Amateur radio – Page 5 – Daniel Estévez

Anatomy of Blockstream Satellite

This is another post about GRCon22’s Capture The Flag (see my previous post). One of the challenges in the Dune track submitted by muad’dib was called Heighliner. It consisted of a short recording of Blockstream Satellite, as we might guess from the challenge description below, especially if we had watched Igor Freire‘s talk about gr-dvbs2rx and Blockstream Satellite (I’ve heard that the fact that the talk and the challenge had the same topic was just a coincidence).

A heighliner just passed through “folded space” and it has sent a secret message to the remaining members of House Atreides on the surface of Arrakis. The communication protocol was historically used for sending visual propaganda films and archival files, recently however, Duke Leto had his engineering guild repurpose the transmission unit for financial transactions. It’s the perfect place for a covert message, the Harkonnens would never think to look there… The original transmission was on Frequency 12.0164GHz. Our groundstation receiver downconverted to 1.2664GHz.
Heighliner challenge description

I didn’t manage to solve this challenge, mainly because I was looking in the wrong place. I was focused on looking at the Bitcoin blockchain chunks, but the flag was in a Blockstream Satellite API message, and I wasn’t aware of the existence of API messages back then. After the CTF ended, a few of us were discussing this challenge in the chat. None of us really understood all the details about how the Blockstream Satellite system works. Since the intended way of solving the challenge was setting up and running the Blockstream Satellite receiver tools, an in-depth understanding wasn’t really necessary.

I have some interest in satellite filecasting systems since I reverse-engineered Outernet back in 2016, so I’ve been taking some time after the CTF to look at the details of how Blockstream Satellite works. While attempting to solve the challenge, I found that detailed enough documentation wasn’t available. There is some high-level documentation, but for the details you need to go to the source code (which is a typical situation).

In this post I describe the details of how Blockstream Satellite works, using the recording from the CTF challenge as an example. I will mainly focus on the Blockstream Satellite API, since I haven’t been able to understand all the details of the Bitcoin blockchain FEC blocks.

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.

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.

QO-100 orbit determination

In a previous post, I showed my experiment about measuring the phase difference of the 8APSK and BPSK beacons of the QO-100 NB transponder. The main goal of this experiment was to use this data to do orbit determination with GMAT. Over the last week I have continued these experiments and already have started to perform some orbit determination in GMAT.

Here I give an update about several aspects of the experiment, and show how I am setting up the orbit determination.

Calculating the QO-100 beacons frequency separation

In my previous post I set out to measure the phase difference between the QO-100 8APSK and BPSK beacons. One of the things I mentioned is that the frequency separation between these two beacons was approximately 1.6 Hz larger than the nominal 245 kHz.

A frequency error of a couple of Hz is typical when working with SDRs unless special care is taken. Many SDRs allow choosing the sample rate and centre frequency with great flexibility, but the drawback is that the frequencies that are achieved are often not exactly the ones we indicated. Fractional-N synthesis PLLs are used to generate the sampling clock and local oscillator, so there are small rounding errors in the generated frequencies.

With enough knowledge of how the SDR hardware works and how it is configured, it is possible to determine these frequency errors exactly, as a rational number \(p/q\) that we can calculate explicitly, multiplied by the reference frequency of the SDR. Then we can use this exact value to correct our measurements.

I have asked Mario Lorenz DL5MLO and Kurt Moraw DJ0ABR the details of how the beacons are generated in the Bochum groundstation. Two ADALM Pluto‘s are used: one generates the CW and BPSK beacons, and the other generates the 8APSK multimedia beacon. With the data they have given me, I have been able to compute the frequency separation of the 8APSK and BPSK beacons exactly, and the result matches well my experimental observations.

In this post we will look at how the fractional-N synthesis calculations for the Pluto can be done. Since the Pluto uses an AD9363 RFIC, these calculations are applicable to any product using one of the chips from the AD936x family, and to the FMCOMMS3 evaluation board.

Measuring the QO-100 beacons phase difference

Since a couple months ago, the QO-100 NB transponder has now two digital beacons being transmitted continuously: the “traditional” 400 baud BPSK beacon, and the new 2.4 kbaud 8APSK multimedia beacon. This transponder is an amateur radio bent-pipe linear transponder on board the Es’hail 2 GEO satellite. It has an uplink at 2400.25 MHz, a downlink at 10489.75 MHz, and 500 kHz bandwidth. The two beacons are transmitted from the AMSAT-DL groundstation in Bochum, Germany, with a nominal frequency separation of 245 kHz.

In some posts in the last few years (see this, for instance), I have been measuring the frequency of the BPSK beacon as received by my grounstation in Madrid, Spain. In these frequency measurements we can see the daily Doppler curve of the satellite, which is not completely stationary with respect to the surface of Earth. However, we can also see the frequency variations of the local oscillator of the transponder (including some weird effects called “the wiggles“). For this reason, the frequency of the BPSK beacon is not an ideal measurement for orbit determination, since it is contaminated by the onboard local oscillator.

If we measure the frequency (or phase) of the 8APSK and BPSK beacons and subtract the two measurements, the effects caused by the transponder local oscillator cancel out. The two beacons have slightly different Doppler, because they are not at the same frequency. The quantity that remains after the subtraction is only affected by the movement of the satellite.

Bochum and my station use both references locked to GPS. Therefore, the phase difference of the two beacons gives the group delay from Bochum through the transponder to my station. This indicates the propagation time of the signal, which is often known as three-way range. The three-way range is roughly the sum of distances between the satellite and each groundstation (roughly, but not exactly, due to the light-time delay). It is a quantity that is directly applicable in orbit determination.

In this post I present my first results measuring the phase difference of the beacons and the three-way range.

Trying to observe the Vega-C MEO cubesats

On July 13, the Vega-C maiden flight delivered the LARES-2 passive laser reflector satellite and the following six cubesats to a 5900 km MEO orbit: AstroBio Cubesat, Greencube, ALPHA, Trisat-R, MTCube-2, and CELESTA. This is the first time that cubesats have been put in a MEO orbit (see slide 8 in this presentation). The six cubesats are very similar to those launched in LEO orbits, and use the 435 MHz amateur satellite band for their telemetry downlink (although ALPHA and Trisat-R have been declined IARU coordination, since IARU considers that these missions do not meet the definition of the amateur satellite service).

Communications from this MEO orbit are challenging for small satellites because the slant range compared to a 500 km LEO orbit is about 10 times larger at the closest point of the orbit and 4 times larger near the horizon, giving path losses which are 20 to 12 dB higher than in LEO.

I wanted to try to observe these satellites with my small station: a 7 element UHF yagi from Arrow antennas in a noisy urban location. The nice thing about this MEO orbit is that the passes last some 50 minutes, instead of the 10 to 12 minutes of a LEO pass. This means that I could set the antenna on a tripod and move it infrequently.

As part of the observation, I wanted to perform an absolute power calibration of my SDR (a USRP B205mini) in order to be able to measure the noise power at my location and also the power of the satellite signals power, if I was able to detect them.

Real time Doppler correction with GNU Radio

Satellite RF signals are shifted in frequency proportionally to the line-of-sight velocity between the satellite and groundstation, due to the Doppler effect. The Doppler frequency depends on time, on the location of the groundstation, and on the orbit of the satellite, as well as on the carrier frequency. In satellite communications, it is common to correct for the Doppler present in the downlink signals before processing them. It is also common to correct for the uplink Doppler before transmitting an uplink signal, so that the satellite receiver sees a constant frequency.

For Earth satellites, these kinds of corrections can be done in GNU Radio using the gr-gpredict-doppler out-of-tree module and Gpredict (see this old post). In this method, Gpredict calculates the current Doppler frequency and sends it to gr-gpredict-doppler, which updates a variable in the GNU Radio flowgraph that controls the Doppler correction (for instance by changing the frequency of a Frequency Xlating FIR Filter or Signal Source).

I’m more interested in non Earth orbiting satellites, for which Gpredict, which uses TLEs, doesn’t work. I want to perform Doppler correction using data from NASA HORIZONS or computed with GMAT. To do this, I have added a new Doppler Correction C++ block to gr-satellites. This block reads a text file that lists Doppler frequency versus time, and uses that to perform the Doppler correction. In this post, I describe how the block works.

LTE downlink: PBCH and PDCCH

This post is a continuation of my series about LTE signal analysis. In the previous post I showed how to decode the PHICH. Now we will decode two other downlink channels, the PBCH (physical broadcast channel) and the PDDCH (physical downlink control channel).

The PBCH is used to transmit the MIB (master information block). This is a small data packet that all the UEs must decode after detecting a cell using the synchronization signals. The MIB contains essential information for the usage of the cell, such as the cell bandwidth and PHICH configuration. The PDDCH contains control information, such as uplink grants and the scheduling of the PDSCH (physical downlink shared channel).

The PBCH and PDDCH use the same kind of channel coding: a tail-biting k=7, r=1/3 convolutional code with a circular buffer for rate matching that performs puncturing and repetition coding as needed to obtain the required codeword size. The remaining aspects of the PBCH and PDDCH are quite different, so they will be treated separately.

As usual, we will be using a short IQ recording from my local cell site. The link to the recording is given at the end of the post.