# Skyfall: Signal Fusion from a Smartphone Falling from the Stratosphere
## 3 Materials and Methods
### Balloon Platform  
The flight system consisted of a 4 kg Totex [58] weather balloon carrying a 2.25 kg instrumentation package (Figure 1). The flight line was a 10 m paracord tether from the balloon nozzle to a Balloon Ascent Technologies HAB Bournder Balloon Cut-Down Device [59]. The Bounder was geofenced to automatically terminate the flight if the balloon approached the Las Vegas metro area. The cut-down device was connected to a fishing swivel tied to the top loop of a 1.21 m Rocketman parachute [60]. The descent rate for the 1.21 m standard parachute at 2.3 kg is specified at 6 m/s (-21 km/h). The fishing swivel allows the payload to rotate independently of the balloon, which tends to prevent violent spinning during ascent. A High-Altitude Science Stratotrack APRS (AutomatedPacket Reporting System) [61] transmitter beneath the parachute was intended to provide one-minute balloon location and ambient temperature readings during flight.  
The key elements of the payload box were a PolarTech insulated shipping carton containing a SPOT Trace [62] asset tracker to report the landing location and a Samsung Galaxy S10 [26] in a protective case taped screen-down to the bottom of the box. The dimensions of the payload box were 38.1 cm long, 33 cm wide, and 25.4 cm high; the rectangular shape of the payload enclosure may have helped stabilize the descent, as the vertical axis remained relatively stable until impact. The payload box was sealed with duct tape prior to launch, but a hole in the side allowed a venting tube to pass air between the exterior and interior of the box.  
There were four independent instruments for estimating location; however, two of the four instruments only provided sparse estimates intended for recovery. The APRS and SPOT Trace were primarily intended to provide redundancy for the payload recovery. Only the Bounder and the S10 smartphone had sufficiently high sample rates to reconstruct the details of the rapid descent trajectory, and both of these instruments had a barometer to provide secondary height estimates. The Bounder relies exclusively on the GNSS for position, whereas the S10 uses both GNSS and cellular location when available. Only the S10 used a cellular network to transmit data in real time when an adequate signal from a cellular network was available.  
The balloon was inflated in a hangar at Desert Rock Airport, then launched at 12:16 UTC on 27 October 2020 into a brisk wind blowing to the south/southwest. The APRS tracker provided the balloon's location every minute until it became too cold during the early dawn ascent. It resumed reporting positions once the sun had risen above the horizon and the tracker had risen high enough in the stratosphere for ambient temperatures to come back into range.  
The balloon burst at 13:47:50 UTC at an altitude of 36,296 m, landing at 14:13:25 UTC at an elevation of 1038 m. Landing was first indicated by the RedVox S10, which reported that it was no longer moving and that the ambient temperature was rising as the payload box warmed up after landfall. The SPOT Trace then confirmed the landing site. The payload was recovered at 17:00 UTC on the same day. The payload box was upside down, and large amounts of balloon material were tangled on the flight line and the parachute (Figure 2). This suggests that the parachute may not have deployed properly and that the payload may have descended faster than planned. Although the box was externally scraped during the hard landing, the S10 was undamaged and has been redeployed in subsequent missions.  
Figure 1: A meteorological balloon carrying infrasound sensors in a PolarTech insulated shipping container in 2015. The parachute is visible between the box and the balloon. The Skyfall set-up is very similar to the one shown and described here.