On 17th of April, 2015, after a year pause since our first test flight we launched a balloon again. The original plan was using the new improved elements of the flight computer during the flight, but due to some unexpected problems during the development we had to make some last minute changes. This flight would be our participation on the Global Space Balloon Challenge so we had strict launch window.
A hybrid flight computer was used in this flight, which used the new improved On-Board Computer (OBC) and the Communication Module (COM) and Electric Power System (EPS) of the previous flight. The OBC was equipped with an integrated camera (ICS) which is able to take and transmit low-resolution pictures via radio. The probe also had a high-resolution camera controlled by the On-Board Computer. The actual position of the balloon was provided by an integrated GPS module and an offline backup GPS-GSM tracker.
On-Board Computer (OBC)
The On-Board Computer (OBC) is based on an STM32F4 AMR cortex microcontroller. The OBC organizes the communication between the modules, collects house-keeping and GPS data and also compiles the telemetry packets. System events are logged and can be downloaded from the SD card of the OBC. The House-keeping data is also stored on the SD card.
- STM32F407VGT6 32-bit ARM Cortex-M4F microcontroller, 1 MB Flash, 192 KB RAM
- System Clock: Real Time Clock (RTC)
- UART connection for GPS modul
- CAN controller + CAN transceiver
- DCMI port for integrated camera control
- Micro SD-card slot
- Diagnostic port
- 2×20 pin stackable UPRA-BUS connector
The new On-Board Computer is produced in the UPRA form factor, in the size of 70x75mm (2.76″x2.96″). This is a huge size reduction compare to the previous model.
GPS coordinates, altitude and speed was provided by an NV08C-CSM module. This GPS reciever was designed for high-altitude operations and has an altitude limit of 50 kms (~32 miles), so we could track the entire flight of the balloon. The NV08C module communicates with the OBC through NMEA standard. Parsing the proper NMEA messages longitude, latitude, altitude and speed data could be processed. Each GPS NMEA word has time stamp which used for synchronize the system time. Time Sync is needed, because the Real Time Clock (RTC) unit of the OBC produce increasing slip due to the temperature decrease.
The internal temperature and the stabilized supply voltage was measured by the microcontroller of the communication unit. These data, the exchange between the COM and the OBC during the preparation of the radio communication action.
Integrated Camera System
The telemetry system has an integrated small resolution CMOS camera module that stores pictures on the SD card of the OBC. The pictures taken by the Integrated Camera System (ICS) are available for every unit of the UPRA system thus the pictures taken could be sent back via the Communication Module during flight.
The Integrated Camera System was tested before and now it is upgraded with a fish-eye lens of a 180° angle of view.
- Resolution: 320×200
- Color: yes
- Native angle of view: 65°
- Modified angle of view: 180°
- Picture format: BMP
- Connection: Integrated (DCMI)
Communication Unit (COM)
Our self made communication unit was designed around an SI4461 FSK transreceivier integrated circuit produced by Silicon Laboratories. It could be widely configured to different solutions. The desired frequency, data rate, modulation and fequency deviation can be set. As for the test flight, the future goal is to involve the amateur radio community. Therefore, appropriet settings was choosen so they could receive data from the balloon. Our transmit frequency was in the ISM range near the 70cm band which is popular among amateur radio operators. An avarege HAM radi ohas a maximal bandwith up to 3kHz, so the data rate, frequency deviation and bandwith had to be chosen that fit into this range. So we used the 625bps data rate and 625kHz deviation. To increase the spectral efficiency, Gaussian FSK modulation was used. Since the communication is done via an additive noisy channel , it was necessary to filter out the erroneous packets , so they are supplemented with CRC error code.
The orientation of the capsule is changeing continuously, so an antenna was integrated into the system, which radiate in all directions as far as possible. Thus, a monopole antenna was connected the probe. Because, according to the ISM standard limited radiated power, the resulting SLOS was compensated by the antenna unit of ground station.
Our main ground station was the control centre of the Masat-1(the first Hungarian CubeSat). On the station four cross-Yagi antenna is located. In order to avoid cross-attenuation they were used in circularly polarized mode.
The ground station equipped with an RF circuit similar to the one in the COM unit. The control software was running on a Raspberry PI minicomputer which processed the data direct through SPI-BUS, so we got the flight information in real-time. The control commands have also been sent through this interface. Setting the directions of the antennas was processed by the GPS coordinates sent back to optimize the data transfer.
The ReHAB-150 capsule prototype was used during the testflight. It has a reinforced aluminium frame which not only provides the antenna fastening, but also allows to connect outer sensors and experiments easily. The probe has a multilayer insulation which built up from a polystyrol box covered with space-blanket.
The parachute is a self made octogonal desing made of lightweight, bright color fabric. In the design and manufacturing, the Pulcsi és Foltmékör fabric processing team helped us.
The designated launch time was 10:00(UTC+2). On the launch day the weather was clear with very light wind. The weather data predicted the arrival of a huge cold front in the afternoon. Due to this rapid weather change our balloon landed outside the predicted landing zone.
One hour before the flight, the laucnh site was prepared behind the “ST” building of the Budapest University of Technology and Economics. Meanwhile at the groundstation the radio communication and the backup GPS tracker was configured. We used PAWAN 1200gr balloons for this flight. Some pre-flight tests were run until the gas tank arrived to the site. Due to some minor problems the countdown stopped several times, so we had to calculate some delay.
10:00 – The gas tank and the filling adapter was prepared by colleague of the Department of Hydrodinamics BME. The inflation of the balloon has begun
10:20 – Inflation had finished. The parachute, backup GPS tracker and the main probe was connected, then the whole payload train was connected to the weather balloon
10:40 – All systems are GO, Groundstation: GO, Mission control: GO
10:50 – Launch
10:55 – After downloading the first picture, the radiocommunication lost with the balloon. Coordinates were updated via the backup GPS tracker. The launch site was broke down
11:20 – Emergency tracking mode: no live radio contact with the probe, using only the simulations, predictions and limited data from the backup GPS tracker.
14:20 – Predicted landing time. First try to reconnect with with the backup GPS tracker
14:50 – Confirmed landing time. First stable coordinates received from the secondary GPS tracker. Landing site Hortobágy National Park, 60kms off course the predicted trajectory
15:20-17:30 – Recovery. Travelling to the landing site. We could approach the site in a 3km radius with car. We continued the search on foot.
Recovery of the probe
The recovery of the probe soon turned to an adventure in the wildlife of Hortobágy. As we headed to the landing coordinates on foot, our first stop was a farm at the edge of the National Park. We tried to gathere information the whereabout of our balloon probe, but unfortunatelly the farmer didn’t see anything,
“Just pass through the fences and look around” – adviced the farmer
“And…these huge animals won’t make any trouble?” – Levi pointed to a cattle grazing near us
“Nooooo way, these are peacful animals”
The words of the farmer gave us confidence so we climbed through the fence and headed to the landing coordinates two clicks away. Meanwhile it started to rain and we all agreed that some boots and changing clothes would have been a good idea to bring. As we passed through two other fences and some no-man’s land we noticed a cattle of 60 in the distance, but it doesn’t bothered us too much. Anyway the farmer said they are peacful animals so we continued our trip…right into the cattle.
We were about 300 meters from the cattle when someone asked:
“When are we going to ask ourselves to turn back avoiding any troubles?”
In that exact moment the cattle of 60 turned to us as one, and those “tiny” animals started to rush towards us. This was more than enough for us to run for our lives to the fences.
We reached the no-man’s land and so the safety for a while, but we noticed that from the other direction another cattle closing towards us. So we were standing between two cattles playing “we the guys in the hood”, mooing and making other noises, and we were trying to figure out how to get to our balloon.
The plan was to split to two teams. One of the teams tried to keep the cattle at the fence while the others recover the balloon. It looked good on paper. The recovery team was in a 50-100m radius of the probe when they had some doubts about this idea. The cattle was between the recovery team and the “cattle free” safe zone…so…there was no reliable escape route in sight. That could be trouble…
And it was trouble. As the recovery team was looking for visual contact with the probe the cattle started to walk towards them. Fortunately this time they were closing slowly, so the team could made some evasive action, and got back to safety. Meanwhile the other part of the team followed the cattle to get the probe.
Levente approached the cattle in about fifty meters and at this point he called for some support via cell phone. He asked his mother (who’d grown up in the countryside) what should we do with “our small problem”. She informed Levi that “the cattle are peacful animals (jeah, we’ve got the experience) and they are just curious. Probably they are more frightened than us.” (We could hardly beleaved this last pice that time). So Levi gathered up all his confidence and he headed for the animals. And the animals headed to him. And we lost our breath. Then the cattle madly fled away as we did it earlier.
As the animals passing by we noticed the bright parachute and the golden probe of ours. Looks like the cattle are really curious animals, and our balloon probe was kind of interesting heavenly gift was for them.
So it was a happy end at last. We found our balloon, didn’t killed by the cattle and even the rain stopped as we were closing to the car.
Unfortunately the flight computer stopped at the beginning of the flight due to a hidden software error. Although we consider the launch as a success: the probe was found and recovered thanks to the backup GPS tracker. Also we gathered a lot of experiences that will help to plan and carry out succesful launches in the future.
We had a lot of help during the preparation and the flight. The groundstation and the backup GPS tracker was provided by Levente Dudás and the BME-GND Team. The balloon filling gas was provided by the Department of Hydrodinamics BME who helped during the balloon inflation too.
We would like to thank all of you, who helped this balloon launch carried out!