ISS reception with RTL-SDR

[f5oeo]

For information : this guy try to have a minimal hardware reception for HamTV ISS

http://www.pabr.org/radio/softdatv/softdatv.en.html

73's Evariste F5OEO

[pabr]

Thank you for the introduction, Evariste !

FYI here is the shoulder-mounted receiver I tested this morning during the Sandringham School contact. This is the current state of my project to build a HAMTV receiver for about 50 EUR (plus a smartphone).

• 24 dB WiFi dish (100x60cm)
• MMDS downconverter (LO 1998 MHz) with built-in linear dipole (I know...)
• DC bias tee and 14.4 V battery pack
• rtl-sdr dongle with R820T2 tuner and TCXO
• scrap construction materials
• Linux tablet.

The software in the left window tracks the ISS and uses the accelerometers and compass of the tablet to display a pointing reticle. GQRX (right) displays the usual spectrum waterfall and records raw I/Q data at 2.4 MSamples/s. Software-based demodulation is not yet real-time. The white smartphone on the right, running an off-the-shelf satellite tracker app, serves as a backup pointing device.

Actually I was only able to finalize the software late last night, and the transmitter was offline earlier this week anyway, so everything failed miserably this morning due to lack of preparation and testing. I am not sure whether video was used during this contact, but I was hoping to at least catch the blank transmission despite the poor elevation (56° from Paris, France). Hopefully I will be able to announce a successful reception in the next few weeks.

P.S. I would like to thank all the amateurs who have been working for years (since 2002, I think) to make HAMTV possible. The technical challenges definitely revived my interest in amateur radio.

[pabr]

I manually tracked an overhead pass of the ISS this morning:

https://youtu.be/p6xCT0BhOmE

The signal was centered at +450 kHz, which means either my SDR dongle or my MMDS downconverter or the ISS was way out of spec. In the end I only got 3.8 MB of error-corrected Transport Stream spread over two minutes. Nice blue line with some kind of interlaced video artifact as usual.

There is still plenty of fine-tuning to do. The tracking system was not even calibrated against the beam pattern of the antenna. I believe this approach will produce usable videos eventually, and at $50-$100 this could be a nice hands-on project for pupils involved in ARISS school contacts or for other STEM outreach activities.

[G8DKC]

Hi pabr,

Good to see someone looking at the problem from a different perspective.
Will look into your project when I get some free time.
So much to do in this fine hobby
Great website, http://www.pabr.org/
Peter

[pabr]

During the ISS-RMS HamTV contact this evening I was able to recover 11 MB of error-corrected TS with my shoulder-mounted 24 dBi dish, MMDS downconverter and rtl-sdr dongle. Maximum elevation was 47° from Paris, France and the weather was wet.

Also, it looks like real-time software-defined demodulation will be possible on my tablet (Intel Atom Z3745 1.33 GHz using only one core out of four) at the cost of reduced sensitivity.

[pabr]

Here is a preview of my software DVB-S demodulator, in case anyone wants to try SDR during the upcoming ARISS HamTV events:
http://www.pabr.org/radio/leandvb/leandvb.en.html

The primary goal was real-time operation on low-power platforms, and this requires a high SNR. Other trade-offs between speed and sensitivity will be implemented later. Also, only FEC=1/2 is supported.

On Linux the quick start guide boils down to:

Code: Select all

wget http://www.pabr.org/radio/leandvb/leandvb.cc
g++ -O3 -DGUI leandvb.cc -lX11 -o leandvb
rtl_sdr  -f 397000000  -s 2400000  capture.iq
./leandvb  --gui  < capture.iq  > capture.ts
ffplay capture.ts

Problem reports, IQ captures and screenshots are welcome, especially with other signals than HamTV. I will resume work on this project in two weeks.

[f5oeo]

Successefull working leandvb receiving SR from 1M downto 250KS without any problem.
CPU on PC is about 20% on a live stream at SR 250K.

Receiving RBDATV from 30MHZ to 1.8GHZ is now possible with just one cheap RTL-SDR usb dongle !
Hope that in a nearr future we can run it on Raspberry Pi which then will be a complete RBTV receiver with embeded MPEG-2/H264 decoder.

Thanks for this great work Pabr.

[g0mjw]

It very nearly works for me but I just can't get it to lock with my RTL-SDR.

[pabr]

If you have a clean signal (ring-shaped I/Q plot as in F5OEO's screenshot), possible problems are:

• Oversampling is too high. Try 2.4 Msamples/s for 2 Msymbols/s, or 1.2 Msamples/s for 1 Msymbols/s, etc. This is a bug; will be fixed in the next version.
• Tuning is off by more than 5% of the symbol rate (e.g. +-100 kHz for 2 Msymbols/s). rtl-sdr dongles with a TCXO are recommended, but some downconverters introduce large errors anyway. You can estimate the offset visually or with a tool called kal-rtl (or kalibrate-rtl), and provide it with "leandvb --tune HZ".
• Modulation is not supported (currently QPSK only).

If you believe the SNR is good but you don't see a ring-shaped I/Q plot, the reason could be:

• Out-of-band interference within the spectrum captured by rtl-sdr. To save CPU, leandvb does not bandpass-filter the signal, therefore nearby transmissions will be picked up. This should be obvious in the top spectrum plot. Again the solution is to reduce the rtl-sdr sampling rate.
• In-band interference. Leandvb does not equalize the signal either. However, it does try to remove birdies (a common problem with rtl-sdr dongles). Ideally the bottom spectrum plot should be free from persistent spikes. By default leandvb tracks only one birdie, but you can start it with "--anf 2" or more. Also, shifting the rtl-sdr tuning frequency by a few kHz sometimes makes the birdies go away.

Thank you for testing !

[pabr]

I have uploaded a new version at http://www.pabr.org/radio/leandvb/leandvb.cc

• Better locking on the IQ recordings that early users have submitted.
• Tested on Raspberry Pi 2 Model B: 500 Ksymbols/s in realtime using only one core (without --gui).
• AGC is disabled by default (hardware AGC of rtl-sdr is enabled by default and should be fine).