HACT - Testing radio communication at the edge of space

Published: 05.08.2019
On the 6th of June 2019, we launched our first official Atmoventus mission named “HACT“ or high-altitude communication test. We tested the radio communication equipment of our Atmoventus H rocket thoroughly at the limits of its abilities by sending it to what by many is already considered the edge of space.
The HACT mission patch The HACT mission patch

The background of the mission

The planning for this mission started back in February when the Satgruppe Langenzenn told us that they would launch another weather balloon. We chose to fly with them to test the radio equipment that we later want to use in our Atmoventus rockets, and a weather balloon was the perfect choice to test the reliability of our connection over long distances without the interference of obstacles like houses, hills and trees to create a comparable situation to a rocket launch.

Mission goal

The mission goal was, to send information over the longest possible distance. We chose to send GPS information to track our balloon live so we could predict its flight path and set up our next receiving station ahead of it, but we also would be able to calculate the actual distance between the two antennas and find out whether they are a viable option for our rocket. We simulated our flight multiple times before launch, but when dealing with weather, calculations are always rather vague. Also, we did not know the exact altitude at which our balloon would burst. This made predictions even harder as the weather balloon reaches a strong jet stream at an altitude of around 25 kilometers that extends further up than our maximum possible altitude. So only a few kilometers in height difference can cause the total flight distance to increase by 20-50 kilometers, making it impossible to calculate a certain landing spot beforehand.

Picture of our probe prior to launch Picture of our probe prior to launch

The tech on board

Our weather balloon probe consists of a styrofoam box with fins for better stability during flight and all the electronics inside. On board, there is a data logger that measures the temperature, humidity, and pressure outside the probe and records them together with the current position on a memory card so we can recreate the flight path after landing. There are also three cameras on board: One 360-degree camera, an HD action camera and a small PCB camera connected to our HACT-radio. We also carry two redundant GPS trackers, that send their position via the GSM network as an SMS in order to help us find the probe after landing. Most of the electronics are powered by a 28Ah powerbank from Amazon. As all the electronics give off quite some heat, the isolating styrofoam box should be able to keep it warm enough for everything to work properly, even during chilly -60 degrees Celsius outside. And finally, the most important thing inside of the probe: Our HACT mission (cf. First test of our radio communication system). It consists of a small Raspberry Pi Zero connected to an Adafruit GPS Breakout v3, a LoRa RFM95W Radio module and the tiny PCB-camera. For the antenna, we used a four-wavelength long wire, that, connected to a glass fiber rod, sticks out sideward from the probe.

The launch of the weather balloon on the 6th of June 2019 The launch of the weather balloon on the 6th of June 2019

The flight day

On the 6th of June 2019 we transported all our equipment to the launch location (the sports ground of the WBG Langenzenn, where the Satgruppe Langenzenn is located) at 9 am on Thursday after finalizing and connecting all of the electronics inside of the probe since 7 am that day. The balloon was filled with helium by 9:30 and at 9:45 all strings were attached, our signal flares were lit, and the radio communication as well as the cameras were running.

The launch went well, and we received GPS information until the balloon traveled 11,5 kilometers and to a height of 2,1 kilometers. At that point, the probe was at an inclination of only 10,3 degrees over the horizon, so our signal was blocked by a ridge and big trees closer to our location. While the balloon picked up speed, we packed our equipment, ran a last flight prediction and set off to the first radio location in the probes flight path north of Bamberg.

After setting up our radio station there at 11:15 we immediately received a signal from our balloon. At this point, it must have already been at an altitude of around 10 to 20 kilometers and we were greeted by our first live picture from high up. While blurry, as the connection wasn’t stable enough to send all packages successfully, we could clearly tell apart the ground from the sky and see a big white cloud in the center of the picture.

Picture from the atmosphere over Bamberg Picture from the atmosphere over Bamberg

What was bugging us though was that we were not receiving any GPS locations, but only an error message telling us that the GPS module inside of the probe failed to establish a position fix. Somewhere between the last message at our launch site and above Bamberg, it had lost its connection to the GPS satellites in orbit, but our radio equipment worked flawlessly.

We moved on to a parking lot close to Suhl, but no matter how hard we tried, we had no success between the high trees growing around. After deciding it was the smartest not to stop while the probe was crossing the mountains of the Thuringian Forest, as a good reception there was very unlikely, we set up the radio receiver far closer to one of the expected landing sites between Fulda and Kassel.

We arrived about 4 hours after our balloon started, with a predicted 90 minutes left until it was supposed to drop from the sky and about two hours away from landing, if the balloon did not burst below its maximum possible altitude. Only seconds after setting up our equipment we received the balloon clearly at what was most likely its maximum altitude of around 40 kilometers. Only for a few seconds, so far too short for an image, but long enough to receive several GPS packages. But they were empty as well. We now knew our probe was still flying somewhere above us, but it failed to reconnect to any GPS satellite.

We waited until we knew that the balloon must have burst and then moved on to a restaurant close by, where we met up with the rest of the Satgruppe to wait for the probe to land, so we could locate the GPS-trackers inside, but the trackers didn’t respond. While waiting, we tried to figure out the landing site based on the balloons climbing speed, the locations at which we received a signal and maps of GSM dead zones in the area, as that would explain the trackers not responding.

We identified a 20 times 30 kilometers big, mountainous area as the most likely landing zone, far too big to find it by just looking around. Nevertheless, a few of us decided to drive there after 4 hours of waiting, just in case we could receive signals from our radio equipment and to be close by in case someone found it, so we could immediately come pick it up. But the trackers did not respond until we knew they by now must have either ran out of electricity or gone into sleep mode, waiting for a wakeup command they can’t receive where they are.

Our mission success

Since we returned home without our probe, and to this date (5th of August 2019) we still haven't been contacted by someone who found it, we have no conclusive information on what happened to our GPS receiver or our trackers. But after all, our mission was a complete success, as we could receive clear signals at multiple points over distances of at least 11,6 km at launch and later at approximately 10-20 and 30-40 kilometers altitude. As our Atmoventus H, which will carry this hardware, will only reach 6 kilometers altitude, we can confidently say that we have proven our hardware to be fitting for this job.