Guide
How WSPR balloon trackers encode telemetry, how protocols differ, and what to expect from a high-altitude float mission.
Types of Trackers
ZachTek
The simplest and most common protocol — standard WSPR2 with altitude encoded in power fields. Most DIY community trackers use this approach.
U4B
Advanced protocol with rich telemetry packed into the WSPR callsign and locator fields. Traquito is the most well-known tracker based on this protocol.
Simplest tracker · No channels · Altitude in power fields
ZachTek is named after the ZachTek WSPR-TX Pico, a popular commercial transmitter kit. It is by far the most widely used protocol in the DIY balloon tracking community — practically every home-built tracker you encounter uses this approach.
Technically, ZachTek does not define a new radio protocol. It reuses WSPR2 — a standard weak-signal propagation mode that has nothing to do with balloons and was never designed for them. What ZachTek actually defines is a single convention: how to encode altitude into the WSPR power fields. Everything else — the two-minute slots, the callsign, the locator — is plain unmodified WSPR2.
What ZachTek adds to WSPR2
All other fields (callsign, locator, timing) are standard WSPR2 — no modifications.
Two-frame structure
Altitude formula
WSPR power levels are discrete (0, 3, 7, 10 … 57, 60 dBm). The smallest possible step in the fine component is 3 dBm, giving a resolution of 3 × 20 = 60 m. When both fields report 60 dBm, the altitude is considered invalid.
Rich telemetry · Channel-based · No suffix
U4B is a dedicated WSPR balloon telemetry protocol that packs a rich set of data into the standard WSPR callsign and locator fields. The most widely known tracker implementing it is Traquito.
Unlike ZachTek, U4B encodes far more than just altitude. All data is packed into the character values of the callsign and locator fields through multi-step arithmetic — the result looks like a normal WSPR transmission but the callsign in Frame 2 is not a real amateur call.
Position
6-char locator
Altitude
~20 m res.
Temperature
°C
Battery
Voltage (V)
GPS
Lock + sat count
Speed
km/h horiz.
Two-frame structure
Channel system
U4B uses a channel system to prevent collisions between multiple balloons operating on the same band. Each channel defines a specific pair of WSPR time slots and an encoded callsign prefix for Frame 2.
/6). No suffix can be carried in the encoded Frame 2.Balloon Floating
Physics, payload weight, precision & luck
Unlike weather sondes that ascend and burst, ZachTek and Traquito balloons are designed to float — finding an equilibrium altitude and drifting with upper-atmosphere winds for days, weeks, or even months.
Typical flight timeline
Ascent rate is typically ~1 m/s regardless of free lift. Survival probability rises sharply after the first 72 hours.
Diagram is illustrative only and does not reflect actual flight data.
The biggest failure point: catching float
The most critical moment of any flight is catching float — the phase during ascent when the balloon slows down and settles into its equilibrium altitude. If the envelope has any weakness, a micro-leak, or was slightly overfilled, this is usually when it reveals itself. The transition from ascent to float puts significant stress on the latex, and if it's going to fail, it most often happens right here.
A slow leak at float means the balloon will gradually lose lift and start descending — sometimes so slowly it's barely noticeable on the tracking map. During the day the sun heats the gas, expanding it and generating extra lift that masks the leak — the balloon can still appear to hold altitude. Then night comes: no sun, the gas cools and contracts, the lift it provides shrinks below the payload weight, and the balloon descends slowly. Sometimes, if it lands in an open area, the tracker will continue transmitting WSPR from the ground — a fortunate ending that at least confirms where it came down.
Why do balloons come down?
The overwhelming majority of descents — ~99.9% of cases — come down to one thing: clouds. Not equipment failure, not bad luck, just clouds. High-altitude cirrus and similar formations routinely reach 14, 15, even 16 km, and a balloon drifting at float altitude will eventually fly straight through one. Ice crystals and moisture coat the envelope, adding weight and disrupting the delicate lift balance until the balloon can no longer stay up.
A common question is whether aircraft pose a risk. They don't — commercial planes cruise at 10–12 km, well below typical float altitude, and military or business jets rarely climb higher. At those altitudes a balloon is statistically far more likely to encounter a cloud than any aircraft. And even in the unlikely scenario where paths cross, the jet would never actually touch the balloon — the wake turbulence alone would scatter it long before any physical contact.
Only after clouds comes everything else: equipment failures (solar panel delamination, cold-induced battery cutoff, solder joint fracture from thermal cycling), envelope degradation from UV exposure slowly embrittling the latex, and gradual micro-leaks. These are real failure modes, but they are secondary — if a balloon avoids clouds long enough, it can fly for weeks.
Surviving the first days
The most dangerous period is the first 24–72 hours. The envelope faces intense UV radiation, repeated temperature cycling (scorching heat in sunlight, extreme cold in shadow), and the pressure stress of the initial float equilibrium settling. Most failures happen here. Once a balloon survives this phase, it becomes significantly more stable and can continue flying indefinitely.
Payload weight matters
Total payload weight (tracker + battery + antenna + balloon envelope) is one of the most critical variables. A payload of around ~10 g typically floats at approximately ~14 km altitude. Lighter means higher and more stable. A single gram can shift the float ceiling by several kilometers.
A precision hobby
Float ballooning is genuinely precise work. There is no room for mistakes in the areas you can control — a wrong helium fill ratio, a gram too much weight, a poor antenna solder joint, or even the wrong knot can end a flight before it properly begins. Every detail matters and shortcuts tend to have consequences at altitude.
Luck plays a big role
And yet — balloon flying has a humbling habit of rewarding beginners and punishing veterans in equal measure. The clumsiest launcher can stumble into a record-breaking float while the most meticulous builder watches their craft descend in the first hour. Once the technical basics are covered, luck becomes the dominant factor. Skill sets the floor, fortune determines the ceiling.
Other Hardware
Weather balloon sondes repurposed for amateur use
Radiosondes such as the Meteomodem M20 and Vaisala RS41 are mass-produced weather balloon instruments that are regularly recovered by hobbyists after official flights. Their onboard GPS, microcontroller, and radio hardware make them an attractive starting point — and people do launch them. Theoretically, under the right conditions, a radiosonde could even achieve a float.
In practice, however, radiosondes are simply too heavy for a meaningful float mission. They were designed to ascend, transmit once, and be discarded — not to float for weeks at altitude. The full assembled unit is far outside the weight budget that makes low-altitude float viable (~10 g total payload).
Better approach
If you have recovered radiosondes, the most practical use is to harvest their PCB components — the GPS module, oscillator, or MCU — and integrate them into a purpose-built lightweight tracker. Launching a radiosonde as-is for a float attempt is rarely worth it when dedicated trackers like ZachTek Pico or Traquito exist specifically for this purpose and weigh a fraction of the mass.
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The content here is work in progress and may be incomplete or inaccurate. If you have suggestions, corrections, or anything you'd like to see added, feel free to reach out.