Nordintown Blog

Rubbish musings, projects and hackerings from Nordin

Space Radio

Mar 212015

Today I drove my small car in an after-work dash to a quiet low cliffside car park in-between the harbour lighthouses.

There I sat intently listening to radio static blasting from my mac. A freshman radio enthusiast, I was planning to receive magic space images from orbit via my magnetic roof antenna plugged into a cheapo SDR radio receiver. The long, drawn sputnik-sounding 'weep weep' with superimposed donkey-like short 'clip clop' of the NOAA Automatic Picture Transmission (APT) signal was my paydirt. After its capture, the audio signal was going to be processed via a simple toolchain to become a beautiful, fully-formed live weather photo from orbit. This exciting outcome was not a given however, as I had found at my last two attempts. I was pretty sure this run would be a success and not a waste of hours. I had brought sandwiches and a banana just in case.

I'd heard about the magic NOAA weather broadcasts from hackerspace radio folk and it seemed like a fun way to use the RTL-SDR and Sky Scanner Rx antenna combo I'd bought on a whim. A number of weeks ago, space members hosted a radio weekend at Aberdeen Uni where folks erected their roof dipoles and made contacts over HF. Making use of the roof also, I played with receiving the NOAA signals, sprinting to the chilly roof when the sats passed overhead. Orbiting National Oceanic and Atmospheric Administration (NOAA) satellites circle pole-to-pole 540 miles above the earth. NOAA have a few sats up there and the three main APT players - NOAA15, NOAA18 and NOAA19 - pass overhead a few times a day. NOAA15 and 18 have their downlinks as 137.6200Mhz and 137.9125Mhz respectively and seem to be the strongest signals for me.

To track sat passover times, I found gpredict [1] to be a great open source tool that can be installed cross-platform. To capture the actual audio via an RTL-SDR once I had a sat overhead, I used GQRX [2] for the mac set to narrow FM, but SDR# does a great job on a windows machine. On the mac, outgoing audio can be directed to an input device via the virtual soundflower device and recorded via Audacity set to a sample rate of 11025hz. On a non-mac, you’ll probably have a ‘stereo-mix’ device in place already. The resulting wav file then needs to be imported into the very cool WxtoImg tool. It does a one-click job of automatically transforming the audio into imagery, even if the signal is incomplete. This was the audio [3] and resulting image [4] of my first attempt. Not great, but a good start. At 38secs the audio is perfect, but it does not last. I sought a clearer, more rural sky, so a few weeks later I drove to the harbour coastal car parks and took a shot there, but the cloudy day seemed to prevent a strong signal. I got the ‘weeps’ but not the‘clops’ :(

So back to the present: there I was in my third attempt, waiting in my little car in a carpark with worlds smallest entry gate (seriously, my Corsa barely fitted past). After waiting a spell, I started to hear shadows of that magic sound. I had run my mac battery down by this point and I did not want it to die at this crucial stage. I plugged in my car charger for more juice. Not long after, I COMPLETELY lost the signal. Having not started the audio capture I was pretty pissed when no tweaking about the freq made any change. The sat was still in full view but I was getting nothing with 3mins till LOS. Infact the entire noise floor had raisen up (that should have been a clue). I checked the usb/coax connectors and roof aerial but no change. The sat passed below the horizon just as I realised that I had started the car engine to make sure my charging laptop and phone would not kill my car battery. Stopping the car brought the noise floor back to normal. DAMIT DAMIT!! Thats how I learnt that using cheap radios from inside a car sometimes work better with the engine and chargers off. Feel free to write that down if you are a radio dumbass like me. On the plus, I guess I forever learnt something new about SDR radio and found a nice radio spot, so not a waste of hours at all.

If *you* want to have the same (or hopefully more successful) raw audio-to-imagery experience, you should hurry. The APT sats are getting pretty old and cannot hold orbit forever. NOAA APT transmissions are scheduled to die from 2017 [5] and will be replaced with something digital. Boo hiss boo. If you do have a go though, for gods sake make sure you bring sandwiches. It seams brains are optional.