# Acoustic Rocket Signatures Collected by Smartphones
## 4 Discussion  
Analysis of the acoustic rocket launch dataset presented in this paper shows both similarities and differences with other studies on rocket launch acoustic signatures. In this dataset, the lowest characteristic frequency of the main launch sequence observed is at 10 Hz, while previous studies [4; 6; 7; 8; 12; 13; 14] have found the characteristic energy of rocket launch signals to be significantly lower. This is unsurprising due to the diminished frequency response of smartphone microphones compared to traditional infrasound microphones, which makes detecting frequencies much lower than 5 Hz with on-board smartphone microphones challenging at low amplitudes. Nevertheless, the 10 Hz peak is sufficiently low-frequency to remain detectable over the full range of this dataset, and studies on far-field data [4; 6; 7; 8; 12; 13; 14] can be referenced for insight into the very-low-frequency features of acoustic rocket signatures. Figure 21: Normalized, start-aligned waveforms of signals collected from Terran 1. Labels on the left-hand \(y\)-axis indicate the estimated propagation distance of each signal. Figure 22: CWT power of start-aligned signals collected from Terran 1 at 17.1 km (**a**) and 26.9 km (**b**) from the launch pad. In addition to the detected characteristic main launch frequency, interesting features were also identified earlier in the launch sequence. Acoustic data from some launches showed a transient signal at approximately 6 Hz leading the 10 Hz signal. Previous work found in conference proceedings also noted the existence of this transient and concluded that it was an indication of solid fuel ignition [16]. Preliminary analysis of the data in this work seems to lend some support to that conclusion, as the 6 Hz transient appeared for 9 out of the 19 launches using solid fuels (47%) and only 17.3% of launches using only liquid fuel. The lack of an observed transient in the other 10 solid fuel launches may be due to any number of environmental conditions on the specific day and does not necessarily contradict the solid fuel ignition hypothesis. However, since the 6 Hz transient appears in data from a number of launches using only liquid fuel (such as the Falcon Heavy launch shown in Figures 8-10, as well as more than two dozen Falcon 9 launches), we believe the origin of the 6 Hz transient signal is either more complex or it has similar characteristics to another signal with a different source mechanism. The exact origin of the 6 Hz transient, while interesting, is beyond the scope of this paper and will be left for future studies. ASTRA was curated and released to the public with the intention of facilitating our own research as well as that of professionals, students, and citizen scientists alike who may be interested in replicating and improving upon the presented results. Preliminary research using data included in ASTRA to train machine learning models to detect acoustic rocket launch signatures in the field in near-real-time has shown promising results and further research into this application is ongoing. The dataset can be accessed and downloaded as a pandas DataFrame [31; 32], with each recording having its own index and named columns containing the following data: raw acoustic timeseries; epoch time of the first sample; sampling frequency in Hz; smartphone make, model number, identification number, and location; launch identification number, reported epoch time in seconds, and location; estimated source-to-receiver propagation distance in kilometers; time-of-arrival estimates; flight number of the launch; and rocket make, type, model number, and number of SRBs used. To replicate the arrival time estimates, the ERA5 2 m temperature and 10 m u- and v-components of wind data [34; 35] are required. The open-source software packages [29; 30; 31] used for data curation and analysis in this work are available online. Conceptualization, S.K.P. and M.A.G. ; Data curation, S.K.P. ; Formal analysis, S.K.P. ; Funding acquisition, M.A.G. ; Investigation, S.K.P. and M.A.G. ; Methodology, S.K.P.