## 2 Wireless Sensor Network
### GNSS Sensor Node  
Figure 3a schematically shows the GNSS (global navigation satellite system) receiver node including its hardware blocks, which we also introduced in [41]. The node basically consists of the same hardware components as the sensor nodes introduced above in Figure 1, but is additionally equipped with a GNSS receiver. The NovAtel OEM615 GNSS receiver module supports GPS, GLONASS, Galileo and Compass frequencies and sampling rates up to 20 Hz. In active mode, the GNSS receiver draws constantly around 400 mA. To reduce the average power consumption, the module is activated only periodically and can be switched off completely during times when it is inactive. Figure 3b shows a photo of the GNSS receiver node. The GNSS receiver module is connected to the wireless sensor node by a 20-pin connector. The satellite antenna is a GPS-701-GG antenna from NovAtel. A UART interface is used to configure the GNSS module and to read out the GNSS data periodically. A GNSS range message generally consists of four fields as depicted in Figure 4. The first 28 bytes are the header that includes information like message type, time, length, etc. The next four bytes' large field gives the number of valid observations; the third 24 bytes' large field includes all valid observations; and the last field is a 32-bit cyclic redundancy check. Altogether, a range message varies in its size from 50 bytes in the case of one valid observation to 516 bytes for 20 valid observation. Figure 3: (**a**) Schematic of the GNSS (global navigation satellite system) wireless sensor node. (**b**) Photo of the GNSS sensor node. At a sampling frequency of 4 Hz, this sums up to 200 bytes per second for one valid observation and 2064 bytes per second for 20 valid observations. At a sampling frequency of 20 Hz, the observation of one satellite results in 1000 bytes per second, and the observation of 20 valid satellites results in 10,320 bytes per second. We use a baud rate of 230 kBit per second (equal to 23 kByte per second for an 8N1 transmission) to transmit data from the NovAtel board to the sensor node. The GNSS data are firstly stored in an internal 14-kByte memory buffer and then transferred periodically onto a 4-GB Micro SD Card. To increase the speed, the MCU is running at 28 MHz during this process. By assuming a sampling rate of 4 Hz and 15 valid observations per message (396 bytes), a 4-Hz logging frequency (1584 bytes per second) for a duration of 30 min (1800 s) adds up to 2851.2 kByte, which have to be transferred wirelessly. At 20-Hz logging, we expect around 14,256 kByte data sampled during a 30-min logging period in the case of 15 valid satellites. An important parameter to consider is the lifetime of the GNSS sensor node, that is the time the node can autonomously operate before it dies due to low energy. As presented in [34], the lifetime is connected to the node's battery capacity and to the times where the node is actively logging (\(t_{log}\)) or communicating (\(t_{com}\)) as the sensor node requires most energy during these periods. During idle times (\(t_{idle}\)), the node requires only marginal power in comparison to active times. Figure 5 depicts \(t_{log}\), \(t_{com}\) and \(t_{idle}\) during a certain time interval \(t_{int}\). To calculate a certain lifetime, we considered one logging interval (\(t_{log}\)) to be 1800 s and the corresponding communication interval (\(t_{com}\)) to be 900 s for a 4-Hz logging frequency and 3600 s in case of a 20-Hz logging. The interval (\(t_{int}\)) is 24 h. This means if the sensor node does one logging in 24 h, it is in idle state for 83,700 s ((\(t_{idle}=t_{int}-t_{log}-t_{com}\)). Figure 6 shows the lifetime of our GNSS sensor node plotted over an increasing number of logging and communication intervals in 24 h assuming a battery capacity of 40 Ah and no energy harvesting. Figure 4: Structure of a range data log.