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1. Containerized APRS IGate with RTL-SDR Dongle

   One of the lessons learned from Launch 3 was we needed way to verify all payloads were working before releasing the balloon. For an APRS transmitter, this means getting the packets to the APRS Internet Service, where packets can then be viewed on many different websites such as aprs.fi.

   Our usual launch site is too far away from an APRS IGate, so we had no way to verify that Kazu's RP2040 APRS transmitter was working for Launch 3. This APRS transmitter only puts out around 5 mW (~6 dBm), which is not enough power to make it through the trees and buildings to the nearest IGate. Unfortunately, we never heard from it on the flight.

   Previous Work

   There are several projects for running an APRS IGate on a Raspberry Pi with a RTL-SDR dongle. However, all of these projects use the entire Pi and/or recommend the desktop GUI version of Raspberry Pi OS. Downloading some random ISO to flash on an SD card didn't seem too appealing to me either, what else is installed?

   Since I'm a big ...

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2. SF-HAB High-Altitude Balloon Launch #4

   The SF-HAB group got together on Sunday 8 Feb 2025 and flew a bursting high-altitude balloon. This was a quick follow-on to Launch 3, which happened just before Christmas 2024.

   Preflight Planning Meeting

   The team got together on Zoom before the launch, to discuss what payloads we wanted to fly and find available dates. We targeted a Jan 25th launch. However, the weather wasn't looking good, and also that weekend was Winter Field Day, so it was decided to push the launch back by a two weeks.

   The week before the launch I kept running flight predictions for Sat Feb 8th. The early week predictions weren't good, but by Thursday the upper-atmosphere winds had slowed considerably and the predictions showed the balloon landing about 12 miles east of Stockton. Not great, but not bad either. We decided to launch.

   

   We flew seven payloads on this flight. From the balloon (top) end:

   • Parachute
   • Two 900 MHz Meshtastic Nodes
   • 431.05 MHz Reprogrammed Vaisala RS41 Radiosonde
   • 433.775 MHz LoRa Tracker
   • 144.39 MHz APRS Tracker
   • GoPro Camera
   • VHF/UHF Cross-band Repeater ...

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3. Mobile Horus Binary v2 and APRS Receiving Station with Raspberry Pi

   After the previous December HAB launch, I realized that I needed to spend some time on my receive hardware setup in the car. I previously had a VirtualBox VM running Horus-GUI and headless horusdemodlib, but it was very much a kludge. As an upgrade, I wanted to add APRS reception for the balloon 2 meter APRS tracker.

   In order to keep track of all the hardware required for this, I created a block diagram using draw.io desktop. Click to make bigger.

   

   The next sections will describe the different parts of this setup.

   RF Filtering

   I quickly realized that having two RTL-SDR dongles in close proximity to a 10 or 50 watt transmitter might cause problems. It would certainly desense the receivers during transmission, but it also might cause permanent damage to the dongles. Permanent damage is hard to diagnose because there's no error message saying "broken," it just doesn't receive weak signals (or any signals at all).

   Filtering the 441 MHz voice transmission out of the 144.39 MHz APRS receiver is pretty easy, as those frequencies are very ...

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4. Building a UHF Band-Stop Stub Filter

   During a recent balloon launch, I communicated with the other chase teams with my mobile rig, transmitting around 441 MHz with a transmit power of 10 watts = +40 dBm.

   For the next balloon launch, I also wanted to put a 144 MHz APRS receiver on my car, but was worried about damaging the RTL-SDR dongle I would be using for receiving. RTL-SDR dongles have a maximum input of around +10 dBm with absolutely no filtering on the input, so I could potentially permanently damage the RTL-SDR dongle when I transmitted.

   Moving and separating the antennas around on the car roof and trunk might give 20 dB of isolation, but we're still at risk of permanently damaging the receiver.

   I needed a band-stop or band-pass filter for my APRS receiver. I already had a band-pass cavity filter for 144-148 MHz, but the 3rd harmonic of 145 MHz is 432 MHz, so this filter won't really keep 441 MHz transmissions out of my 144 MHz receiver. Bummer.

   Open and Shorted Stubs

   Fundamentally, stub filters are based around the concept of constructive and ...

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5. Sondehub Grafana Graphs for December 2024 High-Altitude Balloon Launch

   In December 2024, the SF-HAB group flew a high-altitude balloon from the San Francisco Bay Area. The flight was a partial success, with the payload recovered the next day. Only several weeks later did I find out about this excellent Sondehub Grafana instance, hosted on AWS and directly connected to the Sondehub Amateur database.

   In this post, I want to dive a bit deeper into the Horus Binary v2 telemetry that was transmitted from a reprogrammed RS41 radiosonde on this flight. This entire post is commentary and screenshots from this grafana dashboard, screenshotted here to reduce link rot. All times are in UTC on Dec 15th 2024.

   Balloon Position Graphs

   Due to the under-inflated balloon, we knew it was going to rise slower than predicted, and burst at a higher altitude. This balloon reached a maximum of 37,412 meters (~122.7k feet) at 21:09:57.

   

   This is a graph of the ascent and descent rates for the whole flight. The blue line is what the GPS reports (in meters/sec, left axis), and the yellow line is calculated ...

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6. December 2024 High-Altitude Balloon Launch

   On Sunday 15 December 2024 the SF-HAB group got together and flew a bursting high-altitude balloon. We had a whole crew there, including Martin W6MRR, Kazu AG6NS, Robert K6RGG, John NI6D, Walter K6ATV, Benjamin KO6CNT, and Steve K6WW. Everyone helped out with assembling the payload train, filling the balloon, or documenting the launch.

   Preflight Planning

   The days before the flight we had an atmospheric river in the Pacific Northwest, so the upper winds were very unsettled. The lower-level winds were also blowing all different directions, and it rained significantly the day before. But the weather forecast for launch day was clear and sunny, and the Sondehub predictions showed a general Southeast flight. We picked Walnut Creek as the launch location to have our balloon land on the eastern side of the Central Valley

   

   Balloon Assembly and Filling

   The launch site was at a local park in Walnut Creek, and we met at 9:30am on a very cool and cloudy day. There was no wind at all. The balloon was a standard Kaymont 1500 gram filled with hydrogen.

   

   Unfortunately, during ...

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7. Reprogramming a RS41 Radiosonde for Amateur Radio Frequencies

   I decided to re-fly a Vaisala RS41 radiosonde on an upcoming SF-HAB high-altitude balloon launch. The radiosonde must be reprogrammed on amateur radio frequencies, and I decided to use the RS41ng project by Mikael Nousiainen OH3BHX. This post is a companion post to setting up a Horus Binary receiving station.

   Radiosonde Hardware

   The radiosonde I randomly pulled out of my box was V1920305, launched over a year ago from Oakland on 8 August 2023. I picked it up the next morning from a construction zone in South San Francisco, and the construction guys just gave it to me after I asked about a balloon.

   

   Building the firmware

   On Linux, building the RS41ng firmware is ridiculously easy. You build a local docker container with the build environment, which takes up about 2.2 GBytes of storage (!?). After editing the configuration file, one command builds the firmware inside the docker container.

   Here are the commands I ran, stolen from the detailed linux docker procedures:

   ~$ git clone https://github.com/mikaelnousiainen/RS41ng
   ~$ cd RS41ng
   ~/RS41ng$ vim src/config.h                                                        (edit with your configuration ...

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8. Setting Up a Linux Horus Binary Receiving Station

   The SF-HAB group is talking about doing a High-Altitude Balloon launch in the next few weeks, and asked if I had any payloads to fly. I thought it would be fun to refly all these radiosondes that I have collected.

   Looking around a projects online for reprogramming radiosondes, I came across the the great work that Mark Jessop VK5QI was doing down in Australia. Mark and David Rowe created the Horus Binary protocol, which is a low-power 100 baud 4FSK modulation scheme specifically designed for high-altitude balloons. Mikael Nousiainen OH3BHX wrote the RS41ng project which implements the Horus Binary transmitter on a regular Vaisala RS41 radiosonde.

   The next blog post will focus on reprogramming the RS41 radiosonde.

   Hardware Setup

   The hardware receiving the Horus is pretty much the same as receiving a radiosonde or AIS, except it's a different frequency so a different antenna must be used. While I list a Raspberry Pi 4 on the block diagram (running Debian 12 Bookworm), any Debian-based distribution can be used.

   

   The Mt. Carmel High School Amateur Radio Club (W6SUN) down in San ...

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9. Keflavik, Iceland Radiosonde Receiver

   I spent a few weeks in Iceland in May 2024, working near the main airport in Keflavik. The area was a US Air Force/NATO base from 1951 until 2006, when it was turned over to the Icelandic government. The US Air Force returned in 2016.

   While I was there, I installed a radiosonde_auto_rx station. Iceland has one radiosonde launching site located at the Keflavik airport, and launches two radiosondes a day at 1100 and 2300 UTC. Skew-T plots are published by the Iceland Met Office after every launch.

   Hardware

   The hardware for this receiving site is the standard radiosonde receiving hardware. A Rasbperry Pi 3B+ and a RTL-SDR Blog v3 are the main components, along with an external antenna. All of the hardware sits on a rack shelf in the building.

   

   

   Antenna

   The air in Iceland is very corrosive from the volcanoes and salt spray. Any metal that isn't stainless rusts almost immediately, and even lower-grade stainless rusts after a few years. Since the receive antenna would be placed outdoors, my homemade quarter-wave ground plane antenna would be a ...

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