# Skyfall: Signal Fusion from a Smartphone Falling from the Stratosphere
## 4 Results
### Three-Component Sensors: Accelerometer, Gyroscope, and Magnetometer  
The three-component (3C) sensors provide insight into changes in the orientation of the payload. Accelerometers are included in almost all smartphones to detect when the device is picked up and for gaming apps. Gyroscopes are often bundled in with accelerometers to correct for a smartphone's rotational orientation relative to gravity (e.g., screen orientation) and magnetometers are used for navigation. All the 3C channels are sampled unevenly,  
Figure 13: Sensor payloads for (a) audio, (b) high-pass filtered \(Z\)-component of acceleration, and (c) high-pass filtered barometric pressure. and all show three clear stages of descent: the release of the payload with a freefall in the low-density stratosphere, the transition to the denser atmosphere in the tropopause at ~900 s, and the impact with the ground. The accelerometer suggests the phone was deployed screen down and remained so through most of the turbulent descent. All (X-Y-Z) channels in Figure 14 show sign changes on release, consistent with an initial tumble during the initial stages of descent. The horizontal (X-Y) channels show high accelerations during the stratospheric descent and stabilized penetration into the troposphere. Since the DC offset of vertical acceleration was relatively stable at +9.8 m/s2 during most of Skyfall, we can describe the rotation about the Z-axis as a flat spin. All three gyroscope channels in Figure 15 indicate a stable payload while tethered to the balloon with the initiation of rapid rotation after release. This reinforces the inferences made from the accelerometer channels that the payload initially tumbled after release. The gyration about the horizontal X and Y axes (panels (c) and (b) in the figure, respectively) along the base of the payload box are dominated by fluctuations around the value of 0 rad/s during the descent, while the Z channel (panel (a)) appears to show continuous and rapid rotation of the device about the Z-axis from approximately 300 to 900 s into the descent through the stratosphere. The vertical components of the gyroscope and accelerometer support the description of the primary rotation motion as a flat spin. The maximum rotation rate is 13.43 radians/second (2.14 rotations/s) or ~128 revolutions per minute (RPMs) sustained for ~100 s. A lower but more sustained rotation of ~10 radians/second (~95 RPMs) lasted for ~200 s.  
After the 900 s mark (after entry into the troposphere), the gyroscope readings drop and fluctuate about 0 rad/s. This is also consistent with the accelerometer channels. All channels spike at ground impact, with the polarity changes also indicating a tumble. Data collected by the magnetometer along the three axes of the device are shown in Figure 16. The magnetometer is also a good rotation sensor. A slow rotation about the Z-axis is evident in the X-Y components before release while tethered to the balloon, while the magnetic line component along the Z-axis is stable. All magnetometer channels switched polarity on initial release: the payload tumbled. In the X and Y directions (shown in panels (b) and (c), respectively), readings oscillate around 0 uT during descent. For both the X and Y channels, the frequency is markedly higher from about 350 to 950 s into the descent, slowing down thereafter. The amplitudes, however, are similar throughout the descent for both the X and Y channels. The data in the Z direction are shown in panel (a). Unlike those of the other channels, the readings in the Z direction do not oscillate around the value of  
Figure 14: Acceleration channel payload for (**a**) Z-direction component, (**b**) Y-direction component, and (**c**) X-direction component. 0 \(\upmu\)T, but instead remain near \(-5\)\(\upmu\)T after the initial moments of the descent, indicating that the compass direction of the \(Z\)-axis remained relatively steady during the rapid descent, flipping only upon impact as inferred from the vertical accelerometer channel.