|1 Oct 2017|
Firstly, I would just like to clear up one thing, and that would be the word “near.” Space officially starts at 100km from the surface of the earth, but the method I will describe below only goes up to about 30km, with a cost of just under $650. Just to double that altitude to 60km will cost $200 million! This would be an ideal group project at school or just something awesome to do with your friends that’ll make everyone else jealous.
So, now that that’s out the way, we can finally begin. So, a few questions to think about before you start:
As for the first question, well, that’s not too difficult to answer for most people. It might be along the lines of getting images of the earth (as seen above), or for the more scientifically inclined groups, maybe a profile of the areas that the payload is travelling through. But that’s just the beginning of the problem, now we need to start the work! For this project, the easiest way to divide the constituents is into three parts: 1)The data, 2) The flight, and 3) the recovery.
So, what information will you collect? How is it relevant to the project, and how will you collect this data? The most obvious data are the images. I mean, why wouldn’t you like to see where your payload went?! But then you can collect other data. For example, temperature, pressure, humidity, horizontal and vertical speeds. Another question to ask is: how often would to like to collect this information? Too long of an interval will cause gaps in the data, but too much data can also clog up the system with unnecessary information. To do all of this, what we did is use a raspberry pi, with an extra board attached on top called “pi in the sky,” or PITS for short. We also attached an external board to the PITS board for the external pressure, temperature and humidity.
This is probably the easiest part to complete, and that’s how to send it up and down safely. The easiest answer: a weather balloon. It’s not rocket science! But there are a few things to be aware of. Firstly, the size of the balloon. Most people use either a 300g or 500g balloon with about 900g of helium inside. As the balloon rises, the air pressure outside drops. The helium inside will have the same pressure, so the balloon will expand. If your balloon is too small, it will expand and pop at a shorter altitude, but if you have a bigger balloon, more helium will be needed due to the added weight, so a fine balance is needed. The best way to decide on a balloon is to sort out what you’re sending up and weigh that, so then you know how high and how much lift you’ll need. Also, if the balloon is overfilled, the ascent rate will increase. If you aim for about 5m/s, the data collected from the raspberry pi or onboard computer will have a good spread, but if the balloon is too fast, the computers won’t be able to keep up!
But how about on the way down? How will you stop it from crashing? That is another reasonably easy question to answer, and all you need to do is use a parachute. Our parachute 12-inch diameter with a loop on the top. For the final set up, we used nylon rope to tie the payload 5m below the parachute, with the balloon 5m above the parachute.
This part isn’t really necessary, but most people decide that they want everything they sent up back! Places like the MET office in England send up weather balloons all the time but don’t collect them again (though they probably have a bigger budget than we do). However, there are some backup plans if you cannot find your payload. As mentioned before, we used the PITS board, and that had radio and GPS capabilities built in. That meant that all the data that our pi was taking was being sent back to the ground. But that’s just two radio frequencies (low frequency being a 0, high being a 1). What do you do with them?
We had two aerials which, when used with the correct dongle, can receive all the data being sent out. The software that we used was called SDR sharp, which receives the data and then sends that through the soundcard of the laptop/computer used to another piece of software called dl-fldigi. This software takes these 1s and 0s and converts the binary data packets into ASCII and useable data. This then uploads the information to a website called tracker.habhub.org, which is a website used by high altitude balloonists. It will plot your location on the map, and give an estimated flight path.
What I haven’t mentioned yet is the aerials used, and that’s because we needed two. One was a 70cm YAGI aerial, which is used when stationary. When launched, you can stand still to use this dongle, but it doesn’t work well when driving, which isn’t great for recovery. That is why we used another whip aerial called a magmount. This can find your payload if you’re in a moving car, so it is really useful when recovering the launch materials. Our one was magnetic which allowed us to have it on top of the minibus that we used. Both of these aerials work with a USB dongle called the FUN CUBE dongle, which is just a small dongle that plugs into your PC and works with the software above.
The raspberry pi can be programmed to select an image from the past 10 that it takes and send it down in a series of 20 data packages, so the image can be seen. These images are stored internally on the SD card as well, but if you can’t recover the payload, you’ll still have some images! Also, as a further backup, you can send up an SMS locator. This works just by placing a SIM card into the device, and when you call the phone number, it replies with a text message of its last known GPS location and will send it as a link on google maps. This makes it extremely easy to find if you do end up losing your payload.
And that’s it! A brief introduction to weather ballooning. Remember, test everything! Before you even have a balloon ready, test that the pi works, check if it is sending the image and data packages, buy spare batteries for the launch day, and, most of all, don’t give up! On the day before our launch, the whole pi shut down and we had to start again. It was one of the worst parts, but that what these things are like. With enough and effort, time everything will work out. And, finally, have fun!
A selection of the photographs from the balloon can be found at the following link https://drive.google.com/drive/folders/0ByxBqxd_w3gMb3hjaHMyZjBKeGc?usp=sharing
Sheamol is a STEM student, studying math, further math, physics and computing at Bishop Vesey's Grammar Shool. He enjoys rowing, taekwondo and coding in his free time, and is a talented photographer.
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