
Receiving Weather Satellite Images With An £8 Dongle - mmastrac
http://mattg.co.uk/words/noaa_sdr/
======
olympus
The author mentions that he used "A 2 metre bit of wire jammed in the
screwhole at the base of the tiny antenna" to try and get better reception.
While the signal he is receiving is in the 2-meter band, a monopole antenna
works better if it is 1/4 the wavelength, so he might actually get better
reception with a 0.5 meter length of wire. What he created is usually called a
"random wire" antenna, and may deliver acceptable performance with a strong
signal.

But the NOAA satellites (and quite a few other satellites) use a circularly
polarized signal, and for just a tiny bit more money (perhaps the cost of the
dongle) you can make a MUCH better antenna for circularly polarized signals.
Some designs are just helixes made with coax cable, some are just a couple of
wires in an x-shape.

Google is your friend, and building an antenna is a fun cheap weekend project
and will greatly enhance your results when hunting for satellite signals.

P.S. if you make a circularly polarized antenna and you discover that you made
a LH polarization instead of the RH that you wanted (or vice versa), just turn
the antenna upside down. It may behoove you if you are planning on listening
to signals with both polarities to construct one that is easily flipped over.

~~~
weinzierl

        While the signal he is receiving is in the 2-meter 
        band, a monopole antenna works better if it is 1/4 the 
        wavelength, so he might actually get better reception 
        with a 0.5 meter length of wire. 
    

I always thought a lambda/4 antenna is used because it's almost as good as a
lambda antenna but much more conveniently to handle. I understand that more
complicated antenna designs can be better than a simple lambda wire, but how
can a lambda/n wire be better? I don't doubt what you are saying, I'm just
curious how this can work.

~~~
aexaey
Linear monopole antennas are very easy to reason about.

Imagine (or actually draw) a sine wave graph, and then fit your antenna along
the X axis of the graph. Now look at "signal" amplitude at the antenna's
extremities. You'll notice that 1/4 wavelength antenna will give you the
strongest signal, while 1/2 and full wavelength will apparently give no signal
at all. The only reason that full-wavelength piece of wire picks up anything
at all are build imperfections - wire length not perfectly matched to
wavelength, not perfectly straight, not perfectly aligned with signal source,
signal reflections hitting it from different angle, etc. Thats why olympus
refers to performance of full-wavelength piece of wire as "random piece of
wire"-kind of performance.

On the other hand, linear dipole would be 1/2-wavelength (or 2 times
1/4-wavelength), as it is just two 1/4-monopoles put together.

~~~
weinzierl
This makes sense. Thank's a lot.

------
Fizzadar
This is awesome; am ordering a dongle right away!

Did some digging and there's a whole load of things which can potentially be
picked up, including dead satellites [0]. RTL-SDR.com has a whole load of
awesome looking tutorials [1].

[0] [http://www.rtl-sdr.com/receiving-dead-satellites-rtl-
sdr/](http://www.rtl-sdr.com/receiving-dead-satellites-rtl-sdr/) [1]
[http://www.rtl-sdr.com/category/tutorial/](http://www.rtl-
sdr.com/category/tutorial/)

~~~
xur17
I just ordered one too - this looks awesome.

You can track aircraft (newer airplanes broadcast their locations, ships, etc.

This [0] is the one I ended up ordering, I just wish one day shipping was
available.

[0] [http://www.amazon.com/dp/B009U7WZCA](http://www.amazon.com/dp/B009U7WZCA)

~~~
dima55
$20 is way too expensive for these. Here's the same thing for half the price:
[http://www.ebay.com/itm/Newsky-TV28T-v2-USB-DVB-T-RTL-SDR-
Re...](http://www.ebay.com/itm/Newsky-TV28T-v2-USB-DVB-T-RTL-SDR-Receiver-
RTL2832U-R820T-Tuner-MCX-
Input-/131532641773?pt=LH_DefaultDomain_0&hash=item1e9ff4d5ed)

~~~
TD-Linux
There are a variety of factories producing these at variable quality. I'd
still recommend the Nooelec version if you have the budget to spare.

------
MrBra
Great, just recently I became passionate about anything relating to radio
theory and transmission and I had learnt about these dongles and was ready to
get one.

But before that, I decided I wanted a proper "hardware radio", and bought
myself a portable wide band radio receiver [1] (great value for the price, I
advice it). Then when I was about to get one of these dongles, as usual and as
for the above radio, I got sucked into the reviews suggesting this or that
other model for better sensitivity or extra features. Now I understand the
price range for these cheap sticks is just in the order of a few tenths at
maximum, but maybe there is a chance anyone has investigated a bit more on the
available choices already?

For example.. the one that has been written about in the article has 4 stars
and 159 reviews while the first alternative in the "Amazon recommends" box has
some 940 reviews and same 4 star score for just £ +2 (but I'm not even sure
they do exactly the same things.. sorry). So, just wondering, maybe someone
has already gone through this search and can recommend the best one to get?

[1] [http://www.amazon.com/Tecsun-PL-660-Portable-Shortwave-
Singl...](http://www.amazon.com/Tecsun-PL-660-Portable-Shortwave-
Single/dp/B004H9C4JK)

~~~
olympus
> Now I understand the price range for these cheap sticks is just in the order
> of a few tenths at maximum, but maybe there is a chance anyone has
> investigated a bit more on the available choices?

Thanks to the popularity of this family of DVB dongles, all the information
you need is right here: [http://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-
dongles/](http://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/)

Also look into the Funcube dongle for a sdr dongle that is designed for ham
radio and not just an adapted DVB dongle -- they are pretty much the same
thing though. If you can find an E4000 dongle on ebay it's probably worth
twice as much as the R820T dongles because it gives you a wider frequency
range. I have an R820T dongle and it works just fine though.

One thing to keep in mind is that these dongles only have 8 bit ADCs in them,
so no matter how great your signal path, you will top out at ~48db of
sensitivity (rough formula is sensitivity in db = 6*bits of ADC). A "real" SDR
like the USRP B210 has a 12 bit ADC in it, which means that it will top out at
around 72db of sensitivity. Plus, spending the big money means that you will
get a radio that takes less shortcuts than a cheaply produced dongle, so you
are much more likely to get closer to that theoretical peak. But certainly
start with a dongle, and play with it and see how far you get before you
"absolutely need" some TX capability or have truly maxed out what you can do
with only USB 2.0 and 8 bits of resolution (though there is a guy who was
passively tracking aircraft with FM radio stations and two synchronized
dongles so what you can do with these is pretty crazy).

~~~
CamperBob2
_One thing to keep in mind is that these dongles only have 8 bit ADCs in them,
so no matter how great your signal path, you will top out at ~48db of
sensitivity (rough formula is sensitivity in db = 6x bits of ADC). A "real"
SDR like the USRP B210 has a 12 bit ADC in it, which means that it will top
out at around 72db of sensitivity._

Of course, the ~6 dB per bit rule applies only at the ADC's full (Nyquist)
bandwidth. If the ADC is followed by a decimation and filter chain it can
potentially do a lot better. Do these dongles perform their conversion at the
final signal bandwidth, or do they digitize at a wider IF and downconvert to
baseband digitally?

~~~
olympus
Excellent point. I believe that the ADC samples at 28.8 Msps and then
decimates to the rate requested by the computer (max ~3.2 Msps). So
theoretically it could be doing ~3 bits better, if everything was implemented
properly and those extra bits were delivered across the USB to the computer. I
think the samples delivered to the computer are still only 8 bits so the ~48
dB theoretical limit to the dynamic range remains.

CamperBob you sound like you know what you're talking about and I'm mostly
self taught in this area so if I'm wrong please let me know.

~~~
CamperBob2
Same here, I (try to) spend most of my time out of my depth. :)

It sounds like the bandwidth of the 137 MHz satellite signal is in the 36 kHz
vicinity, going by the article. So one possible strategy would downconvert 137
MHz to somewhere in the first Nyquist zone of the ~30 MSPS ADC (i.e., below
about 15 MHz). The ASIC picks up 3 more bits of process gain by decimating to
3 MSPS, then the host CPU could pick up about 5 more bits by further
decimation to about 100 kSPS. The 36 kHz signal would then have a dynamic
range approaching that of a 16-bit ADC, which is quite adequate for most
purposes.

Of course, IMD artifacts and other spurious tonal responses due to the cheap
RF front end don't experience any such improvement, but you sure can't
complain for $20.

------
RobotCaleb
I did this a few years ago with a similar dongle and a homemade antenna. It's
a pretty cool thing to see something you cobbled together receiving real-time
messages from space.

[http://robotrising.org/2012/10/capturing-weather-
satellite-i...](http://robotrising.org/2012/10/capturing-weather-satellite-
images-using-a-homebrew-antenna/)

------
nly
Last thing I used my R820T for was decoding POCSAG[0] messages here in the UK.
It's a paging system, and the messages seem to be broadcast nationwide in the
clear. Seems to be mostly used by the NHS and vets. I got kind of depressed
reading about dying pets.

[0]
[https://en.wikipedia.org/wiki/POCSAG](https://en.wikipedia.org/wiki/POCSAG)

------
jodrellblank
There's a Reddit for software defined radio, using the cheap Realtek RTL2832U
devices.

[https://www.reddit.com/r/rtlsdr](https://www.reddit.com/r/rtlsdr)

~~~
adestefan
Just be aware that some of the moderators have iron fists and like to go ban
happy.

------
yardie
I did something similar using the same libraries and all using a Raspberrypi
and DVB-T dongle [1].

Then I configured the tcp server, rtl_tcp to forward the packets to my
workstation. This allowed me to put the Pi in a location more suited to
receive transmissions, i.e. not next to RF emitting servers and power
supplies. Then, using Gqrx with remote server, I analysed the results.

[1] [https://www.joechin.com/raspberry-pi-and-sdr-getting-
started...](https://www.joechin.com/raspberry-pi-and-sdr-getting-started/)

~~~
robertduncan
Looks like your SSL certificate has expired.

------
fosk
I would expect the communication to be encrypted, or is it supposed to be open
to the outside world?

~~~
nitrogen
I think weather satellites were meant to be useful to everyone who could
receive the signal.

~~~
fnordfnordfnord
Especially for marine traffic.

~~~
dzhiurgis
WeatherFax has been used for decades now.

This, however, wouldn't be of much use in a regular cruiser as you do not want
to keep your laptop constantly to wait for broadcast. Dedicated devices are
quite cheap and reliable.

~~~
fnordfnordfnord
I think most people are doing this for fun. But if you needed it and couldn't
get your hands on the real thing, adding an Rpi and a screen to it makes a
functional substitute.

------
joushx
I have this one: [http://www.nooelec.com/store/sdr/nesdr-
mini2-rtl2832u-r820t2...](http://www.nooelec.com/store/sdr/nesdr-
mini2-rtl2832u-r820t2.html)

I already received aircraft positions (ADS-B), aircraft radio, ACARS, VDL,
HFDL, satellite pictures, ISS pictures, a ISS astronaut speaking, a bunch of
other satellites (including radio pirates on US military sats), small outdoor
weather stations, a portable radio for children, some AM radio stations, a lot
of ham radio and pagers.

There is also huge amount of signals I can't decode yet.

~~~
joushx
BTW: I'm using an old TV antenna.

------
amelius
I don't understand these signals are not encrypted, for commercial reasons.

~~~
kintamanimatt
It's not a commercial service, but a government one. The NOAA (National
Oceanic and Atmospheric Administration) satellites belong to a department of
the US government.

~~~
amelius
In many countries, public transport is run by the government, but still costs
money.

Governmental does not have to be orthogonal to commercial.

~~~
bjelkeman-again
In Sweden the agency that deals with cartography and land measurements
(Lantmäteriet) charged money for their data. An acquaintance who works with
the government checked their income and where it same from. Something like 80%
came from other government departments and the rest from companies. But the
extra 20% was roughly the cost of running the administration to manage the
billing. So the data got limited use for no financial gain.

In May they started releasing the maps under Creative Commons [1]. Not only me
was really excited about this here in Sweden.

[1] in Swedish [http://computersweden.idg.se/2.2683/1.616018/slapp-
kartorna-...](http://computersweden.idg.se/2.2683/1.616018/slapp-kartorna-
loss-det-ar-var--lantmateriets-databas-ute)

------
kenrikm
You can use the same dongle to tap into ADS-B signals to get realtime ATC and
display it on a map. Also, it's interesting to just sit and listen to SFO ATC
audio channel.

~~~
robterrell
He has a screenshot of this in the article.

------
fsiefken
Nice hack! I wonder what other useful common digital signals can one record
with a dongle? Time and date, teletext (which includes weather forecasts and
news) I am aware of. There is also project Lantern & Outernet which might
deliver 20 MB of data to all continents through nanosats next year. What other
digital signals are transmitted?

------
dm2
Free fast shipping:
[http://www.ebay.com/itm/251240985308](http://www.ebay.com/itm/251240985308)

Most of them seem to have the same two chips in them, RTL2832U and R820T. Does
anyone see an issue with this item I linked to? Some people were mentioning
quality issues with generics.

------
weinzierl
There is also OsmocomSDR[1] which is an excellent starting point into software
defined radio.

[1] [http://sdr.osmocom.org/trac/wiki/rtl-
sdr](http://sdr.osmocom.org/trac/wiki/rtl-sdr)

~~~
MrBra
How does this one differ?

------
SoulMan
All I have done till now is tune to local FM stations. I would like to tune to
AM SW stations all over the world. I think buying a 50 USD RF up converter is
the only option ?

~~~
dkozel
You can do a very simple, though somewhat permanent, modification to the
dongle and receive DC-14MHz. [http://www.rtl-sdr.com/rtl-sdr-direct-sampling-
mode/](http://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/)

~~~
userbinator
There's a software modification here which is also interesting:

[http://superkuh.com/rtlsdr.html#directsample](http://superkuh.com/rtlsdr.html#directsample)

The whole idea of the community figuring out hacks to use this hardware in
ways that were never intended by its original designers, including exploiting
bugs/artifacts, reminds me of the demoscene.

------
exo_stack
I wonder if that would work on a commercial jet @ 30K feet. Would be an
interesting test.

~~~
grkvlt
\-
[https://twitter.com/grkvlt/status/600998588475375616](https://twitter.com/grkvlt/status/600998588475375616)

This is the output I got on a flight from EDI to LHR at about the half-way
point, decoding ADS-B transmissions.

------
ma5ter
done this few years ago [http://imgur.com/a/3GELZ](http://imgur.com/a/3GELZ)

------
jafingi
That's totally awesome! Have to try this myself.

