# Skyfall: Signal Fusion from a Smartphone Falling from the Stratosphere
## 2 Summary of Systems, Sensors, and Signals  
On 27 October 2020, a Samsung Galaxy S10 [26] smartphone was hoisted by a helium-filled mylar balloon over southern Nevada to capture blast signatures from a surface chemical explosion [23]. This S10 model has been extensively tested and validated operationally in nuclear reactor monitoring studies (e.g., [25]). The S10 smartphone collected geophysical and state-of-health information using multiple on-board sensors. Additional location and barometric pressure information was provided by lightweight external sensors specifically designed for high-altitude balloon deployment and payload recovery. The balloon reached an altitude of 36.3 km before the payload was released; its descent through the stratosphere and troposphere was slowed by a parachute. Although the landing was abrupt--as expected from the non-steerable parachute canopy--the smartphone was recovered intact and has been deployed in subsequent missions. We focus on the environmental and geophysical signals recorded by the smartphone during its rapid descent until its ground impact \(\sim\)30 min later. The smartphone was running the Android version of the RedVox application available through the Google Play Store [27] (an Apple version is also available through the App Store [28]). Signals from the surface explosion are explicitly excluded in this work as they are part of separate studies [21,23]. The choice of smartphones as our low SWaP-C system platform is worth a brief note. We presently experiment with single-board computers; after adding a wireless communication system, various external sensors, tuning the analogue-to-digital parameters, adding a touch screen for navigation, and calibrating and bench testing the system, we end up with a bulky stand-alone digital data acquisition system with a price point comparable to a smartphone. In contrast, anybody with a reasonably modern smartphone can download an app and start recording useful environmental data. Some of the advantages of the smartphone platforms are their ubiquity, accessibility, and immediacy. However, the types of sensors and processors on board a smartphone vary widely and evolve rapidly. All smartphones have a microphone [29] and most of them use micro-electromechanical systems (MEMS) [30; 31]. Original equipment manufacturers (OEMs) often obscure the specifications and configurations of microphones to protect their supply lines and retain their competitive advantage. With tens of thousands of smartphone models available in the wild, standardizing calibrations is a challenging task. The microphones are designed for the recording and transmission of speech, which requires an enhanced response across the 350-4800 Hz passband. [PERSON] and [PERSON] [30] described the pressure reciprocity calibration of MEMS microphones commonly used in smartphones across 100-10,000 Hz. Their measurements showed the MEMS microphone responses to be within manufacturer specifications. [PERSON] and [PERSON] [29] measured 1/3-octave sound pressure levels and reverberation time with iPhone applications (apps) and compared them to a Bruel & Kjaer (B&K) Hand Held Sound Level Meter Type 2250. This study found that iPhone and B&K sound pressure levels were typically within 5 dB of each other, and that iPhone internal microphones had a limited dynamic and frequency range; namely, the iPhone internal microphone had a poor response below 200 Hz due to high noise floor. Further literature describing the frequency magnitude response of smartphone microphones is limited. Examination of various smartphone sound apps by [PERSON] and [PERSON] [32] found some of them accurate and reliable in occupational noise measurements. In follow-up studies [33; 34], the accuracy of these apps when used with two different external calibrated microphones was examined. The authors showed that the accuracy and precision of smartphone measurements can be significantly enhanced with the use of external microphones.