One of the most exciting things of each of Passepartouts flights is the recollection of the balloon – as it can come down more or less everywhere! There are two possibilities to find the balloon: A team can be sent out into the predicted landing area and then be directed via the received flight data to the actual landing spot, or we just leave an address on the balloon so any potential finder can get into contact. Both methods are applied with Passepartout. But how do we know, where the balloon is headed to and where is its actual landing spot? This will be discussed in this article.

A radiar reflector hangs directly below the payload of Passepartout

Before take off, the most likely direction of flight can be predicted to send a rescue team ahead. In principle, this prediction is based on some straight forward principles: the rate of rising is faster the greater the difference between the surrounding gas and the gas in the balloon is. This enables us to calculate the rate of rising, which along with measurements of airflows allows the prediction of a likely landing zone. However, real life is more complicated: the density of the air changes with hight, along with its temperature changes which again influence the speed with which the balloon rises. These parameters are subject to continuous changes, which therefore leads to uncertainties in these predictions. Up to now, these errors were between 5 and 40 km.

There also exist several possibilities to directly follow the balloon, either by using GPS, transponders or radar reflectors. GPS uses the fact that electromagnetic waves travel with the speed of light. Basically, the signal consists of the time at which it was sent out, which enables the receiver to calculate the time difference since it has been emitted. If this is done with three signals, this already defines a location – to be sure, however, a fourth signal is needed to really get a reliable 3D position. However, this system has some possible errors: lets assume we are receiving a GPS signal inside a city, and one of the signals has been reflected from a building before reaching the receiver: this will result in a longer time difference, which even if it is only 0.000000003 seconds off, can lead to differences of meters! This is why using GPS within buildings is not sensible.

Why is there talk about a European system, instead of just continue to use the American GPS? The main reason is the fact that GPS is a military system, but nearly everything today depends on it in some way or the other. What would happen if the US ever decide to turn it off for any other users?

Galileo Satellites (c) ESA

Transponders are very widely used in airtravel. The basic principle of the secundary radar system is very similar to the idea of GPS. The difference is that a signal is only received from a small area, which takes the position of the sender out of the equation. This is why the radar is always turning around 360° (as is seen in many movies – along with the well known radar screen). Such a transponder can also send its identification, which is very helpful along with its position and height.

The last system is a primary radar: Again, it is based on a transponder idea, with the difference that the sender itself is passive. This system is used in shipping and also weather balloons. They apply radar reflectors, which use the principle of reflection: If light hits a mirror, its reflection angle is the same as the infalling angle. For radar waves, this can be done with simple metallic surfaces. Since the signal has not only to travel from a transponder to the receiver, but have to cover twice the distance (back to the transponder) this system needs more energy. The advantage is, that for example planes can be found even if they do not have a transponder onboard. The system which is in use in Austria is known as “Goldhaube” (golden hat) and cooperates with each of Passepartouts flights.
On the 13th of August, all those different methods to track the balloon are applied: GPS, primary radar, APRS (Automated Packet Reporting System, which is used in amateur radio communication, see our next article for more details!) and Globalstar (an alternative to GPS).