Listening to Space

I’ve been playing around with radio astronomy and satellite stuff lately. As usual, this is with the cheapest / free-est (is that a word?) gear I can build or scavenge. So far I’ve made a very very basic “radio telescope” out of an old TV dish and security camera mount. I’ve also managed to listen to passing weather satellites with some bits of wire. This is another project I’ll be working on more in the near future. I’d like to be able to do more with the dish (maybe pick up free NASA TV), and more with the weather satellites (maybe geostationary next). Stay tuned for updates!

Some weather images I received from NOAA satellites using the V-dipole antenna:

Listening to NOAA weather satellites as they pass overhead is relatively simple! Actually getting imagery decoded from the transmissions took a little more effort, as I learned in the process of making this video. Below are some of the resources and guides I found helpful for this project:

Basic intro to satellite reception: https://publiclab.org/notes/sashae/06-26-2020/diy-satellite-ground-station

The antenna design I used: https://lna4all.blogspot.com/2017/02/diy-137-mhz-wx-sat-v-dipole-antenna.html

Website for tracking satellites and determining upcoming overhead passes (As of early 2021, the ones to watch are NOAA 18, 19, and 15): https://www.n2yo.com/

Info on the NanoVNA I used for antenna tuning: https://www.youtube.com/watch?v=QJYeFpiqY8c

This is a decent Software Defined Radio that’s capable of hearing these satellites. You can get them with or without lots of extras like antennas and amplifiers: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

The software I used to interface with the SDR and record audio files (for Linux / Raspberry Pi): https://gqrx.dk/ (I have heard SDR# is good for Windows, but have not tried it: https://airspy.com/download/)

The software I used for decoding images: https://wxtoimgrestored.xyz/

Some “gotchas” I ran into when working with saved audio files (not always obvious from online guides):

-WXtoIMG is ancient abandonware and barely works on modern computers. The Linux version has some display issues and freezes when trying to update Keplers, at least on my system. On Windows I found that the beta version works better, the “stable” release won’t install at all). It also doesn’t like modern sound drivers, so if you can’t decode live signals you may have to record and decode later (see below for even more tricks with this!)

-Gqrx saves recordings in stereo at the wrong sample rate. You’ll need to open the saved audio file in Audacity (or some other audio editor), collapse the file to mono and change the sample rate to 11025. You’ll probably also want to Normalize the file after each step. See here for more details: https://lucasteske.dev/2016/02/recording-noaa-apt-signals-with-gqrx-and-rtl-sdr-on-linux/

-WXtoIMG It is very finicky about date/time stamps and you may need to fiddle around a lot to get your recording to match a known satellite pass. I had no luck using the filename to specify recording time as the faq recommends. I had to download a file attribute editor (Or this software can change timestamps: https://noaa-apt.mbernardi.com.ar/ ). You need to change the “Modified” date and time (not the “created” attribute) to the time when the recording started (because once you process the file through Audacity, the timestamp will be different). I then had to manually adjust the map overlay in WXtoIMG (fortunately I had a visible reference point, if it’s all clouds you might be out of luck!) See https://wxtoimgrestored.xyz/faq/ for some info on this.

Hopefully all that helps! There are a lot of guides online for how to do this, some are more complete than others. There are problably also plenty of videos better at explaining this than mine, I was just trying everything out for the first time.

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