## 1 Introduction  
Industrial, Scientific and Medical (ISM) bands are those portions of the electromagnetic spectrum that are made available for non-telecommunication applications. Typical uses of the ISM bands are for domestic appliances, such as microwave ovens and door openers, and for the omnipresent Wi-Fi and Bluetooth networks. As per Reference [1], \"... Radiocommunication services operating within these bands must accept harmful interference, which may be caused by (ISM) applications... \". Due to their unlicensed status, the popularity of ISM bands is growing very fast in the telecommunication sector, in spite of the possible interference issues. The use of ISM bands is especially appealing for Internet of Things (IoT) applications [2], where interference issues are limited by the low power, short burst nature of messages and by the use of spread-spectrum techniques. Several terrestrial operators plan to deploy IoT networks over a geographical area using Low-Power Wide Area Network (LP-WAN) protocols, such as LoRa [3] and Sigfox [4], that use the ISM sub-GHz frequency bands (868 MHz in Europe and 900 MHz in the USA). During its early phase, Google's Project Loon made use of 2.4 and 5.8 GHz ISM bands to link stratospheric balloons to each other and with the ground [5]. Recently, several initiatives have been started aiming at defining new IoT protocols suitable for the satellite environment, and several attempts are underway to adapt existing terrestrial technologies to the satellite environment. The possibility to use portions of the terrestrial ISM bands for telecommunication between the spacecraft and ground terminals is a potential disruptive factor for the development of satellite-based IoT networks. However,as a result of the growing popularity, the number of devices operating in these frequency bands is rapidly increasing, potentially leading to further congestion of this portion of the spectrum to the extent that communication performance could be seriously degraded. Several studies exist on the ISM interference in a terrestrial setting, e.g., References [6; 7; 8; 9]. However, in order to design and operate satellite communications in the ISM bands, one needs to have a full understanding of the background electromagnetic noise at orbital altitude due to terrestrial ISM emissions. The work reported here was carried out in the frame of a recent study by the Italian company MBI for Eutelsat, aimed at gathering experimental data to investigate orbital altitude ISM noise, as part of an ongoing study related to the feasibility of a Low Earth Orbit (LEO) constellation for IoT applications [10]. The approach chosen was to take measurements at stratospheric altitudes of about 25 to 30 km using high-altitude balloons (HAB), from which one can estimate the background noise level at LEO by extrapolation. The most common and most cost-effective implementation of HABs is the sounding balloon, also known as weather balloon. Such vehicles are commonly used to collect meteorological information--they are actually the primary source of data for weather forecast worldwide [11]. With about 900 launches worldwide twice a day, the sounding balloon technology is mature, well-understood and reliable. Sounding balloons, made of natural latex or neoprene film, are inexpensive and easy to procure and setup, and can be operated by a small crew with minimal training. The relevant authorizations from air traffic authorities can be obtained easily and quickly in most countries worldwide. When properly instrumented with state-of-the-art onboard electronics, HABs can effectively act as miniaturized high-altitude platforms (HAPs), being to the larger HAPs what Cubesats are to larger satellites. In addition to their well-established role in scientific research, they may prove very valuable as a test bench for HAPs technologies and, to some extent, for satellite technologies as well.