Society of Robots
Search and Index Search Here

 Parts List
 Robot Forum
 Member Pages
 Axon MCU
 Robot Books

 How To Build
  A Robot




 Robot Journals
 Robot Theory

    High Altitude Balloon Tutorial
    == Space Balloon Photography ==


    Photography from a Balloon in Space
    Nothing is more satisfying than looking at pictures from space - which you yourself took. It's the take home trophy from weeks/months of hard work and preparation. No flight can be considered successful without those images.

    As such it would be depressing to have your balloon reach space, but the camera system failed to take any good shots. And that's the point of this tutorial: to make you aware of the issues you may encounter.

    Camera Spin and Rocking
    Both camera spin and rocking are the #1 problems of putting a camera on a balloon, making video very nauseating and of low quality. The entire balloon package not only spins due to wind, but it also rocks up and down due to the pendulum effect.

    Even if the resultant video doesn't give you motion sickness, it still plays havoc on the cameras' auto-brightness and auto-contrast features: a rocking camera swaps between pointing at the darkness of black space, occasionally the bright sun, and the whiteness of the clouds on Earth. Depending on the camera this may also cause issues with auto-focusing. Some video compression formats aren't very good with handling fast motion, either.

    There are four ways to prevent spin and rocking:
    1) increase string length
    2) use a rotating swivel
    3) increase rotational moment of inertia
    4) decrease wind resistance
    5) use an active control system

         String Length is the distance between the balloon and your package. If the balloon is spun or pushed at the top by wind, that action is translated down to the package by the string. A long string can dampen the twisting effect by storing the twist in the string. It also provides for a longer pendulum arm, reducing the maximum rocking angle of the package as wind pushes the balloon. A longer string is also great if your parachute gets stuck in a tree, allowing you to reach the package and cut it loose.

    The one disadvantage to a long string is after your balloon reaches space, where there is little wind, that string may still have twists stored up. The rotations might not stop before the balloon pops - it's a trade off.

    My recommendation is a string no less than 50 feet, and preferably ~100 feet. I based these numbers subjectively from results of previous launches.

         Use a rotating swivel so that balloon spin doesn't translate down to the package. It should be relatively frictionless even under strain. Heck, use two of them in series just to make sure. More info on choosing a swivel can be found on the space balloon assembly page.

         Rotational Moment of Inertia is a measure of an object's resistance to changes in its rotation. There are two ways to increase the RMoI: increasing mass, and moving the mass further away from the center of rotation.

    For a balloon, increasing mass is bad as it results in a lower maximum altitude, and at some point exceeds the legally permissible weight limit. The other method to increase RMoI without increasing mass is to make your package in the shape of a donut, as this moves as much mass as possible away from the center of rotation.

    The disadvantage do this is that it's much easier to make a cubic-shaped package.

    I've seen other teams attaching long poles with small masses at each end, far from the center of rotation to increase RMoI. They've had good results.

         Decreasing Wind Resistance . . . If you don't want your package blown around by the wind, make it aerodynamic. Enough said.

         An active Control System is the final option to ensure steady video. This could be a multi-access gyro stabilizer, or two servos to form a pan and tilt. The issue of course is the added weight and complexity, plus the difficulty of determining orientation when your motion sensors are being tossed violently in the air.

    Post Processing Manipulation
    Besides stabilizing the video during flight, you can also use software to stabilize it afterwards. There are various ways you can do this. With less advanced image editing software this involves cutting off the edges of your video and then manually re-centering it every few seconds. (I use Sony Vegas Movie Studio to do this).

    If you search online you can find other video stabilization software packages. More recently Youtube added a video stabilization feature. You push a button and it does it automatically for you. But I personally find this feature distorts the video too much so I rarely use it.

    Lastly, if your camera filmed at a high frame rate (~50fps or above), you could put the video in slo-mo. This creates a false impression that it isn't rocking and swaying nearly as much as before. For video above the clouds I find this works really well.

    Video Length
    The entire flight will likely last ~3 hours, plus the time it takes for you to launch and retrieve it. That's going to be a huge video file! Make sure you calculate how big of a memory card you will need, and that your camera won't run out battery power before the flight ends. Some cameras also won't take videos more than X minutes long or Y gigabytes large. We used a 32GB high speed memory card.

    Test your camera and memory card combination for the entire expected flight time beforehand at home. Do this test in a realistic situation, such as while in the insulated package - to test for thermal shutdown due to overheating.

    You can also use this test time to figure out what software you need to open such large video files - so you can watch the video immediately after retrieval.

    Using Two Cameras
    We almost always have a camera fail. Either due to file size limit issues, overheating, or the off button getting accidentally pressed. Fortunately we always have two cameras on board. One is dedicated to high resolution video, while the other takes high resolution stills every ~10 seconds (the camera has a programmable timer).

    The camera which takes the still images is programmable. We've tried playing around with the various light settings, but haven't had much success on improving average image quality beyond default settings.

    These are the cameras we've flown:

    • Kodak Zi-6 HD Video Cam
    • Canon A530 Still Cam
    • Canon Powershot A480
    • Canon Powershot SD300 Digital ELPH

    The Kodak camera had serious overheating issues. We had two of this model - the first took more than an hour to overheat and the other failed almost immediately. We've since ditched it.

    Live Video Transmission
    In the old days video wasn't stored and recovered - it was transmitted live. We didn't do this and I have no experience with it, but a quick google search should get you more information. The general concept is to couple the A/V output of a camcorder or digital camera to an ATV or SSTV system. Here are some links to help you out:

    Slow Scan (SSTV)
    Fast Scan/Amateur Television (ATV)

    To receive SSTV, you'll need a receiver. You can then attach it to your laptop and view the video using the following software.

    note: Transmission of any video requires having a HAM license.

    Calculating Distance to Horizon
    The higher a near space capsule ascends the greater the distance to its horizon. A simple rule of thumb is that the distance to the horizon in miles equals the square root of one and a half times the altitude in feet. In other words:

    distance in miles = sqrt( 1.5 x altitude)

    You can also calculate Earth's angular diameter using this equation:

    angular diameter of the Earth = arcsine[Re / (Re + altitude)]

    Re is the radius of the Earth (3963 miles or 6378 km).

Get Your Ad Here

Has this site helped you with your robot? Give us credit - link back, and help others in the forums!
Society of Robots copyright 2005-2014
forum SMF post simple machines