High Altitude Balloon Tracker WSPR APRS

Solar powered RP2040 based circuit for transmitting 20 meter and VHF telemetry while circumnavigating the globe at ~50,000'

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Development of a tracker for Amateur Radio balloon flights. This has been going on for some time primarily with AVR based chips, and we are now launching RP2040 based hardware with functional 20 meter WSPR capability. APRS transmission in the VHF band are in development and coming soon.

Our current test flight AG6NS-11 was launched Saturday morning from Hayward California. Live tracking (updates when it's in full sunlight):!mt=roadmap&z=11&call=a%2FAG6NS-11&timerange=604800&tail=604800

Raspberry Pi RP4040 based HF/VHF GPS tracker Concept:

  • this board is intended for educational, STEM and research purpose
  • RP2040 based, same pin assignment as Raspberry Pi Pico
  • support both WSPR (HF) and APRS (VHF) on Si5351
  • user can modify / extend the functionalities in Python (MicroPython)
  • only using components that are available (and in stock) in JLCPCB parts library (most of them are "Basic Parts")
  • use JLCPCB's "Economic PCB Assembly Service" for assembly (no need to deal with tiny SMD components)
  • once we will confirm that this tracker is working during actual flights, we will publish EVERYTHING (EasyEDA project file including schematic and PCB layout, gerber and BOM files for JLCPCB order, all the firmware source code) under GPL (firmware) and Creative Commons BY-SA-NC (hardware) license

Vital Details:

  • RP2040, 12MHz XO, 16Mbit QSPI NOR
  • GPS, Temperature & Pressure sensor
  • Si5351 based VFO with 26MHz TCXO
  • 2 LPFs for 2 different bands (all L and C for LPF are 0805 size SMD for hand soldering)
  • 3.3V LDO (not the buck-boost one used in RaspPi Pico)
  • LED
  • FET switch for GPS power
  • FET switch for Si5351
  • Solar Cell voltage monitoring
  • GPS 1PPS signal width measurement
  • Si5351 clock frequency measurement
  • all the unused pins are available at header pins (9 GPIOs and 3 ADCs)
  • UART0 for logging and CLI
  • SWD for debugging
  • USB for power, serial for CLI and flashing firmware
  • USB connector portion of PCB can be detached for launch
  • weight is 3.00g (not including any components for LPF) and size is 48mm x 29mm (after detached the USB connector portion of PCB)

Detailed Details:

The RP2040 is a dual ARM Cortex-M0+ running at 133MHz but can slow down to save power.  The two cores are partitioned so one core is devoted to operating the tracker while the other is free for other purposes, like student experiments.   

The tracker core firmware is written in C and assembler.   It will send GPS position report and other data over VHF/APRS and HF/WSPR. The other core runs MicroPython.   An API will provide the MicroPython environment with balloon information: GPS status, location, etc.  Only a small set of the RP2040 user GPIO is needed for the tracker and the remainder is available for MicroPython programs.   That allows for communications with a wide variety of sensors and actuators.

The tracker is small and lightweight, and is aimed at pico/high altitude ballooning.  They would well for other platforms like the San Diego Mount Carmel High School Amateur Radio Club's (MCHSARC) amateur radio amateur radio marine buoy marine buoy.

The tracker generates a signal by modulating the Si5351 rapidly.  It can produce VHF APRS AFSK 1200 BPS RF directly as well as HF WSPR < 0.5 BPS RF.  Other modes have been demonstrated with this technique, like SSTV, but the tracker will only support APRS and WSPR for initial release.   Other modes like FT8, more, RTTY and voice are of interest for reporting data.   Other modes are being explored for sending more detailed telemetry.


Tracker log from pre-launch testing through 7am Pacific Time April 25th 2023

plain - 600.00 kB - 04/25/2023 at 14:10



V 0.2 Tracker

Portable Network Graphics (PNG) - 13.17 MB - 04/25/2023 at 02:32



New AG6NS V0.1 RP2040 tracker/old W6MRR AVR tracker

JPEG Image - 126.33 kB - 04/25/2023 at 01:55


  • Good Morning Hackaday Prize 2023!

    xBeau04/25/2023 at 08:35 0 comments

    As the sun rises on the West Coast, AG6NS-11 nears Solar Noon two-thirds of the way through it's Transatlantic Crossing at the start of it's 4th day of being a High Altitude Balloon.

    The 20 meter WSPR transmission, only being powered by a small solar panel, is being heard from Alaska to Australia!

  • Up up and away, a new beacon rises!

    xBeau04/25/2023 at 07:10 0 comments

    Plans were made to launch the first RP2040 based 20 meter WSPR beacon transmitter for Saturday morning 8am on April 22nd 2023. The test flight of Kazu's latest design given the call sign designation AG6NS-11. We had chosen the primary launch site in Hayward California where we had previously done our first latex floater launch

    Freshly flashed board, it blinks, ship it! Additional components to come for APRS.

    • 07:59 soldering antenna to balloon payload (in the trunk)
    • 08:09 positioning for launch in the park
    • 08:10 power test with solar panel angled directly to sun
    • 08:11 un-spooling/de-tangling antenna wires
    • 08:12 initial float of balloon
    • 08:14 verifying WSPR signal transmission
    • 08:17 launch attempt 1
    • 08:20 reposition for launch
    • 08:21 launch attempt 2
    • 08:23 launch attempt 3 (success)
    • 09:12 first WSPR spot reported from CM87wp (S.W. of launch)
    • 11:36 new grid square spot CM86xw (off the coast of Santa Cruz)
    • 16:22 last spot for the day CM95ws (between Monterey & Bakersfield)
    • 21:25-21:46 moonlight ping? DM05co

    Tracking the first day of flight via relayed from and

    K6RGG listening to whispers, tuned to the 20 meter band.

  • GPS Exchange @ Electronics Flea Market

    xBeau04/25/2023 at 07:10 0 comments

    Kazu started assembly of boards and was preparing for a launch. There was just one critical component missing, a GNSS/GPS receiver. I had a stash of these still on hand from W6MRR's AVR based boards and we made plans for a rendezvous at the return of the Electronics Flea Market

    This event is a long standing Silicon Valley tradition that has mostly been on hiatus for the last several years. If you've never found yourself in an empty parking lot well before dawn, as HAMs from all over come out of the woodwork to bond over coffee, donuts and the age old question of how many more of these things that I don't particularly need (right now) do I actually absolutely have to have? Kazu debated this with me for a few in our particular case of modules. I think they have 5 or 6 boards to build, and settled on 7 or 8 modules as a good number to have on hand.

    This is the log jam of sellers still being processed at dawn as things started to get going around 6am in the main lot. Some people were already there at 4am, ahhhem, so I hear.

    One of the many bags of mystery to be found, pro-tip, the good stuff is often at the bottom. As it turns out I have a feeling will be seeing a lot more of these.

    Space grade high quality solar panels manufactured in Japan. Likely destined for use on a commercial satellite, and never even opened. They have been carefully looked after and stored for a weeee while, aged like a fine spirit. Among the undoubtedly long and prestigious chain of custody holders was Anchor Electronics the last standing brick and mortar provider of fine vintage, new and weird components.

    I got a look at some of Kazu's latest handy work, a v0.2 tracker that's been under test.

    After a thorough trip up and down and all around the fine wares on offer, it was time to call it a day and explore the wonders that I had acquired from a WWII era "Pipsqueak" to a doughnut shaped solar panel array. My pocket was a few grams lighter of cash and GPS modules bound for the sky, and replaced with pounds of new to me gear.

  • January 2023 Meeting Recap

    xBeau04/25/2023 at 06:03 0 comments

    We had an excellent gathering of exceptional Amateur Radio Operators talking about various projects from Martin's thoughts on elevation control, to current assembly and testing of AVR based PCB's and experiments with new solar panel configurations.

    General Agenda

    • New Tracker/Kazu
    • Solar Array/David, Beau, Rikke
    • Manufacturing Trackers/Beau
    • Controllable Altitude Balloon/Martin, David
    • Lightweight Crossband Repeater/Martin
    • Chabot Space and Science Center/Martin
    • Calaveras County Public Schools/David
    • Drone Rules Explained/Robert

    Continuing on from our first experimental floater launch last year, the focus shifts to 2023 and the development of a totally new design! This was posted earlier at

    Kazu AG6NS and W6MRR have been working on a couple of trackers. One is a dual band 2/20 m APRS/WSPR with other modes coming. It is based on an ATMega1284 processor, which is a 328 (used currently) with more memory and and running at 10 MHz, the safe limit for 3.3 V operation. Kazu has come up with code for it and a revised PCB is being worked on. The other is a Raspberry Pi Pico based tracker. This is a longer range project. It will use the same Si5351 based modulator and filters for 2/20 m. The Picos are attractive as they have a dual core ARM Cortex-M0+ @133MHz which is far more powerful than the ATMegas. The RP2040 chip has instructions for DIYing a board. However, the Raspberry Pi Pico as is, was the easiest path to start working on a design. Due to the chip shortage, it may end up using through hole components for the output filters.

    We are looking at a lot of fun things that can be done – SSTV data and cam, voice ID over VHF, crossband repeaters, and latex floaters.

    The ongoing work and interest continues with Kazu's coding and PCB development to use a RP2040.

    While this is the start of a new project, you might have caught wind of some of our balloons at the last Supercon

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