# The Design of an Energy Harvesting Wireless Sensor Node for Tracking Pink Iguanas
## 5 Energy Consumption Tests  
In this section we present the experimental results about the energy consumption of the board with reference to its specific operating modes. Figure 12 shows the simplified finite state machine implemented in the CC1310 controller. In the IDLE state all power sections are switched off and the only power absorption is due to the LTC3106 and NCP170 LDO. The current absorbed by the main chip is 100 nA. In the position state, the GPS is active (absorbing 89.1 mW) during the attempts to acquire the position. Then, the sensors are activated and the data are collected and saved in the flash memory. Periodically, the system checks the flash memory to verify if there are still data to be transmitted. In this case, it attempts to transmit the data and waits, in the receiving state, for an acknowledgement packet. Table 1 reports the board average power consumption in the various states. The acquisition of the position by the GPS is the most power-consuming activity. The GPS unit needs a large amount of current to power the transceiver. Such high power consumption is caused by the fact that the chip has to be active for a long time, of the order of seconds, needed to increase accuracy in retrieving the position. Figure 11: Behavior of the voltage of the supercapacitor during continuous operation. Figure 10: Behavior of the relative humidity corresponding to the temperature in Figure 9.  
The time to retrieve the GPS position is then a crucial parameter for energy management. For this reason it has been carefully evaluated in a repeated procedure of position acquisition and reset. Measurements were collected placing the device in a open sky condition. The response time of the device has been defined as the time necessary to receive the first valid NMEA message of the type GPGGA from the GTOP PA6C GPS receiver (GPS fix). Figure 13 shows the complementary cumulative distribution function of the GPS response time. The distribution shows that about 40 s is the typical time necessary to get the first data, however,there is also a non negligible tail of the distribution that push this time up to 52 s.  
After the first fix, the accuracy of the position increases. The accuracy is measured by the HDOP parameter (Horizontal Dilution of Precision) [20] which is embedded in the NMEA message. In Figure 14 the value of the HDOP found in the tests is plotted as a function of the time from the first fix. A value of HDOP less than 1 is associated to the highest possible confidence level of the position estimation. The average of the HDOP after the first fix is only slightly larger than 1, however the wide dispersion in the parameter makes the first fix scarcely reliable. After 30 s from the first fix the mean value of the HDOP converges to 0.8 with a negligible dispersion. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**State** & **Power Consumption** & **Duration** & **Description** \\ \hline _Idle_ & 0.054 mW & - & In the _Idle_ state all the power sections are switched off. The power absorbed can be accounted to the LTC3106 and to the LDO. It is worth to note that the CC1310 is in the stand-by state, retains information on RAM consuming about 100 nA. \\ _GPS On_ & 89.1 mW & 60 s & The greatest part of the power consumption is accountable to the GPS modules that absorbs about 25 mA. \\ _Sensor On_ & 0.61 mW & 5 s & The main absorption is accountable to the UV sensor (0.1 mA). \\ _Flash On_ & 128.7 mW & 0.1 s & The flash memory writing power absorption is about 35 mA. \\ _TX On_ & 1254 mW & 0.5 s & The power consumption of transmission mode has been measured using the continuous wave. \\ _RX On_ & 29 mW & 3 s & The receiving state mode absorbs about 6 mA. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average power consumption. Figure 12: Simplified Finite State Machine of the controller. The energy requirement of the GPS unit affects the dimension of the super-capacitor.