Making tracking of the high-altitude balloon flight OH3HAB 7 a community effort. Watch the epic landing in a river!

The high-altitude balloon flight OH3HAB #7, launched on September 2nd 2023 by amateur radio club OH3AA in Hämeenlinna, Finland, was a special flight with two new technical experiments: a RadSens radiation sensor based on a Geiger tube and an image transmitter using the Wenet modulation. The flight payloads included a Vaisala RS41 radiosonde that was repurposed for amateur radio use and a Raspberry Pi Zero W with a Uputronics LoRa board to transmit photographs. With the transmissions of the position and image data available for anyone to receive easily, tracking of the flight became a true community effort!

This article is also available in Finnish on the OH3AA website.

Tämä artikkeli on luettavissa myös suomeksi Hämeenlinnan Radioamatöörit ry:n nettisivuilla.

Videos from the flight and of the flight preparations

An overview of the flight launch, balloon burst and landing. Watch the epic landing in a river and recovery of the flight payloads using a boat:

Flight launch preparations (filmed by Martti OH1ON), audio in Finnish only:

Unedited videos from the flight

Flight launch (3 minutes):

Balloon burst (3 minutes):

Landing in a river (3 minutes):

Raw video of the full flight (2.5 hours):

Flight transmitters: making tracking of the flight a community effort

The idea behind choosing the flight transmitters was to make tracking of the flight a community effort, where anyone — with suitable, cheap radio receiver hardware — could participate in tracking the flight. This time we also included a transmitter for photographs, as while it is great to be able to track the flight of a high-altitude balloon on a map, it is even more interesting to be able to see photos taken from the balloon — in real time, during the flight!

In addition to transmitting photos from the flight, we included a radiation sensor in the flight as an experiment to measure the level of ionizing radiation at high altitudes. Measuring radiation intensity on a high-altitude balloon flight is particularly interesting, because the intensity is considerably higher at high altitudes than at ground level where the atmosphere absorbs much of the radiation. The sensor readings could be monitored by anyone tracking the flight.

The transmitter devices were a Vaisala RS41 radiosonde transmitting the balloon GPS position and sensor telemetry data, and a special image transmitter sending photographs taken during the flight.

An important detail regarding the way the receiver software for both the position data and the images work is that they send the received data to centralized services, which then make the data available on the Internet for anyone to view.

These centralized services are:

We wrote down detailed instructions on how to set up a receiver station for each type of transmission and also provided links to the different websites that could be used to follow the flight even without a radio receiver. We advertised the flight on several different social media platforms, mailing lists and instant messaging groups, so that as many people as possible would know about the flight and could participate in tracking it.

In the end we had 25 Horus 4FSK stations in Finland tracking the balloon position, which is a great achievement, and a couple of Wenet image receiver stations! There were also some APRS stations tracking the flight.

Vaisala RS41 radiosonde with a radiation sensor

The flight used a Vaisala RS41 radiosonde again as the main flight tracker. The radiosonde was modified to transmit on the amateur radio 70cm band (432.500 MHz) and to send both Horus 4FSK and APRS packets with the GPS position and telemetry data from the sensors. The Horus transmission can be received by anyone with an SSB receiver for the 70cm band and a computer running the Horus GUI software!

RS41 radiosondes are reusable and launched daily by meteorological institutes around the world (FMI in Finland), which makes them great for tracking amateur radio high-altitude balloon flights. Read more about repurposing radiosondes for amateur radio high-altitude balloon tracking.

The RS41 radiosonde was equipped with a Geiger tube-based sensor for ionizing radiation, called RadSens, and a Bosch BME280 temperature/humidity/pressure sensor. Both of the sensors were connected to the I²C bus of the radiosonde.

See the results from the radiation sensor measurements near the end of this article!

Vaisala RS41 radiosonde with the BME280 and RadSens sensors
RadSens radiation sensor with the Geiger tube

The RS41ng open-source amateur radio firmware for the radiosonde was adapted to support the additional sensors. For more information on how to reuse radiosonde hardware and how to track and to find them, please read the RS41ng firmware documentation.

Reprogramming the RS41 radiosonde with the RS41ng open-source firmware using an ST-LINK v2 USB dongle

Wenet image transmitter: Raspberry Pi Zero W with LoRa board

The second transmitter was a Raspberry Pi Zero W with a Uputronics LoRa board to transmit photographs using the Wenet modulation. The Raspberry Pi Zero W was running the Wenet transmitter software.

The way Wenet works is that the transmitter software reads a JPEG image file taken by the Raspberry Pi camera, encodes it in multiple small packets using the SSDV encoder and then transmits it using the LoRa radio chip in 2FSK mode (and not the LoRa modulation).

The payload enclosure for the Raspberry Pi Zero-based Wenet photo transmitter and the video camera

The Wenet receiver software then receives the image and displays it in a web browser. The Wenet receiver software also sends the received image packets to centralized server, which collects image packets from multiple receivers and combines them into a single image. This way anyone can view the photos from the flight, even without having a Wenet receiver.

Wenet receiver software displaying received photos in a web browser

Flight preparations and launch

The weather forecast for the launch weekend was quite challenging, because there were a lot of rain areas nearby and the winds in the upper atmosphere were strong. However, Saturday morning September 2nd was sunny and calm, so we agreed to launch the balloon at 9:00 AM local time. We were at the launch site already before 8:00 to start preparations for the flight. The next rain area would reach the launch site around 11:00 AM, so we didn’t have time to waste!

At 7:30 AM there were already balloons in the air. In the foreground there are the antennas of the OH3AA amateur radio club. Mikael OH3BHX spotted a total of 8 hot air balloons on the way to the clubhouse. It was probably some kind of special event. (Photo: Mikael OH3BHX)
The styrofoam enclosure for the Wenet transmitter. The hardware consists of a Raspberry Pi Zero, Raspberry Pi Camera v2, LoRa transmitter HAT (which actually transmits 2FSK and not LoRa) and a QFH antenna for the 70cm band. There is also a small action video camera. The power source is a pair of 18650 rechargeable batteries with 5V upconverters and USB connectors. (Photo: Sami OH3EYZ)
Mikael OH3BHX and Jari OH3UW tying the flight payload box to the rope attached to the balloon. In the foreground, there are water weights with which we measured the required lift for the balloon. (Photo: Sami OH3EYZ)

We calculated the required lift for the balloon using the Sondehub burst calculator tool. We used a Hwoyee 600g latex balloon for the flight, and the weight of the flight payload (transmitters and parachute) was also about 600g. We chose the desired ascent rate to be 5 m/s, so that the flight would not take too long and the balloon would not fly too far from the launch site. Our goal was also to reach an altitude of about 30 km — by using more hydrogen gas the balloon would burst at a lower altitude.

Sondehub burst calculator tool for calculating the required lift for the balloon.

The burst calculator tool (above) showed an altitude forecast of about 29 kilometers and a required lift of 1431 grams. We weighed the required amount of water with a regular kitchen scale in a container and filled the balloon until it was able to lift the container.

Sami OH3EYZ and Jari OH3UW filling the balloon. In the background, there are Harri OH3YG observing and Martti OH1ON filming. (Photo: Mikael OH3BHX)

We launched the balloon from the Miemala area in Hämeenlinna, next to the OH3AA clubhouse. The core team responsible for the launch was Jari OH3UW, Sami OH3EYZ and Mikael OH3BHX. Martti OH1ON arrived to film the launch. Club members Harri OH3YG and Timo OH3CT were also watching the launch.

Jari OH3UW and Mikael OH3BHX tying the balloon to the rope. (Photo from the Wenet transmitter)
Jari OH3UW and Mikael OH3BHX ready to launch the balloon. There was not much wind on the ground, which made it easier to launch the balloon. (Photo: Sami OH3EYZ)
The balloon just after the launch. There is a parachute below the balloon to slow down the descent, then the Vaisala RS41 radiosonde repurposed for amateur radio use, and at the bottom the Wenet image transmitter and a video camera. (Photo: Sami OH3EYZ)

Photos from the flight

The approaching rain front produced some beautiful cloud formations. There is no accurate information about the altitude and location of the images, because the GPS receiver of the Wenet image transmitter did not work.

You can click a photo to display a larger version of it.

It is worth noting that the Raspberry Pi v2 camera does not distort the horizon much, because it is not a wide-angle camera, so the curvature of the horizon in the images is quite realistic!

The last photo taken by the Wenet transmitter before the payloads fell into the river Nokianvirta. The water splash from the fall blurred the camera lens, so there are no images of the balloon payloads floating in the water.

Flight path and landing

The flight path forecasts from the flight prediction service were again quite reliable. The prediction indicated that the maximum altitude for a 600g latex balloon and a payload of about 600g with an ascent rate of 5 m/s would be just over 30 kilometers. The upper atmosphere winds on the morning of the launch day would take the balloon to the northwest direction, towards the city of Tampere. The final forecast on Saturday morning showed the northwest part of the city of Nokia as the landing site, while earlier in the week the forecasts predicted the Kulju area as the landing site. The challenge in landing in the Nokia area is the large number of water bodies, which would make it more likely for the balloon payloads to fall into the water.

The predicted flight path right before the launch would take the balloon to Nokia and the balloon would burst between Lempäälä and Valkeakoski at an altitude of 30 kilometers. The flight path is the solid black line -- the dashed line is a collection of alternative landing sites at different times later in the day.
A more detailed view of the flight path prediction landing site, near the river Nokianvirta, very close to the actual landing site west of the hydropower plant.
The actual flight path Miemala (Hämeenlinna) to Nokia. The maximum altitude of the flight was 32 685 meters.

The balloon reached an altitude of 32.6 kilometers above Lempäälä and continued its descent towards Nokia. The flight payload fell into the river Nokianvirta, west of the Melo hydropower plant, contrary to the predictions and our expectations. However, the transmitters did not turn off when the payloads fell into the water, as the styrofoam enclosures caused them to float well.

The actual flight path Miemala (Hämeenlinna) to Nokia. The maximum altitude of the flight was 32 685 meters.
A map view of the flight path at the landing site. The APRS transmission (the purple icon with text OH3HAB-11) was cut off at the moment the payload fell into the water. Mikael OH3BHX and Jari OH4NDU had mobile receivers for the Horus 4FSK transmission in their cars, so the green path for the Horus transmission (with text OH3HAB-4FSK-V2) indicates the payloads floating along with the flow of the water and the path during recovery of the payloads.

Recovery of the payloads

The flight payloads were recovered from the river Nokianvirta by Mikael OH3BHX (from Hämeenlinna), Jouni OH3CUF (from Tampere), Jarmo OH3BSG (from Ylöjärvi) and Jari OH4NDU (form Lempäälä).

The recovery team for the flight payloads: Jarmo OH3BSG, Jouni OH3CUF, Jari OH4NDU and Mikael OH3BHX. (Photo: Mikael OH3BHX)

With help of the GPS position data, we ended up driving to a road near the river bank, west from the Melo hydropower plant. However, the shore was mostly steep and wooded terrain, so we walked to the nearest cottage we spotted. Near the cottage we met Tommi Lahdenperä, the resident of the house nearby, who immediately agreed to help us. We were able to look out to the river from his pier, from where we saw the balloon payloads floating a few hundred meters away.

Mikael OH3BHX on the pier looking for the balloon payloads in the river. (Photo: Jouni OH3CUF)
Jouni OH3CUF and Jarmo OH3BSG, with his 'sonde hunter dog' Laku! (Photo: Mikael OH3BHX)
The parachute, the RS41 radiosonde and the styrofoam box of the Wenet image transmitter floating in the river. (Photo: Mikael OH3BHX)
Jouni OH3CUF and Tommi during the short boat trip. (Photo: Mikael OH3BHX)

Tommi took us with his motorboat to the other side of the river where the balloon payloads were floating, and we were able to recover the devices and the parachute intact. The devices were not damaged at all, only the Raspberry Pi camera of the Wenet transmitter was slightly wet.

The transmitter devices and the parachute finally in the boat. (Photo: Mikael OH3BHX)
Mikael OH3BHX after the successful recovery operation. (Photo: Jouni OH3CUF)

Amateur radio club OH3AA would like to thank everyone tracking the flight and especially those who helped in the recovery of the flight payloads!

Flight sensor data

The RS41 radiosonde had two additional sensors attached: a Bosch BME280 sensor for temperature, air pressure and humidity, and a RadSens v2 radiation meter based on a Geiger tube. The radiation meter provided readings on the "micro-Roentgens per hour" (µR/h) scale.

The following graphs visualize the sensor data from the flight. The graphs are available on the SondeHub Grafana dashboard for the flight for anyone to view.

Altitude (meters, green) and ascent/descent rate (meters per second, yellow)
External temperature (Celsius, blue) and air pressure (hPa, yellow). The temperature rises at high altitudes, where the air pressure is very low.
Radiosonde battery voltage (green) relative to the internal temperature (yellow) and external temperature (blue). The voltage rises towards the end of the flight as the temperature of the batteries rises.

Detected radiation intensity and comparison to ground-level radiation

The intensity of ionizing radiation measured with the Geiger tube (µR/h, green) relative to the altitude (kilometers, yellow). The radiation intensity at an altitude of 10 km is over 30 times higher than at ground level.

The amount of background radiation according to this Wikipedia article is about 3-6 millisieverts (mSv) per year.

When compared to the radiation intensity measured by the sensor:

  • radiation intensity at ground level was about 20 µR/h ~ 0.175 R per year, which is 1.537 millisieverts per year

  • the radiation intensity peak measured at an altitude of 20 km was 750 µR/h ~ 6.57 R per year, equalling 57.62 millisieverts per year, which is already a significantly higher dose than the average background radiation (over 10 times higher)

The measured radiation intensity relative to the altitude follows a phenomenon called the Regener-Pfotzer maximum, where the peak of ionizing radiation intensity is at about 20 kilometers above sea level. This article has more information about the radiation intensity in the atmosphere.

It is important to note that the above discussion is about annual radiation doses and that short-term (hours or days) exposure even to the highest radiation level measured (750 µR/h) is not a significant health risk for humans. For example, this table documenting the strength of ionizing radiation shows that 50 millisieverts per year is the limit for people working in the US nuclear power industry. The highest radiation level measured by the balloon would correspond to a dose of about 0.16 millisieverts per day, which is in the range of a couple of X-ray examinations.

Horus 4FSK and Wenet reception details

The OH3AA club receiver was a long cross-yagi antenna with a preamplifier. We used the same yagi to receive both the Horus and the Wenet transmissions. The technical specifications of the yagi were excellent for receiving Wenet images, but for some unknown reason the Wenet receiver station did not work at all. There was a problem with the receiving equipment or some possible local transmission was interfering with the Wenet reception.

The cross-yagi of Jari OH3UW ready for reception. (Photo: Mikael OH3BHX)
Jari OH3UW at the Horus 4FSK receiver station of the OH3AA club. The Linux PC on the right was responsible for the Wenet reception. Martti OH1ON in the foreground. (Photo: Mikael OH3BHX)

Below are statistics from the stations that participated in tracking of the OH3HAB #7 flight.

There were a total of 25 Horus 4FSK receiver stations, an excellent number of participants!
The number of Horus 4FSK packets received by each station
The web service collected images from the Wenet receivers for everyone to view. The stations that received most Wenet image data packets were jii (Jouni OH3CUF), OH3MCK and OH3UW. For some unknown reason, the Wenet receiver station of the OH3AA club did not work at all.

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