## 2 Experimental Setup and Mission Architecture  
### The ISM-HAB Platform  
The core of the experimental vehicle is a commercial latex 1200 g HAB filled with industrial-grade helium. The balloon is released from the ground and climbs up to the stratosphere, generally attaining a height between 18 and 40 km. The amount of helium is calculated in order to have a pre-determined amount of lift at ground, so to carry the balloon up to the desired altitude at a speed of about 3 to 5 m/s.  
The balloon lifts a group of objects to the stratosphere, including the scientific instrumentation, a small parachute and the relative suspension lines. The objects attached to the balloon are often referred to as the \"payload train\" (Figure 1). In the configuration chosen for ISM-HAB, the train includes a 1.5 m diameter parachute, a radar reflector and the custom payloads designed to perform the ISM measurements and the characterization of the gondola mechanical oscillation. The payload package also includes tracking devices, sensors and cameras.  
As it ascends through the atmosphere, the balloon expands up to a certain maximum size as a consequence of the decrease in external pressure. Upon attaining a certain maximum altitude, the balloon bursts because of structural failure of the latex membrane under the internal pressure. Figure 2 shows the moment of bursting as captured by the onboard camera of one of the ISM-HAB flights. After bursting, the payload train starts free-falling until the parachute is fully opened and the descent speed decreases down to the limit (typically 4 to 5 m/s) imposed by the size of the parachute and by the attached mass.  
Figure 1: The Industrial-Scientific-Medical High Altitude Balloon (ISM-HAB) sounding balloon flight train. (**a**) Immediately after release at launch. (**b**) Schematic.  
In all of the five flights performed, the payload included the following main systems:  
* The tracking payload, used to track the HAB and to collect data from the onboard sensors.
* The RF payload, used to perform the measurement of the aggregated RF power received over the bands of interest.
* The dynamics payload, including the IMU and the associated data handling and storage systems.  
Some components of the payloads are attached directly to the support base, consisting of a triangular wooden frame, namely the SPOT Globalstar modem, the cameras, the external sensor of the on-board computer and the RX antenna of the RF payload (Figure 3b). All other components are installed within a Styrofoam box (Figure 3a), the only exception being the Automatic Packet Reporting System (APRS) transmitter that is attached to the cord connecting the parachute to the box. APRS is a standard protocol mostly used by radio amateur operators to broadcast Global Navigation Satellite System (GNSS)-based location data and other environmental information. Such data is essential to help track the balloon in near real-time and recover the payload after landing.  
Figure 3: The ISM-HAB payload gondola: (**a**) the main payload Styrofoam box, and (**b**) arrangement of the box and other components on the base frame.  
Figure 2: Burst of the balloon, as captured by the onboard camera.  
The active components are powered by means of standard power banks (Aukey PB-N28, mainly used to power the RF payload, and Li-Ion 3.7 V used to extend the lifetime of the internal battery of the camera) and battery packs with AA batteries. This architecture is able to power all the components for at least 5 h, a duration much longer than the average duration of a mission. The overall architecture of the ISM-HAB system is schematically shown in Figure 4. The four blocks depicted correspond to the four locations of hardware in the ISM-HAB setup: inside the gondola Styrofoam box, on the external surface of the gondola box, on the base frame below the gondola and along the flight line.