The National Oceanic and Atmospheric Administration (NOAA) manages a few satellites in low earth orbit. There are three actively transmitting APT signals at the moment, NOAA15, 17, and 18. Each of these satellites passes overhead a few times a day. I’ve been interested in learning how to receive their signals for a while now, and I’ve finally succeeded!
A bit ago, I bought a “SoftRock” SDR (Software Defined Radio, read the excellent 3-part article by Bob Larkin at the ARRL site.) receiver kit from Tony Parks. (A note about his site, he puts a few kits up for sale a few times a month, so he’s almost always sold out.) I think SDR is really, really interesting. I don’t want to get too bogged down in the details of it, because it’s not the point of this post, but I’m going to briefly discuss it. Basically, the idea is that you want to have some minimum amount of electronics to deal with the antenna; letting your computer handle the rest. This can take a variety of forms, but the simplest is the QSD, or Quadrature Sampling Detector.
Neat, you can pull down the images on your own, very cool!
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This project is using the VHF version of the Softrock receiver. These softrock radios are well done, but because the Softrock project refuses to expand to outsource kitting and fulfillment, they are VERY hard to obtain (demand far outstrips supply, which is from one person only, Tony KB9YIG – who can’t keep up). I consider the fact that the softrock kits are so hard to obtain as a disservice to all. This is the really sad part about the Softrock radios.
Maybe Adafruit can breath life into this project by kitting and fulfillment at a reasonable profit margin. But be ready to be FLAMED by suggesting it! There is an active Softrock Yahoo Group – that’s the go-to place for Softrock Community.
The VHF Softrock receiver used for this APT weather thing is really not optimized for 137MHz AFAIK. Look at the comments on the project link, there is another link on that page from the project author that sort-of addresses this.
APT at 137MHz is being deprecated – it won’t be around in the long-term. So this project has limited life.
The antenna and receiver front-end (LNA) issues are IMO poorly addressed. A decent antenna is critical and not trivial to source or build (Google APT "eggbeater" or "turnstile). For any significant cable run to the antenna one should consider a low noise amplifier (LNA) at the antenna feed-point properly powered through the coax or by using RF Bias Tee’s.
There is little said about the computer sound card needed for this project. Typically, you need a sound card that is of relatively high performance. Roughly greater than 100+ SNR and 24 bits. That’s the case with any direct-conversion I/Q "receiver" like this. I would choose a sound card with at least 96kHz sample rate (192kHz better). Really good sound cards for this DC-SDR stuff can get quite pricey, and you have to pay careful attention to ground loops (the latest Softrock receivers FINALLY address this, but it took seemingly forever!)
The Softrock radio used in this project needs to set the frequency via USB. I don’t think this is properly addressed. The receiver uses an ATtiny85 running a soft USB stack with Fred’s (PE0FKO) Silicon Labs Si570 part used as a local oscillator. There are other solutions. I doubt the software cited in this project can set the Si570 oscillator frequency MUCH LESS compensate for doppler shift, which is optimal IMO.