# The Design of an Energy Harvesting Wireless Sensor Node for Tracking Pink Iguanas
## 4 Sensors Calibration  
In this section we present the various issues we tackled calibrating the different sensors available on the board. Accelerometer and GPS have been used on the basis of the characteristics provided in the respective datasheet. Thus, no specific calibration has been performed for these devices. Different was the case of the UV sensor, in this case, a possible UV attenuation by the PVC case could occur and then the actual correspondence between the signal provided by the sensors and the intensity of the radiation in the UVB band has been measured. In previous reptile studies [6] the UV-B intensity was measured with a Digital Ultraviolet Radiometer (Model 6.2201 UVB) made by Solartech Inc. This is a portable instrument based on a silicon carbide photodiode which is rather insensitive to the UVA spectral band with a peak of responsivity in the range 240-320 nm [18]. This device was shown to be sufficiently accurate for the scope and it has been used as reference to test the embedded sensor. The digital ultraviolet radiometer has been used to measure the transmission of light in the UVB band of PVC during exposure to sunlight. Measurements were performed in sunny days in Rome at a latitude of approximately 41\({}^{\circ}\) North. It is worth to consider that Galapagos islands are close to  
Figure 4: WSN inside and outside the packaging. Figure 5: On the left, animal with the device glued. On the right, the attachment of the WSN on the iguana back. the equator with a latitude of about \(-0.7^{\circ}\). Figure 6 shows the power intensity (W/m\({}^{2}\)) measured exposing the ultraviolet radiometer to the direct sunlight versus the power intensity measured behind the PVC case. Different intensities correspond to different hours of the day. The attenuation is of the order of 50%; however, the transmission is rather constant in the range from 20 to 200 W/m\({}^{2}\) of incident radiation. The linear regression of the collected data provides the estimation of the intensity in the UVB range from a measure of the light intensity from the inside of the case. The relationship between measured and estimated intensities is \(I=A\cdot I_{meas}+B\) where \(A=2.42\pm 0.02\) and \(B=2.51\pm 1.25\). The ratio of the intensity of the solar radiation in the UVA (\(\lambda\) = 320-400 nm) and UVB (\(\lambda\) = 280-320 nm) ranges has been evaluated to be about 20 for most of the part of the day, and in particular when the elevation of the sun is larger than 60\({}^{\circ}\)[19]. At low elevations, due to the atmospheric refraction, the ratio quickly increases exponentially. Considering that the absorption of UVA is negligible in PVC and the ratio of UVB/UVA of the solar light is constant for a long part of the day, instead of measuring directly the UVB intensity attenuated by PVC, we decided to measure the more intense UVA radiation and to estimate the UVB intensity. For the scope a UVA detector (VEML6070 Vishay Semiconductors) has been used. This is an integrated UV sensor that besides the photodiode include, in a single chip, also the amplifier and the digital interface. The VEML 6070 is temperature compensated and shows a linear responsivity to the UV light with a saturation at 328 mW/cm\({}^{2}\). The peak of responsivity lies in the UVA range. The VEML6070 UV light sensor has been calibrated with respect to the UVB intensity measured by the ultraviolet radiometer. For the scope, the sensor and the ultraviolet radiometer have been exposed to the sunlight. Results show a good reproducibility at different intensities of sunlight and at different weather conditions. Figure 7 shows the calibration data fitted by a straight line. The linear fit achieved \(R^{2}=0.96\). The UVB intensity has been estimated from the calibration curve. Figure 8 shows the behaviour of the intensity of UVB radiation in a partly cloudy day. Samples have been collected every 2 s during a typical partly cloudy day starting from 10:00 to 16:40.