Home-brewing your own NOAA weather satellite receiving
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,
many ways in which it can be assembled. As a guide to an aspiring
builder, the following notes describe how we put together our
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
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
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
age. When a NOAA satellite receiving system is working properly,
simply observing its routine day-to-day operation can be interesting
Once a receiving station is displaying cloud
and routinely, it can also be configured to automatically publish its
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.
are many bright easy-to-spot satellites such as the International Space
the Hubble Space Telescope (HST), the
Space Shuttle (SST), Iridium satellites, NOAA satellites... to name a
few... and of course numerous large luminous hunks of space junk.
excellent resource to start visually observing satellites can be found
at Heavens Above.
registering at this website and logging in, one can obtain highly
forecasts for visible satellite passes, tailor-made for one's
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
Observer's website also specialises in observing naked-eye-visible
and is very good.
Another very useful tool for observing satellites, and for
getting a feel for how orbits work, is a computer program called SatScape. With superb graphics
animations, this program is an excellent teaching tool for children and
alike. It's also free. The only minor problem with
is that it is written only for the MS Windows operating system.
However SatScape may be run under Linux via the Wine utility, with some minor loss
of functionality and speed.
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
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 (ARRL). 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
tips for satellite receiver antennas and the like. Likewise the
current edition of the ARRL
Handbook and ARRL
Antenna Handbook can be recommended.
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
137.50, 137.62 or 137.9125 MHz, FM. You may have noticed that
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
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
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
one can hear much higher pitched musical notes than AM radio. In
fact broadcast-FM can potentially carry audio frequencies which are
hearing can actually detect. And so it also is with the FM
from NOAA satellites. Although many common scanning radio
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
apparent when the APT 'tick-tock' sound is converted into a picture by
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
of the WXtoImg website
, which is a good place to commence researching suitable receivers.
is the clever computer program which drives our satellite web
at the WXtoImg web site there is also good information concerning
137MHz antennas. The receiver that we use for satellite
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.
In early 2004, a new cheap kitset NOAA APT receiver has become
available in Australia. The receiver was described in the
Australian 'Silicon Chip' magazine, December
2003 edition, and the complete kitset is now
for sale at Dick Smiths, stock number K3226. Recently I
received an email
from David Perry, who had just built one of these receivers.
David has a web site
through which he may be contacted, and he will repond to technical
queries regarding the Silicon Chip receiver. Hopefully these
queries, and their answers, will form the basis of a future FAQ page at
his website, addressing a few of the problems that have been reported
with this particular kitset receiver.
There is much misunderstanding about how antennas work,
and how different types compare with each other. Section 10 of
Satellite Handbook has some good suggestions for basic antenna
that can get one started with receiving NOAA APT signals.
the Hardware section
of the WXtoImg website has many good 137MHz designs, of which the
Helix antenna is generally acknowledged to be 'king' of 137MHz
receiving antennas (in fact the NOAA satellites themselves transmit
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
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
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
from a NOAA satellite, the spacecraft will be at a distance somewhere
3000km. So the received NOAA signals are generally weak, and
interfered with by local radio noise generated by computers, TVs,
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
to place our particular Lindenblad antenna at a relatively radio-quite
we used 110 metres(!) of coax, along with a 'Kingray' brand of masthead
(commonly available at electronics stores in Australia).
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
sound, and generally that function goes unused. But even a basic
PC sound card is equipped with a powerful analog-to-digital converter,
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
receiver, into numbers which can then be converted into a picture.
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.
So in selecting a computer which will be suitable, one doesn't require
anything particularly fancy. Just as long it contains a Pentium I
processor or better, a sound card, and a reasonable amount of memory
and disk space (say 64Mb+ RAM, and 2Gb+ disk space). As for
computer operating system, I can't really comment on MS Windows
products as I'm a Red Hat Linux
kinda guy, but
Windows 95/98/ME/XP/2000/NT are all suitable. A standard
feature of Linux is its ability to remotely control other Linux
computers. I find this feature particularly handy when I need to restart
WxtoImg from a distant computer. I really can't
comment on Mac computers as I've rarely used them, but I understand
that MacOS X
(or better) is suitable. With a computer's half-life now around
three years, there should be no trouble locating an older Pentium-class
and sparing it from the dumpster for another couple of years. But
of course, one could choose to use a 'good' computer to run a receiving
station, as the load on the PC's
processor is quite modest, and one
will not notice a decrease in computer performance when WXtoImg is
running quietly in the background. The only catch with using a
'good' computer, is that it really needs to run full-time in order
to catch all available NOAA satellite passes, and non-stop operation
may shorten a computer's life.
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
corrected, and the author is open to suggestion for how WXtoImg
be improved. The few minor suggestions of mine that were
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
(even when the incoming sound is attenuated using the computer's sound
controls). If the receiver's output is too powerful for the sound
card, an attenuating cable will need to be bought or made.
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
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
carefully controlled orientation of the spacecraft relative to the
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
can accurately place a map overlay onto a freshly received satellite
But how accurate does a PC clock have to be? Consider that
satellites orbit with a ground speed of around 7km/second. Also
a case where a PC's clock was in error by 10 seconds. In this
the computer software would misplace the overdrawn map by 70km, which
not good. To get high quality images, where the map outlines are
always correctly placed, requires accurate timing to ±1 second
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
but this gets a little tedious after a while.
For computers permanently connected to a LAN network, obtaining
time is fairly easy, by synchronising a PC 's clock to a local Network
Time Protocol (NTP) server. But for people who
only occasionally connect to internet via a modem, NTP isn't an option.
But there is another option, to use a GPS receiver. GPS
receivers, as well as showing one's geographic position very well, also
generate accurate time. Many modern GPS receivers also have an
output port which may be connected directly to a PC's serial or
USB port. There is a standard GPS interface protocol 'language'
known as 'NMEA-0183', and technical folk may read all about it
if they wish. But all one needs to know is that many GPS
receivers can be simply connected to a computer, with the GPS antenna
placed against nearby window. With the installation of a suitable
program to read the NMEA data stream coming from GPS, one can then
very accurate PC time.
Yet another on WXtoImg's many clever functions, is to manage a GPS
receiver that has been connected to a PC's serial port, and to use this
data to periodically correct the PC's clock. "Rather an expensive
clock?!...", you may say. Not necessarily. GPS receivers
have been around for a few years now, and modern ones are getting
smaller, cheaper and cleverer. So what's happened to all those
older bigger dumber GPS receivers? They're out there somewhere,
and if one's lucky, one might come across a near-dead one that supports
NMEA-0183. It doesn't matter that the unit might be
large and clunky, with a battered case and nearly dead display; if its
port works, it's a good one!
Then again, these days GPS receivers aren't all that expensive.
I synchronise my home PC clock using a 'GPS Mouse', a simple
GPS receiver with no display, which connects to my PC via a USB port.
One useful feature of this device is that it gets its power via
the USB cable, meaning no extra power supplies are necessary. The
'GPS mouse' sits
quietly by the window and my PC's clock is always perfect (providing
is also running). Such GPS computer clocks may also be run by a
of good free/share-ware, available from websites such as tucows.
Antenna Masthead Preamplifier
These days one gets good bang for one's buck with antenna preamps.
The advent of surface mount electronics, low-noise high-gain
semiconductors, and mass production, mean that even modestly priced
can still be good performers. In Australia I have been impressed
by the 'Kingray' brand of TV masthead preamp, available at most
One of the nice things about NOAA satellite receiver antennas, is that
they don't necessarily need to be perched high on poles or buildings.
In fact placing an antenna low-to-the-ground in one's back yard
preferable when located within high-radio-noise urban environments.
high or low, if one's antenna is on the end of a long feeder cable, a
lightning strike may cause large ground currents and electric fields,
which may induce a spike of electricity to run up the cable and zap the
If one lives in a lightning prone area, a coaxial cable lightning
could be useful. With the advent of Cable TV, cable isolating
are commonly available and relatively cheap (see image below
left). Within Australia, coax isolators
sold at Dick Smith's, stock number L4665. The following
images show a coax isolator combined with a coaxial
50:75ohm impedance transformer. This arrangement gives our
receiver a very high degree of protection from lightning induced
nasties, as well as providing the correct impedance match to our
receiver. I haven't measured the insertion loss of this
arrangement, but I have measured the impedance transformation, and it's
close to perfect.
If one is using a
masthead preamplifier which is powered by AC or DC current via the
coax feeder, note that this isolator blocks DC and low frequency
electrical current, and therefore it cannot be inserted in series with
th main feeder. But whether a masthead preamp is being used or not, the
best place for the isolator is directly
connected to the receiver's antenna input.
USB Sound Card
Some computers lack a sound card, or occasionally their ISA and
PCI card slots are so full that there is no extra room for a sound
In this case, a 'sound card' facility may be added by using a USB
audio interface. Our particular Linux computer had no spare room
for a sound card, so we added a Telex
P-500 Digital Audio Converter. This gizmo works well and all
our NOAA imagery is received through it. It has standard 3.5mm
stereo microphone and headset jacks, and great care must be taken
when connecting a radio receiver output to a microphone input (as
explained above in the 'A
cable between...' section above.
A friend of mine recently mentioned to me that he had purchased a USB sound interface
device manufactured by Xitel, and
was particularly impressed by the performance and the relatively modest
cost. A device like this would also be quite satifactory for
satellite recording purposes, and would require less fiddling than
trying to send the audio signal into a 'Plug in Power' microphone jack.
Getting it together
Final interconnection of the radio, PC, antenna and preamp has been
mostly covered above. Operation of the WXtoImg software is fairly
straight forward in general, and you should not have much difficulty
satellite image of some sort. And that's where the educational
usually starts; optimising WXtoImg settings, antenna location, audio
timing, interference minimisation etc... etc...
Putting together a system is fun and educational.
Even with a
hands-off fully automatic station, I still get a thrill actually
hearing live NOAA satellite transmissions, and watching live imagery
scrolling onto the computer screen as the satellite is passing
overhead. At these times one can easily imagine being aboard the
spacecraft, looking down on the Earth passing underneath. The
fact that these images are 'live', and the cloud pictures can
be directly related to real-time out-the-window cloud observations,
allows one to
become quite good at forecasting the local weather, or at least the
cloud cover. The ability to interact with live
earth-observation satellites, using familiar looking equipment, brings
immediacy and technical demystification to a student.
Certainly I learned a lot, and still am...
Siding Spring Observatory
Radio Amateur's Satellite Handbook (1998), by Martin Davidoff.
Available from ARRL
publications. In Australia on sale at Dick Smith's.
ARRL Handbook for Radio Communications (current edition).
Available from ARRL
ARRL Antenna Handbook (current edition), edited by Dean Straw.
Available from ARRL
Page last modified - 2005-06-12. This page is still under
development. Please feel welcome to advise
me of any typos, broken
web links, clumsy language, and/or suggestions for improvements.