[lug] decoding weather data via radio Was: Time sync w/GPS or radio

[Nate Duehr]

On May 11, 2008, at 8:16 PM, Mike Stanczyk wrote:

> On Sun, 11 May 2008, Jason Vallery wrote:
>
>> Hi Mike,
>>
>> By weather data I'm assuming your talking about APRS on the amateur
>> bands.
>
> No actually.  I have a Oregon Scientific outdoor wireless temperature
> station that the basestation has died.  There's a Davis wind/rain  
> gauge
> on the roof across the street I'd to listen to as well.
>
>> Typically you need a TNC or radio modem which is a device that
>> converts the analog signal to digital.  These are bi-directional.  I
>> know there is also a project out there called AGWPE that attempts to
>> do this with a sound card.  You might take a look at that if you  
>> don't
>> want to invest in new hardware.
>>
>> -Jason
>
> That's kinda close.  I can tune my old Radio Shack scanner to the  
> right
> frequency and plug the scanner into the soundcard.  I need software to
> convert into bits...  I'll Google AGWPE and see what it can do...

Unlikely to work.  I SERIOUSLY doubt Oregon Scientific is using AX.25  
(Amateur Radio modified X.25 protocol) to talk from their proprietary  
weather gathering device to their base stations.  AGWPE is a tool for  
hams to copy that protocol (the 1200 bps variety) with an audio cable  
from an FM radio to a PC sound card.

If you tune your scanner to 144.390 MHz in most populated areas,  
you'll hear what 1200 bps AX.25 sounds like.  There's quite a bit of  
traffic on that frequency of a special type called "APRS".  You can  
look that up on the Net if you're interested in what that is.  To me,  
AX.25 "squawks"... to my ear anyway.  I spent hours and hours  
listening to it in the early 90's when we were encapsulating IP inside  
it to route things over radio links... at 1200 bps.  Painful, but  
anyone who watched the TCP 3-way handshake at 1200 bps HALF duplex,  
hundreds of times, understands it pretty well!

If the sounds coming from the Oregon device are EXACTLY the same,  
that's amazing... but unlikely.  AX.25 is far too much overhead and  
causes long transmission times for battery-operated devices like the  
weather stations.  They're going to want to densely pack the data into  
a VERY small packet sent "not too often, but often enough to update  
the screen inside" to have a much lower transmitter duty-cycle and  
thus, longer battery life.

You'll have to either find someone who has completely reverse- 
engineered the product, or ask Oregon Scientific if they'll share the  
modulation type and the protocol with you.  (Doubt it.)

Just because your scanner will receive its signal, doesn't give you  
the rest of the protocol.  That'd be kinda like tapping into a random  
wire running from the computer in your car that talks to the  
transmission, and thinking you could read the data... if it's ASCII  
serial at a normal baud rate, perhaps... but even in that case, you  
have less unknowns.

To intercept a digital radio system you need to know its modulation  
type, how it's encoded, and then the packet protocol encapsulated in  
that encoding.  You're looking at a lot of guesswork.

AM, FM, Pulse Modulation, Spread-Spectrum?

Huffman coded?  BCD?  Binary proprietary format?  ASCII (unlikely)?

Which bit in the bitstream represents what if it's binary?

Some "hints" might be who makes whatever RF chipset Oregon uses...  
many manufacturers making low-end/low-priced products are going to  
often pick a standard off-the-shelf RF module and then put their own  
data packets or stream of information over those... so you can get the  
modulation type, speed, etc... from the datasheets for the chipset,  
but you still won't know which bits are temperature, humidity, wind  
speed... nor how they encoded them... that will require bench time,  
and a lot of experimentation to reverse-engineer it.

Unless... the magic of Google shows you where someone else has already  
done all of that work and published it.  Or if Oregon is an open  
company about such proprietary things as their little weather  
stations.  Most companies making products that are priced anywhere  
from 3x to 10x the worth of the components (most Oregon Scientific  
circuit boards are pretty simple) aren't going to make information  
easily available about how to intercept their protocol... they'd  
rather sell you another receiver, and another for the other room, and  
another for the garage... you get the idea.

--
Nate Duehr, (Since you asked for hams... WY0X)
nate at natetech.com