The picture you see up there is a radiofacsimile image of the Earth, beamed down by NOAA 15 as it flew over my house this morning. I received it on the vertical antenna I built for my home VHF station. That antenna is vertically polarized, meaning it is a horrible candidate for receiving the circularly-polarized signal from NOAA satellites. That doesn’t mean it’s impossible to receive on it, though, and with a little post-processing, cloud tops and even bodies of water are perfectly visible. Here’s a GOES capture from this morning for reference:
I say this is “sort of” a success because the picture is of really poor quality compared to what is possible for amateurs. Once I build a proper antenna for this particular use (and maybe a band-pass filter and amplifier as well), I should be getting pro-quality images.
Here’s where you might expect me to point out all the super-fancy and expensive gear I used to make this happen. Well, it probably would have taken a much larger investment to do this 20 or 30 years ago, but today you can do it with a $35 Raspberry Pi and a roughly $30 RTL-SDR device. (In fact, there’s no real reason to use the Pi except for the convenience of not having to leave my laptop hooked up to this stuff.) Granted, I already had the antenna, but it was cheap as well. It’s just an SO-239 connector and some 3/32" brass rods. There are plans for cheap but sturdy quadrafilar helix (QFH), double-cross, and turnstile antennas online, all good for receiving circularly-polarized signals such as those from NOAA birds. In this day and age, with software-defined radios having hundreds of surface-mounted ICs and extremely intricate construction, antennas are about the only thing most amateurs can still build on their own.
The RTL-SDR is a miniaturized software-defined radio packaged in a case like a regular USB drive. The other end has an SMA socket where you can connect an antenna. The Pi is a bare-bones server running a special version of Debian and is hooked into my home network so I can SSH into it from my computer. The RTL-SDR is plugged into the Pi and is connected to my outdoor VHF antenna. Scheduling, recording, and post-processing of weather satellite images is all done automatically with a toolchain I glued together using POSIX shell.
I don’t want to turn this post into a technical lecture. The main idea here is “I built a satellite-tracking station in my spare time and actually pulled imagery down from space!” I think that’s pretty cool.
Stay tuned; I’ll write up a tutorial with detailed steps on how to replicate this process for anyone interested. Use a better antenna than me and it’ll serve you fine!
Topics: radio technology