On March 15, 2012, Felix Baumgartner moved a little closer to a record-breaking jump. He left a ballon at an altitude of 71,581 feet. The goal is to get to the 120,000-foot altitude. He plans to break the following records:
- Highest exit altitude for a skydive.
- Greatest distance of free fall without a drogue chute.
- Fastest vertical speed without a drogue.
If you want to break a world record, you need to somehow measure the feat. How do you determine the speed of a falling skydiver? In this case, the extreme height and speeds makes the process rather challenging. This is the problem that the Red Bull Stratos team along with Brian G. Utley of the National Aeronautic Association had to face.
Recording Options
So, what do you want to measure? At the very least you need to know the position and time of the skydiver. Here are some options you could consider to accomplish this.
Pressure. This is essentially the method normal sky divers use - called an altimeter. The device is basically just a calibrated pressure gauge. As you move down in the atmosphere, the air pressure increases (just like going deep in a swimming pool). Based on this change (and some assumptions about the air) you can determine the height. This isn't an exact measurement since it depends on the air and air can change. For the Red Bull jump, the changes in pressure are even more extreme. Overall, this altimeter method might work for normal skydives but it wouldn't be accurate enough here.
Accelerometer. What if you had a device like a smartphone with an accelerometer in it? As the sky diver falls, the device could record the acceleration of the fall. (Well, it would record the g-force from which you could determine the acceleration.) From the acceleration data, you could numerically integrate to determine the velocity and do it again to determine the position. Here is an older post with the details of using an iPhone to determine altitude of a skydiver. But there are a couple of problems. First, the integration of the acceleration data will tell you the change in altitude. You have to know how high you start. That is a problem. Also, when you integrate twice, you introduce some error into the measurement.
Radar. I don't know why this wouldn't work, but I don't think the Red Bull team uses radar to measure the altitude. I guess you could though. Just take a radar and point it at the skydiver. There must be some reason that this isn't practical.
Global Positioning System. This is in fact what the Red Bull Stratos team used to record the altitude. Even so, it has some problems. GPS really wasn't intended to be used this way. The US Department of Defense places restrictions on commercial GPS units so that they can not be used at heights above 60,000 feet or at speeds over 1000 knots. Why? Well, since GPS devices are so cheap now they would work great in some type of guided missile, wouldn't they?
There are some devices that will work beyond these limitations, but they are classified as "munitions" by the U.S. government and not able to be exported. The Stratos team chose the Garmin 18X-5 for use in the test jumps. Besides just being a "munition", the 18X-5 also supports the Wide Area Augmentation System - a protocol to increase the resolution of the GPS device. to around the 3-meter range.
Would GPS Work?
You don't want to just assume this will give you nice data. To test the data collection process, the team first dropped an unmanned test pod.
So, you put this on a balloon, take up to 90,000 feet or so and drop it. Record the data and compare the values to what you get from a doppler radar (which they did use along with the test pod). The answer is "yes," it seems to work.
But the GPS gives position and time. What about the velocity? Once you have position, you can find the velocity (and speed) by looking at how far the jumper moves in between each GPS position measurement. Really, it is a little bit tricker than that -- but you get the basic idea.
