# Scheduling Sparse LEO Satellite Transmissions for Remote Water Level Monitoring
## 2 Materials and Methods
### Swarm Satellite Transmission  
To minimize transmission power consumption requires understanding how the satellites, transmission, and data plans work. There is not always a satellite overhead, nor are the elevation angle and environmental conditions (e.g., background RF noise) always suitable. Data rate is limited, and frequent transmissions consume energy. These factors critically impact how we orchestrate transmissions.  
The nature of Swarm satellite passages is disclosed by their Web-based tool that lists upcoming satellite passes, their times, durations and max elevation angles for a given location. Elevation angles observed in Montreal, Quebec, Canada range between 15 and 85 degrees, and pass durations typically range between 10 min and an hour. In reality, even with a satellite pass, the modem might not always be able to transmit. There are many factors impacting this: satellite pass \"quality\", RF background noise, environmental conditions, antenna setup, and many others.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Mode** & **Typical Current at 5 V/3.3 V** & **Typical Power at 5 V/3.3 V** \\ \hline Transmit & 550 mA/850 mA & 2.8 W/2.8 W \\ GPS Acquisition & 45 mA/45 mA & 230 mW/150 mW \\ Receive & 26 mA/26 mA & 130 mW/86 mW \\ Sleep & \textless{}110 \(\mu\)A/80 \(\mu\)A & \textless{}550 \(\mu\)W/260 \(\mu\)W \\ \hline \hline \end{tabular}
\end{table}
Table 1: DC power characteristics of 4 modes of operation for 5V/3.3V power supply.  
Figure 4: State machine of the Swarm M138 operating modes.  
The first factor, satellite pass quality, is due to the pass duration and maximum elevation angle. Swarm gives no guidance on what factors impair successful transmission, and one objective of this paper is for each sensor to construct an empirical model for quantifying the likelihood of a pass leading to successful transmission. The second factor, RF background noise, does have guidance provided by Swarm, Table 2, by which noise intensity of -93 dBm or lower is expected for successful transmission.  
There are also the constraints imposed by Swarm data plans, priced at USD 60 per year per data plan, with up to four data plans stackable onto a single modem. Each data plan permits up to 750 packets per month, or about 25 packets per day, or about one per hour. These constraints imply that for finer temporal resolution (e.g., every 15 min), one must either bundle measurements, or pay to stack multiple data plans. The later, costly option also reduces the battery life, while bundling reduces the number of packets and possibly the cost.  
The high-level view of the two main processes is shown in Figure 5. The process on the left produces and inserts the data into a circular queue. Since the Swarm modem's internal queue can drop packets after 48 h, the circular queue needs to contain 48 h of data. Each cycle of waking from sleep, acquiring GPS, listening for a satellite, and transmitting uses a lot of extra energy. In addition, due to environmental variables, there is inherent uncertainty as to how long one can expect the modem to be awake before transmitting successfully. This precise question is examined in the rest of the paper.