Strato
balloon
A KFK project.
© Gergely Attila, Rusu András.
A very important part of the balloon's "cargo", in addition to cameras, telecommunications devices and data storage, are various sensors. The sensors were selected so that with their help we can examine various characteristics of the Earth's atmosphere as accurately and as widely as possible, and afterwards compare the measured values with other existing atmospheric models. Perhaps also contributing to a more accurate understanding of the atmosphere.
That is why, among other things, temperature measuring sensors were installed, with which we want to examine the dependence of the temperature of the atmosphere on altitude. We also use humidity and pressure sensors in the project. The pressure sensors play a dual role in the mission. In addition to measuring atmospheric pressure, we can use the data they measure to determine the height of the balloon above sea level.
Also, we found it interesting to examine the carbon dioxide concentration in the atmosphere, the concentration of dust and the UV index of the sun's radiation. In addition to the GPS coordinates which is mission-critical data, the built-in accelerometer, gyroscope and magnetometer take care of knowing the orientation of the balloon.
Sensors used in the project:
BMP280 – temperature and pressure sensor
SHT21 – temperature and humidity measuring sensor
DS18B20 – temperature sensor
GP2Y1014AU0F - dust sensor
MH-Z14A – carbon dioxide sensor
MPU6050 – accelerometer and gyroscope
MiniIMU-9 v5 – accelerometer, gyroscope and magnetometer
During the mission, the operation of the sensors is ensured by an ESP32 and an Arduino Nano microcontroller. We built the electrical unit as modular as we can so we could change our parts in the case of a malfunction, or if we want to upgrade it in the future.
The main role of the box is to protect the electronics from the environment: external low temperatures, landing impacts, possible precipitation or unhealthy effects on the circuitry from falling on water.
The box plays a very important role in keeping everything working properly, so it makes a difference what material you use, how you build it, how you fit the parts together. The insulation material used for the box is 5cm thick XPS polystyrene, which is an excellent choice because it is durable, waterproof and a good thermal insulator. The sides are made in a serrated design similar to a puzzle to increase stability. We used Polyurethane Foam to glue the panels together, bearing in mind that the temperature can be as low as -40 C, so ordinary insulating tapes are not suitable in this case.
As the roof is not directly attached, the parachute cannot be attached to it, so the whole box was tied with two ratchet straps and the parachute strings were attached to these. Several sensors are placed on the outside of the box to measure atmospheric data. To provide a view for the cameras, we had to cut holes in the walls, which compromised the waterproofing and thermal insulation of the box, which we compensated for with Plexiglas.
Every experiment or measurement project needs some already existing or in some cases even new theoretical models so with their help the studied phenomena and its relevant aspects will become predictable. The practical use of theoretical models and the checking of the data-series most commonly happens with some simulation.
The most important job of the Theory and simulation group (T&S) was that to create an as-realistic-as-possible trajectory predicting simulation. We considered the work of Gai et al. [2] as the basis for this. The ascent and descent model presented in the paper is constructed on some general physical assumptions, while the lateral displacement is determined by using real weather forecast and meteorological with simpler or complicated interpolations between the datapoints.
The main equation of the ascent model:
\[ m_{tot}a_z = g\rho_{air}V - g(m_{gross} + m_{gas}) - \frac{1}{2}C_D\rho_{air}\dot{z}^2A_b, \]where:
\( m_{tot}a_z = g\rho_{air}V - g(m_{gross} + m_{gas}) - \frac{1}{2}C_D\rho_{air}\dot{z}^2A_b, \)
\(m_{added} = \frac{1}{2}\rho_{air}V\) - the mass of the air moved by the balloon during the ascent
\(C_D\) - drag coefficient
\(A_b\) - balloon reference cross-section
The rise in volume is the main responsible for the rise in the balloon vertical velocity, commonly called ascending rate, because in the equation of motion the acceleration is observed to depend on volume. The temperature appearing in the original model, presented in [2] is a rather sophisticated term, because it depends not only on the balloon's expansion but also on different convection rates (air-balloon material, balloon material-helium, radiation convection rates). These represents other differential equations and data that is usually location and/or time specific for which there is either no database or their measurement exceeds our possibilities. These will appear in our model either as constants or some approximations (eg. linear) or they are neglected.
We propose two possible methods to describe the pressure inside the balloon. First we considered the ascension process to be adiabatic. In this case the pressure is given by:
\[ p\cdot V^{\gamma}=const. \]The other variant is based on the ideal gas law:
\[ V = \frac{m_{gas}}{M_{gas}} R \frac{T_{gas}}{p_{air}} \]At the moment we are still investigating which will prove better.
The horizontal movement of the balloon can only be determined based on real weather databases. We analysed different available data packages, like NCEP (National Centers for Environmental Prediction) or ECMWF (European Centre for Medium-Range Weather Forecasts). Crucial criteria were the good temporal and spatial resolutions. For this we have chosen the ECMWF database. The spatial data, however, has not fine enough resolution, here interpolation methods will be applied.
Beside predicting the trajectory, other tasks were to model the air density, pressure and temperature in function of the altitude. Because the balloon will reach a maximal 24-30 km altitude, a single approximation yields serious errors. Hence using different approximations in different altitude regions is required. The altitude regimes and the corresponding approximations can be found in detail in [1] and these are implemented in the simulation and temperature-, pressure-, air density-altitude predictions.
In conclusion the T&S dealt with the above mentioned trajectory and other parameter predictions. The responsibles for the coding part were András Kuki and András Rusu. All the coding was done in Python 3 language. The theoretical models were compiled, applicable data was found and the literature was studied in detail by Zsuzsa Bálint and supported by Kristóf Benedek. The rest of the group was assigned to these two main fields and some will take part in the data processing after the balloon's flight.
Future plans are to perfect the trajectory model. After the balloons flight we will compare the empirical GPS data with our initial prediction and in case we will try to explain the differences and propose a more exact approach. Big part of the T&S will also take part in the data processing which means the interpretation of the data received from the sensors placed on the balloon.
References
Gai, Mario & Guglieri, Giorgio & Lattanzi, M. & Lombardi, A. & Mana, M. & Masserano, L. & Musso, Ivano & Navone, P.. (2014). A scientific mission based on a high altitude stratospheric balloon. Int. J. Aerosp. Sci.. 3. 18-29.
Morris, A. L. (1975). Scientific Ballooning Handbook (No. NCAR/TN-99+IA). University Corporation for Atmospheric Research. doi:10.5065/D6G73BM1
Farley, Rodger. (2005). BalloonAscent: 3-D Simulation Tool for the Ascent and Float of High-Altitude Balloons. 7412. 10.2514/6.2005-7412.
Gallice, A. & Wienhold, F. & Hoyle, Christopher & Immler, Franz & Peter, T.. (2011). Modeling the ascent of sounding balloons: Derivation of the vertical air motion. Atmospheric Measurement Techniques Discussions. 4. 10.5194/amtd-4-3965-2011.
International Standard Atmosphere - Wikipedia article
Density of air - Wikipedia article
According to the project's goals and to the Sensors chapter, the balloon will have as payload a number of measuring devices that will communicate real-time data via a radio unit. The main task of the Data-Processing group is interpreting and evaluating the received information.
Our job begins with the data separation with respect to the sensors and state parameters. In a next step we plot and compare the systematized data with the predictions of the Theory and simulation group. This comparison is in fact the pinnacle of the project's theoretical part. By doing this we will be able to state relevant conclusions regarding the used models and possible hints to improve our models also will be found.
Our current models divide the atmosphere into different layers where different approximations are applied to describe the temperature, pressure and other state parameters. The data-processing hopefully will reveal the validity of these assumptions.
Some more exotic measuring devices will be "on board" too: UV, CO2, humidity, dust concentration sensors, magnetometer and accelerometer. These assure a possibility to investigate some new, creative questions beyond verifying meteorological and other models like the connection between humidity-temperature-altitude or humidity-CO2-UV parameters.
The necessary scripts and plots will be written and made in Python3.