• GRCon22 Capture The Flag

    I have spent a great week attending GRCon22 remotely. Besides trying to follow all the talks as usual, I have been participating in the Capture The Flag. I had sent a few challenges for the CTF, and I wanted to have some fun and see what challenges other people had sent. I ended up in 3rd position. In this post I’ll give a walkthrough of the challenges I submitted, and of my solution to some of the other challenges. The material I am presenting here is now in the grcon22-ctf Github repository.

  • Decoding the STEREO-A space weather beacon

    STEREO-A is a solar observation satellite in a heliocentric orbit with a period of 346 days (slightly less than the Earth). It was launched in 2006 together with STEREO-B, which failed 2016. STEREO-A is still operational and producing science data. Whenever the spacecraft is not being tracked by the DSN, its X-band downlink at 8443.530 MHz transmits the so-called space weather beacon. This is a low data rate (~633 bits per second) downlink that contains a summary of the instruments data and that can be received by smaller stations (such as AMSAT-DL’s 20 metre antenna in Bochum, which is one of the stations used to track STEREO-A).

    Yesterday, Wei Mingchuan BG2BHC shared some recordings of STEREO-A done with a 13 metre antenna in Harbin Institute of Technology. A large portion of these recordings contains the space weather beacon signal, but there is another part where the transmission first goes carrier only and then transmits wideband data (although the SNR and the recording bandwidth are not enough to work with this signal). Apparently, STEREO-A was being tracked by DSS-35 in Canberra between 7:25 and 10:30 UTC, more or less at the same time that Wei was recording.

    In this post I analyse the space weather beacon signal in these recordings.

  • Connecting the Pluto SDR to an Android phone

    I have a couple of ideas in mind that involve connecting an ADALM-Pluto SDR to a phone or tablet. Usually, the Pluto is connected to a PC through USB, and the Pluto acts as an Ethernet device, so that network communications between the PC and Pluto are possible. I want to have the same thing running with my Android phone, which is an unrooted Xiaomi Mi 11 Lite (model M2101K9AG, if anyone is curious).

    As usual when trying to do something slightly advanced with Android, this hasn’t worked on the first go, so I’ve spent some time debugging the problem. Long story short, in the end, the only thing I need to make this work is to run

    # fw_setenv usb_ethernet_mode ecm
    # fw_setenv ipaddr 192.168.89.1

    on the Pluto once and reboot (these settings are saved as uBoot environment variables to persistent storage), then enable Ethernet tethering on the phone every time that I connect the Pluto. I can go to the web browser in the phone and check that I can access the Pluto web server at 192.168.89.1.

    ADALM-Pluto web server browsed from Android

    Hopefully the rest of this post will give useful information about how everything works behind the scenes, as your mileage may vary with other Android devices (or if you try with an iOS device, of which I know next to nothing).

    I haven’t seen many people doing this, so the documentation is scarce. PABR did a set up with LeanTRX, the Pluto and an Android phone, but they were running the Pluto in host mode and the Android phone in device mode, and I’m doing the opposite. Note that when you connect a Pluto and phone together, the roles they take will depend on the USB cable. My phone has USB-C, so I’m using a USB-C plug to type-A receptacle cable (USB-C OTG cable) together with the usual USB type-A plug to USB micro-B plug cable (the cable provided with the Pluto). There is also this thread in the ADI forums, but it doesn’t really say anything about Ethernet over USB.

  • More QO-100 orbit determination

    In a previous post, I showed my orbit determination experiments of the GEO satellite Es’hail 2 using the beacons transmitted from Bochum (Germany) through the QO-100 amateur radio transponder on-board this satellite. By measuring the phase difference of the BPSK and 8APSK beacons, which are spaced apart by 245 kHz in the transponder, we can compute the three-way range-rate between the transmitter at Bochum and my receiver in Spain. This data can then be used for orbit determination with GMAT.

    I have continued collection more data for these experiments, so this post is an update on the results.

  • Writing GUPPI files with GNU Radio and using SETI tools

    GUPPI stands for Green Bank Ultimate Pulsar Processing Instrument. The GUPPI raw file format, which was originally used by this instrument for pulsar observations, is now used in many telescopes for radio astronomy and SETI. For instance Breakthrough Listen uses the GUPPI format as part of the processing pipeline, as described in this paper. The Breakthrough Listen blimpy tools work with GUPPI files or with filterbank files (basically, waterfalls) that have been produced from a GUPPI file using rawspec.

    I think that GNU Radio can be a very useful tool for SETI and radio astronomy, as evidenced by the partnership of GNU Radio and SETI Institute. However, the set of tools used in the GNU Radio ecosystem (and by the wider SDR community) and the tools used traditionally by the SETI community are quite different, and even the file formats and some key concepts are unalike. Therefore, interfacing these tools is not trivial.

    During this summer I have been teaching some GNU Radio lessons to the BSRC REU students. As part of these sessions, I made gr-guppi, a GNU Radio out-of-tree module that can write GUPPI files. I thought this could be potentially useful to the students, and it might be a first step in increasing the compatibility between GNU Radio and the SETI tools. (The materials for the sessions of this year are in this repository, and the materials for 2021 are here; these may be useful to someone even without the context of the workshop-like sessions for which they were created).

    In this post I will show how gr-guppi works and what are the key concepts for GUPPI files, as these files store the output of a polyphase filterbank, which many people from the GNU Radio community might not be very familiar with. The goal of the post is to generate a simulated technosignature in GNU Radio (a CW carrier drifting in frequency) and then detect it using turboSETI, which is a tool for detecting narrowband signals with a Doppler drift.

    Before going on, it is convenient to mention that an alternative to this approach is using gr-turboseti, which wraps up turboSETI as a GNU Radio block. This was Yiwei Chai‘s REU project at the ATA in 2021.


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