This was my fifth “Pi In The Sky” flight and originally was to be an altitude record attempt using an extremely light photographic payload, however such a flight this time of year is likely to end up in the North Sea. Been There Done That don’t want to do it again. Now, most flights (nearly all of them actually) follow a simple profile – go up –> burst –> come down, but there’s another option which is to go for a “float” (in the air, not the sea). The simplest method of achieving a float is to underfill a large balloon, and what happens is that as the balloon approaches the stretch limit of the latex, the tiny amount of free lift reduces to zero at which point the balloon stops ascending continuously and instead just bobbles up and down. If the balloon survives until sunset then it cools and drops substantially in altitude to float at a lower level, before ascending again after sunrise. Typically the balloon then bursts but it is possible for it to float another day.

As far as I’m aware, no previous floaters have carried cameras, and my plan was to do just that with a Raspberry Pi camera using the live image transmission system (SSDV) that I’ve used before. I decided to take fairly large images and to send those at 300 baud which is known to work over large distances, but to use 2 radio transmitters to effectively double the bandwidth. Essentially the hardware was the same as in my Raspberry Pi-shaped tracker (which will fly very soon) but in a better insulated container with more batteries for the longer flight. I was expecting a flight beyond Poland and for the tracker to run for about 24 hours by which time it would be out of range of our receiver network anyway.


With a workable flight I contacted Liz and Eben Upton of the Raspberry Pi foundation to see when they were free to to launch with me – something all of us have wanted to do for some time. Saturday was that day, and with predictions from my home looking a bit risky (a fly-past of some minor provincial airfields called Luton and Stansted) I asked Steve Randall if it would be OK to use his site in Cambridgeshire (convenient for the RPi guys of course!). He said yes, and Anthony Stirk was available to come and launch his AVA payload, so the plan had come together. I love it when that happens …

The weekend approached quickly and I didn’t get time to make the tracker till the Thursday, and then built the payload container on the Friday evening. No rush then! I even managed to find time to program in a late Easter Egg (more on that later). Liz publicised the launch on the RPi site and Lester Haines did the same on The Register. I didn’t get much sleep on Friday night, having to get up in time to finish packing the 4×4 and then leave for Cambs at 7am. We needed to get there in time to launch in the morning as the winds were going to get up in the afternoon.

We arrived first, a few minutes early to find the wind rather stronger than expected. Anthony was next and we waited for Steve to arrive as he’s launched there many many times before and knows the best spots to launch according to the wind direction. Here he is using his patent-pending wind-direction-finding technique:


Eben and Liz were next, then Tom from Bloomberg who was recording the event for a news item, Graham (another balloonist) and James (Pi Camera Coder).


With the video streaming and tracking stations set up, we put our payloads together. Here Eben’s holding PIE5 for me whilst Anthony works on his AVA payload at the front:


With payloads working and tied to the parachutes, it was time to get inflating those balloons. You’d have thought that a hydrogen cylinder would be lightweight what with all that hydrogen in it, but apparently not …


Now, a normal flight (one intended to burst) has a “neck lift” in the range 1-5kg, typically about 2kg, and those are usually easy to measure by hanging a suitable weight from the balloon filler. here though we wanted a neck lift of around 300g, which becomes a problem when your filler weighs nearly twice that! So we had to use the scientific method known as “guessing”, followed up by removing the balloon and hanging a weight (2 reels of duct tape!) below. Here Anthony is helping me at a delicate part of the procedure:


We filled Anthony’s balloon first, and then asked Eben to hold it whilst we worked on my balloon:


Once my balloon was ready and both payloads tied to their respective balloons, I handed my balloon to Liz and we got the two of them to pose for us:


After checking that both payloads were still running fine, we were ready to launch. Steve called Air Traffic Control for clearance, and they said we had 5 minutes to launch in or wait a short while. We were ready, and after waiting a minute for the wind to drop we both launched:

(thanks to Alex Eames of for recording the video stream for us).

What you don’t see in the video is the resulting excitement and panic. Whilst my flight ascended at the expected rate, Anthony’s didn’t. See his exciting write-up for details, but suffice to say that it just cleared a tree 500 metres from the launch point!

With that over I did a piece to camera:


and Eben did a piece too. We all then packed up, and Anthony, Liz, Eben, Julie and myself made our way to Milton Keynes (why, you may ask?) to an excellent Dim Sum restaurant that Liz and Eben recommended (there’s your answer) and is probably the only reason to visit that place.

In between the many, many rounds of dishes we talked ballooning and Raspberrying, and kept tabs on progress of our flights. PIE4 was out over the North Sea transmitting some nice images as it went.


We then noticed that AVA had stopped transmitting and that uXABEN (Steve’s flight) had lost GPS. That was running a pre-preduction unit and a failure wasn’t a big surprise, but we were disappointed that AVA had hit trouble so early. After the meal we all set off to our respective homes, and during a short stop for fuel Julie and I noticed that AVA had updated on the map again and was near the Dutch coast. AVA carried 2 radio transmitters one of which is not allowed over the UK and was programmed to switch itself on only when legal to do so. We called Anthony with the good news, then went home.

At home I spent pretty much all the rest of the day watching the progress of both flights. PIE5 settled into a float at around 40km, gently going up and down with a peak altitude of 40,350 metres, comfortably above my previous “live images” record of just under 40km!


The images weren’t all as good as I’d hoped, principally because I’d used auto exposure and that faces an enormous challenge to find the correct settings with the vast difference in luminance between the clouds and the blackness of space. The flight was over a lot of cloud which makes that task almost almost impossible, and I’ve seen many poor images before with other cameras in this situation. It would have been better if I’d used manual settings, which I will do next time! Also, the flight seemed unusually stable (normally the payload swings around a lot) resulting in a lot of images with a lot of black and a little cloud. So in the even it would have been better to have the camera aimed downwards slightly. Nevertheless there were some interesting images including these apparently of a gigantic raspberry!



By now AVA had apparently died altogether, with neither RTTY or APRS transmitters running. After sunset my balloon descended as I’d hoped, achieving an excellent second float:


After that I waited eagerly for 9pm to arrive. Why? Because knowing that the sky would be completely black by then, I’d copied some specially chosen space-related images to the SD card the night before, and programmed the Pi to select and transmit images from the set. Needless to say this caused some confusion, consternation and amusement, as intended of course!



The missing segments are down to the fact that by now the flight was a long way from home and we only had a couple of listeners in range. I’d expected the flight to go towards Poland, but it had floated so high that the winds brought it south over Switzerland instead! When I awoke in the morning the last listener reported that PIE5 had finally stopped transmitting data, and that would have been because the batteries had given up. Here’s the last image it managed to transmit:


By now it was still over Switzerland and beginning to rise in the sun. Most likely it then burst, with a landing across the border in France. It does have contact details on it so it’s possible though unlikely it’ll find its way home!


Of course, if you do find the payload or know of its whereabouts, please contact me (details in the About page!

With PIE5 “lost in space”, attention turned to AVA which wasn’t as doomed as it had seemed. It suddenly appeared on the map over the Czech Republic, and was then tracked down into Austria where it burst and landed atop a 1500m high hill! Here’s the map showing the paths of both balloons – note how different they are despite both being launched from the same place at the same time!


We all assumed that was that, but we weren’t banking on International Rescue arriving in Austria in the form of a Slovak balloonist called Radim and his intrepid team. After driving from Bratislava they hiked up the snow-covered hill to rescue AVA! See Anthony’s account.

So, an awesome ballooning weekend, with records broken (highest live images, highest landing spot) and the most incredible recovery I’ve seen. Now we just need PIE5 to be found!

For more photos see my flickr set

9 Replies to “Watching A Big Cheese Over Switzerland”

  1. Excellent blog post, I love what you are doing with the Pi. Did anyone else have a flashback to the 60’s and those creepy balloon things that popped up out of the sea and chased Patrick McGoohan? “I am not a number, I am a free man!”

  2. As long as you’re going to include an Arduino, it seems worthwhile to have the Arduino run a low-duty-cycle recovery beacon on the last dregs of battery power, after the Pi+camera cuts out. Although in that case, I wonder if extra circuitry is needed to fully disconnect the Pi, to stop current drain when it is in an almost-but-not-quite-working state.

    1. Yes, definitely. I didn’t get time to do that but it’s worth it. It’d need some circuitry to switch the Pi completely off, and also the GPS would need to move from the Pi to the Arduino. At that point the Arduino would need to do all the telemetry and the Pi would just feed it with image data. All do-able just needs some effort. Remember that for this flight I thought that the Pi would be out of range anyway before the batteries ran out (otherwise I’d have simply added more batteries!). I guess the big thing to learn from the flight is to plan for the unexpected!

  3. This was the first time I’ve followed yours, XUABEN (heard his signal too) and AVA’s balloonflight. I didn’t knew you guys where doing such interesting things with balloons! Wow, I was so amazed! Someone told me by mail. The whole day I was monitoring your balloon (wouldn’t even go to bed hi), and while it reached the Netherlands, I hurried to download the software to decode the RTTY data on my 70cm transceiver. After a while, it worked (had to put decode in reverse on USB)! So I helped a little little bit by sending the SSDV parts to the server 🙂 I should have had an better antenna… Probably next time! But this was also a very interesting day for me. Thanks!
    Greetings from the Netherlands,
    (Ham radio operator PE1RQM)

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