# Skyfall: Signal Fusion from a Smartphone Falling from the Stratosphere
## 4 Results
### Three-Component Sensors: Accelerometer, Gyroscope, and Magnetometer
High-passed filtered channels are shown in Figure 17, with their corresponding estimated short-term Fourier transforms in Figure 18. The waveform plot amplitudes are normalized relative to the maximum values. The three main stages of the Skyfall are clear in all channels: initiation, transition to troposphere, and ground impact. The accelerometer, gyroscope, and magnetometer provide a cumulative nine channels of data and can be integrated with the high-resolution height estimates from the barometer to provide a cohesive Skyfall narrative with a high level of confidence. The signal fusion reveals that throughout the flight and descent, the payload was in a flat spin along the vertical axis, with different rates of rotation at different stages: slow before descent, frantic (>10 rad/s) in the stratosphere, and slowing down in the thermosphere. The payload only tumbled twice: at the onset of Skyfall and after ground impact. The impact was abrupt but relatively moderate at <10G due to the parachute reaching stability before landfall. All channels show an interesting transition as the S10 enters the troposphere, where it is hit by strong  
Figure 16: Magnetometer channel payload for (**a**) \(Z\)-direction component, (**b**) \(Y\)-direction component, and (**c**) \(X\)-direction component. Figure 15: Gyroscope channel payload for (**a**) \(Z\)-axis component, (**b**) \(Y\)-axis component, and (**c**) \(X\)-axis component. The maximum rotation rate is 2.14 rotations/s (\(\sim\)2.14 Hz) about the \(Z\)-axis. shear winds and where the parachute may have malfunctioned. Although it would be interesting to zoom into the tropopause penetration stage around the 950s timeline, it will substantially increase the length of the manuscript to no clear advantage. The reader is welcome to explore the data and tools at their disposal. The tonal structure in the spectra in the rotation-sensitive channels (Figure 18) is a measure of the flat-spin rotation frequency, and there is no evidence for tumbling outside the initial and terminal states. The spectra also support the narrative that the payload tumbles only briefly after release and after impact. The gyroscope spectra reinforce the time-domain representation, as expected. More interestingly, the X and Y components of the magnetometer spectrograms confirm the  
Figure 17: High-pass filtered (hp) waveforms for sensors: audio, barometer (Bar), accelerometer (Acc), gyroscope (Gyr), and magnetometer (Mag). X-Y-Z coordinates are shown in Figure 5. The vertical axes are normalized to the peak absolute magnitude of each channel. Figure 18: Short-term Fourier transforms of high-pass filtered (hp) waveforms for sensors: audio, barometer (Bar), accelerometer (Acc), gyroscope (Gyr), and magnetometer (Mag). The color scales to the maximum amplitude. The vertical axes represent logarithmic frequency in hertz to the sensor Nyquist. The standardized minimum frequency scales with the record duration and the sample rate [56]. -2 Hz maximum revolution rate of the payload in the stratosphere obtained from the gyroscope. The magnetometer was sampled at ~100 Hz, with a Nyquist frequency at 50 Hz. The base-2 logarithm (bits) [56] of the spectral magnitude of the X component of the magnetometer is shown in Figure 19. The peak spectral magnitude envelope matches the vertical component of the gyroscope in Figure 15, with a peak revolution rate of ~2 Hz. The flat spin loses its speed and stability after 950s when it enters the troposphere, as evidenced by the smearing of the spectral energy to lower frequencies. It would be possible to further decrease the uncertainties in rotation speeds with signal fusion in the frequency domain if desired. For example, standardized multiresolution time-frequency representations [56] would increase the temporal resolution of the lower frequency bands and permit direct comparison between signal modalities. Raw data and Python code are provided to reconstruct the results in this paper, as well as produce additional results not presented herein.