! You can get live images on your computer by receiving radio transmissions from weather satellites (often abbreviated to wxsats.)
There are two types of weather satellite from which it is possible to receive images: orbiting satellites and geostationary satellites.
Geostationary satellites stay in a fixed place in the sky, and can be received all the time. However, they are further away so their signals are weak. They transmit at microwave frequencies. You need a dish antenna and some quite expensive equipment to receive images from them. Assembling the hardware

So you've finished thinking and watching, and want to get on and build a NOAA Weather Satellite receiving station.  The three big items one must now consider buying or building are the radio receiver, the antenna, and the software to run it all.  But before buying any of this, I strongly recommend the purchase of the Radio Amateur's Satellite Handbook, published by the American Radio Relay League.  Like most ARRL publications, one gets a lot of bang for one's buck, and this particular book is chock-full of useful information and practical construction tips for satellite receiver antennas and the like.  

The Receiver
The radio receiver is the most expensive item that one needs to buy, so careful consideration must be given.  The imagery presented on our main web page, is NOAA APT imagery, which is transmitted by NOAA satellites at radio frequencies of 137.50, 137.62 or 137.9125 MHz, FM.  You may have noticed that your broadcast FM radio tops-out at 108MHz.  If you could keep spinning your radio dial beyond 108MHz, and on up to 137.50MHz, then several times a day you would hear this curious sound.  This distinctive 'tick-tock' sound is typical of NOAA APT transmissions, with each half-second-long 'tick-tock', encoding a single line of the satellite image.

If a regular el-cheapo broadcast-FM receiver could be tuned up to 137MHz, it would be fine for the purpose of receiving NOAA APT signals, as the NOAA satellite radio signal is FM-modulated in much the same way as regular broadcast-FM radio signals.  But modifying a broadcast-FM receiver to work at a higher frequency is outside the scope of most people, and buying a purpose-built unit is an easier option.  As any listener to FM radio knows, the quality of FM sound is much higher than broadcast AM radio.  This is because FM signals have better immunity from radio interference, and also because of the much higher bandwidth of broadcast-FM transmissions.  On broadcast-FM, one can hear much higher pitched musical notes than AM radio.  In fact broadcast-FM can potentially carry audio frequencies which are higher than human hearing can actually detect.  And so it also is with the FM transmissions from NOAA satellites.  Although many common scanning radio receivers can tune to 137MHz FM, the receiver's bandwidth may be too narrow to receive the full signal, as indicated by poorer sound quality and absence of high-pitched tones.   Narrow receiver bandwidth becomes apparent when the APT 'tick-tock' sound is converted into a picture by a computer.

So a wide bandwidth FM radio receiver is desirable, and so is one that can be controlled by a computer.  Computer control is particularly useful if one wishes to have a fully automatic station which can periodically switch between the frequencies used by different NOAA satellites.  So if one decides that wide bandwidth and computer control are essential, then the choice of available radio receivers becomes much narrower.  Given my limited knowledge of how good various receivers are, and whether they represent good value for money, in honesty I can't recommend any particular receiver.  But I can direct folks toward the Hardware section of the WXtoImg website , which is a good place to commence researching suitable receivers. WXtoImg is the clever computer program which drives our satellite web page, and at the WXtoImg web site there is also good information concerning suitable 137MHz antennas.  The receiver that we use for satellite reception, is an Icom PCR1000, and we have been very pleased with its performance.  We purchased it from ATRC in Sydney, Australia, and we liked their sales service.



The Antenna
There is much misunderstanding about how antennas work, and how different types compare with each other.  Section 10 of the ARRL Satellite Handbook has some good suggestions for basic antenna designs that can get one started with receiving NOAA APT signals.  Likewise the Hardware section of the WXtoImg website has many good 137MHz designs, of which the Quadrafilar Helix antenna is generally acknowledged to be 'king' of 137MHz satellite receiving antennas (in fact the NOAA satellites themselves transmit their APT signal from the spacecraft using a Quadrafilar Helix antenna ).  However for 137MHz LEO satellite reception, I have a personal preference for an old design of antenna (circa 1930's), known as a Lindenblad Antenna.  I have made a number of Lindenblads over the last few years, designed for various frequencies, and particularly like their simplicity, robustness and effectiveness for LEO satellite communications.  They may not be the ultimate NOAA 137MHz antenna, but they're pretty damn good in my opinion.  If you wish to have a go at making a Lindenblad for 137MHz, please check out my construction notes.

The big advantage to using TV standard antenna equipment, is that it's relatively cheap.  Good quality low-loss coax cable, TV masthead preamplifiers and 75-ohm coax connectors are mass produced and readily available.  NOAA satellites transmit their signal with around 5 Watts of radio power, and when a signal is being received directly from a NOAA satellite, the spacecraft will be at a distance somewhere between 800 to 3000km.  So the received NOAA signals are generally weak, and easily interfered with by local radio noise generated by computers, TVs, VCR's, broadcast-TV transmitters and the like.  To avoid this interference, it may be necessary to locate one's antenna well away from the sources of noise, and a long coax feeder may be required.  When coax feeders longer than ~20m are used, it is advisable to also use a TV-grade masthead VHF preamplifier.  In order to place our particular Lindenblad antenna at a relatively radio-quite site, we used 110 metres(!) of coax, along with a 'Kingray' brand of masthead preamp (commonly available at electronics stores in Australia).

The Computer
We perhaps take for granted the processing power of modern PCs, compared with computers of even a decade or so ago.  These days any bog-standard computer now contains a 'sound card', or some device to process audio sound.  For most of us, the sound card is just something that permits our computers to make occasional noises, or perhaps to play music.  Few people use their PC's sound card to routinely record sound, and generally that function goes unused.  But even a basic quality PC sound card is equipped with a powerful analog-to-digital converter, which can digitally sample audio signals at rates of 10kHz and higher... in stereo!  This facility to digitise audio signals is what allows the easy conversion of the NOAA APT 'tick-tock' sounds from a radio receiver, into numbers which can then be converted into a picture.  If one goes back just a few years, reception of APT signals required a rack-load of expensive dedicated electronic equipment, whereas these days any old Pentium-class PC (or the like) equipped with a sound card, is easily up to the task.




The Software

From my limited survey of the software available to run a NOAA APT receiving station, there is one program that is head-and-shoulders above the rest in my opinion, and it is known as WXtoImg.  This one comprehensive computer program can take the raw NOAA APT 'tick-tock' sounds, and convert them into a variety of visually stunning images, and even automatically publish freshly received imagery straight onto the Web.  The program is easy to use and pleasingly bug-free.  But the thing about WXtoImg which impresses me most, is the superb user service provided by WXtoImg's author.  Minor bugs that I have spotted were promptly corrected, and the author is open to suggestion for how WXtoImg may be improved.  The few minor suggestions of mine that were implemented have made the WXtoImg software somewhat more useful for our particular application.  To be perfectly honest, I've never experienced software user service quite like it.

WXtoImg may be downloaded as freeware from the WXtoImg Website. There are versions for most popular computers and operating system platforms.  The freeware version is quite powerful, has no use-by date, and will perform most tasks that people might wish.   But once you have determined that this program does what you wish, I strongly advise payment the US$58.95 'Standard Edition' licence fee. This makes the program even more versatile, and software and software support of this quality, needs to be supported.

Odds and ends

A cable between your receiver and computer sound card.
Most computers have a 'line-in' and 'microphone' sockets, which accept 3.5mm audio phone jacks.  Most radio receivers have a 3.5mm audio socket which will accept headphones and the like.  Some up-market receivers may also have a 'line-out' socket as well as a headphone/speaker socket.  And this is where it can get a little complicated.   Some receivers have a 'mono' headphone/speaker jack.  Likewise the output from some receivers might overload the 'line-in' input to the sound card (even when the incoming sound is attenuated using the computer's sound mixer controls).  If the receiver's output is too powerful for the sound card, an attenuating cable will need to be bought or made.  These are fairly readily available at electronics stores such as Radio Shack, Tandy, or stores specialising in audio.  

If one has a sound card which only has a microphone input (as is the case with many laptop computers) then one will need to severely attenuate the radio receiver audio signal before feeding it into a microphone jack.  Moreover, the microphone inputs on most modern sound cards, are actually 'hot mics' (often labelled with the phrase 'Plug in Power').  That is, DC electrical current from the computer is fed up the microphone cable in order to power the microphone.  If one was to connect a radio receiver's output directly to a PC sound card's microphone input, then DC electrical power could feed back into the receiver, which is not good!  So if one wishes to feed signals directly into a computer's microphone jack, one needs to block this DC current with a capacitor, as well as severely attenuating the audio signal with some kind of resistor voltage-divider network.  If you are not familiar with these terms, it's time to hunt down your friendly neighbourhood electronics geek, or radio ham, for advice applicable to your particular equipment.

A GPS clock
Downloading NOAA satellite imagery requires good timing.  The position of a satellite is defined by the so called Keplerian elements, a series of numbers which precisely define a satellite's orbit and whereabouts.  A satellite's known orbit, coupled the very carefully controlled orientation of the spacecraft relative to the ground, mean that a computer program can precisely calculate what strip of the Earth is being observed by a NOAA satellite at any given moment, so long as the time is known very well.  This is how a program like WXtoImg can accurately place a map overlay onto a freshly received satellite image.  But how accurate does a PC clock have to be?  Consider that NOAA satellites orbit with a ground speed of around 7km/second.  Also consider a case where a PC's clock was in error by 10 seconds.  In this case the computer software would misplace the overdrawn map by 70km, which is not good.  To get high quality images, where the map outlines are always correctly placed, requires accurate timing to ±1 second or better.  WXtoImg permits misplaced map overlays to be manually corrected, but it's preferable to not get it misplaced in the first place.  One can periodically set one's computer clock manually, but this gets a little tedious after a while.



References:

Radio Amateur's Satellite Handbook (1998), by Martin Davidoff.  Available from ARRL publications.  In Australia on sale at Dick Smith's.