Automatic Dependent Surveillance-Broadcast (ADS-B) is one of the key components of the next generation air transportation system. Since ADS-B will become mandatory by 2020 for most airspaces, it is important that aspects such as capacity, applications, and security are investigated by an independent research community. However, large-scale real-world data was previously only accessible to a few closed industrial and governmental groups because it required specialized and expensive equipment. To enable researchers to conduct experimental studies based on real data, we developed OpenSky, a sensor network based on low-cost hardware connected over the Internet.OpenSky is based on off-the-shelf ADS-B sensors distributed to volunteers throughout Central Europe. It covers 720,000 sq km2, is able to capture more than 30% of the commercial air traffic in Europe, and enables researchers to analyze billions of ADS-B messages. In this paper, we report on the challenges we faced during the development and deployment of this participatory network and the insights we gained over the last two years of operations as a service to academic research groups. We go on to provide real-world insights about the possibilities and limitations of such low-cost sensor networks concerning air traffic surveillance and further applications such as multilateration.