First of all I’d like to thank Outernet project creators for getting people interested in radio, satellites and out of the box thinking. It is a very cool project to DIY (or should I call it “assemble it yourself”?) and it’s a perfect “gateway drug”.
Short version of the story: fully autonomous receiver is a very expensive project, it’s going to cost you hundreds dollars if not more, so for now I decided to focus on better reception giving up the autonomy idea. Below I list the challenges I’ve met, perhaps somebody could share their experience solving them.
Now the long version.
I got interested in radio in general and SDR in particular recently and thanks to RTL-SDR blog learned about Outernet project. I decided to make fully autonomous receiver because it sounded like a very cool idea.
I’ve ordered a DIY kit without a power bank (because I’ve already had two), a waterproof case and a 20W waterproof solar panel.
My findings so far:
The receiver works 5-6 hours on a 10 000 mAh power bank. Therefore if you want your receiver to be autonomous and you’re not living too close to the poles of the Earth, you’ll need at least twice as big power bank or multiple power banks. I think that the biggest power bank RAVpower sells (it’s 22 000 mAh) is still slightly below adequate capacity. To be on the safe side you need two such power banks, so it won’t be very small or light, but the price is reasonable (below $100 for two).
I’ve read that in cloudy weather solar panels work at 10%-25% of their maximum power rating. In reality 20W solar panel can’t charge any power bank or phone I’ve tried in cloudy winter weather, even if it looks like it’s bright enough outside. It shows the charging sign but the current is very low so it doesn’t charge at all (or charging so slowly that it doesn’t make any difference). Perhaps it can charge a power bank specifically designed to work with low power solar panel but I’m not sure about that. I’ve bought a USB dongle to measure current, I’ll post my findings as soon as it arrives. Unless I have a faulty solar panel, at the moment I think that you need at least 200W solar panel to fully charge a small (20000 mAh) power bank in a cloudy day. Such panel is going to be pretty heavy, big and expensive (few hundred dollars). It’s probably better to give up on solar-charging solution and better think about big enough power banks that you can swap and charge in some other ways.
Cheap ($15) waterproof case (made of polycarbonate) works excellent. Rain or snow, no problem at all. Can be a problem in sunny summer weather though - the RTL-SDR is getting really hot even in sunny winter weather. Perhaps a white colored case would be ideal, but the clear one is OK for occasional use. A black one is probably going to get really hot. With $2 stand for a tablet from dollar store it’s really easy to point it where you want.
Reception outdoors (far away from buildings) is not a problem at all, pointing is easy, the reception is good enough in any weather. I’ve tried it when it was raining with real heavy clouds on the sky and I was getting 5-7 dB SNR (signal to noise ratio). In my area reception indoors or close to a building is pretty much impossible for me. I’ve made a small research and learned that there is so much RF noise indoors in all frequencies and particularly 1540 MHz range that it makes reception impossible. I thought that the unshielded LNA is the problem but my tests proved that all the noise is coming through the antenna, so there is really no way to filter it out. What is worse, that my RTL-SDR dongle (a shielded V.3 version) still “receives” something without antenna attached. Surprisingly, the shielding completely removes broadcast FM but doesn’t remove strong signals in higher frequencies (like 900 MHz range). Some of that could be internal noise, but some is clearly GSM and pager signals. Not sure how this problem can be solved. I still have to investigate how much noise is coming through the USB port, but anyway it’s silly to expect $10000 performance from $30 dongle. I’m going to try a FM band-stop filter to see if it affects anything at all. It would be nice to have a band-filter just for L-band. Just out of curiosity I’ve compared RTL-SDR V.3 with no antenna attached and SDRplay RSP with no antenna attached and they both shown similar noise, SDRplay having some FM broadcast visible. Wrapping SDRplay tightly in aluminum foil didn’t affect anything at all.
CHIP is a great tiny computer, everything works surprisingly well and it boots super quickly. I wonder how much power it looses through the bright white led, it looks almost like a small flashlight. it would be nice to have it blinking once every 5 seconds instead of being always on. The only problem is that the W-Fi antenna is tiny and weak so it can be a bit tricky (but not too hard for sure) to place all your components in a small case without something interfering with the Wi-Fi connection. Just make sure you don’t place anything over the Wi-Fi antenna (it’s on the PCB side opposite to USB ports, on the “far end” of the CHIP board).
Overall I am very happy with the project, I’ve learned many exciting things while working on it. Taking into account how much Inmarsat equipment and subscriptions cost, the Outernet is a great way to get familiar with satellite communications without investing much.
Just as a side note, I have photos and screenshots of all the stuff described above, so if there is any interest I could add them here. For now here is a photo of my receiver: