# Acoustic Rocket Signatures Collected by Smartphones
## 2 Materials and Methods
### Data Alignment
Previous studies [4; 7; 8; 12; 13; 14] have shown that characteristic acoustic frequencies of rocket launch signals are higher than 0.5 Hz, and preliminary observation of the data showed a decrease in the signal-to-noise ratio below approximately 50 Hz, thus the frequency limits of the passband are set to 0.5 Hz and 50 Hz. As the main phase of the rocket launch signature persists over a timescale on the order of 10 s to 100 s, a rolling median filter is applied to the absolute value of the windowed and bandpassed waveform with a kernel duration of 15 s. The peak of the median-filtered  
Figure 2: Normalized waveforms collected at three different stations during NASA’s Artemis I launch, plotted relative to the start-aligned estimated time of arrival at each station. A vertical green line indicates the arrival time estimate, and labels on the left-hand \(y\)-axis indicate the estimated propagation distance of each signal. time series is then determined, the corresponding epoch time of which is the second arrival time estimate. We call data aligned this way \"peak-aligned\", and the associated arrival time estimate is labeled as such in ASTRA. Aligning the dataset this way is useful for observing characteristics of the signature that are more stable over the propagation path, such as the frequency of peak energy. Unlike start alignment, the accuracy of peak alignment is partially dependent on the signal-to-noise ratio of the waveform, but it is less reliant on the accuracy of the travel time estimation and reported launch time. The peak selection process is visualized in Figure 3, and an example of peak-aligned data is shown in Figure 4.  
Figure 4: Normalized waveforms collected at three different stations during NASA’s Artemis I launch, plotted relative to the peak-aligned estimated time of arrival at each station. A vertical green line indicates the arrival time estimate, and labels on the left-hand \(y\)-axis indicate the estimated propagation distance of each signal. Figure 3: A visualization of the peak alignment estimated arrival time selection process. In panel (**a**), the normalized, unfiltered waveform is plotted in black, the selection window is indicated by green shading, and a dashed green line marks the closer-range station’s estimated arrival time, which is used to place the window. In panel (**b**), the windowed, bandpassed, and re-normalized waveform is shown, with the selected median-filter window indicated with darker shading. In panel (**c**), the result of median filtering the absolute value of the bandpassed waveform from panel (**b**) is shown, with the selected peak indicated by a solid green line. Which type of alignment should be used depends on which phase of the rocket launch sequence is of interest. For example, if analyzing the ignition signal, start alignment is more useful, as propagation effects cause the signal to become more emergent as it travels and peak-aligning the signals will misalign the ignition signals (see Figure 4). In contrast, if the goal is to analyze the characteristics of the signal generated during the rocket's ascent, peak alignment may be more useful.