For several years, systematic research has been carried out on the pollution of the night sky by artificial light in the city of Toruń11,36,38. The main objective is to monitor this phenomenon, including its spatial and temporal variability and the most important factors affecting it. Based on previous experience, an internal measurement device was built to automate the data acquisition process.
Genesis of the project
The first measurements related to the phenomenon of pollution of the night sky in Toruń were carried out in the fall of 2017, followed by regular observations using a portable photometer SQM (Unihedron, Canada) within the framework of a project implemented in 2017-2018. To this end, a permanent measurement network distributed throughout the city has been set up, consisting of 24 locations. During an overnight measurement session taking place during the astronomical night (there is no such period at the latitude of Toruń in summer), sky brightness was measured at all sites. Spot monitoring results were plotted using interpolation methods and visualization tools available in GIS systems11.39, which determined the spatial distribution and extent of light pollution in the night sky. The intensity of this phenomenon at each of the points studied was also explored in relation to the categories of land use and the types of urban development distinguished.
Repeatable measurements performed regularly over such a long period of time had significant limitations. A measurement session was very time-consuming, as it lasted about two hours, during which all measurement sites were visited, covering a distance of almost 50 km each night by car. Despite meeting all deadlines and adhering to the fieldwork plan, measurements were not performed simultaneously at all sites, which affected the results, especially during the night with changing cloud cover. Although the measurements were carried out with great consistency and care, they were carried out within a spatial buffer of approximately 5 m, which could unintentionally slightly affect the results obtained. An additional limitation was also a single nightly measurement at one point, instead of a whole series of measurements at specific time intervals. Inaccuracies in readings during a single session could have been caused by sudden changes in weather parameters. In the procedure adopted, it was not possible to carry out simultaneous measurements under identical time and weather conditions on all the sites, not to mention the involvement of personnel at each round of the measurement network points.
Based on the experience gained and after an analysis of the constraints identified and the technical capacities available, work began in 2019 on the development of an automatic remote monitoring network for night sky light pollution in Toruń, based on in-house designed recording devices.
Design, functional and utility features of the device
To enrich research on light pollution in the urban space, work was initiated on the construction of a device that would perform automatic measurements, be mobile, battery-powered and use long-range wireless communication. All the above features are in line with the Industry 4.0 strategy and the modern solutions offered under the Smart City concept.
The concept of Industry 4.0 assumes the increasingly common use of process automation as well as the processing and exchange of data with the use of new transmission technologies26. LoRaWAN is one of the solutions used for communication of Internet of Things (IoT) devices, which supports the development of smart cities in the field of smart environment. As a result, the interactivity, frequency and range of measurements made in urbanized areas are increased.40.41.
According to the project developed, the device was to serve as a light meter of very low intensity observed in the night sky. In this regard, it was necessary to use a sensor with technical parameters suitable for very precise light intensity measurements. In order to locally check the weather conditions occurring during the operation of the device, it was decided to carry out additional simultaneous measurements of other environmental parameters, temperature and moisture content. Analysis of the spatial coverage of the study area indicated that 36 measuring devices should be deployed to provide complete coverage of Toruń. The concept of creating an urban measurement network involves the selection of points covering the whole city in a relatively homogeneous way and representing different types of subdivisions and elements of land use. It was assumed that the measurements would be made only at night, between 9 p.m. and 6 a.m. the next day, at 15 min intervals, and in addition, the weather conditions would be recorded twice a day.
Construction and technical parameters of the device and selected characteristics of its components
A prototype device meeting all the predefined functionalities was built from the available electronic modules. The B-L072Z-LRWAN Discovery development board from STMicroelectronics42 was selected as the main electronic component providing wireless communication. This board has a built-in LoRa communication module, enabling low-power wireless messaging, and also allows the board to enter a low-power state during hibernation, and thus target powered long-term operation by battery. This module is fully programmable, allowing future expansion of the set with other features. AMS’ TSL2591 light sensor, which has high sensitivity and recording accuracy, was selected as the component implementing light intensity measurement. Its great advantage is a wide measurement range from 188 μlx to 88,000 lx, a sensitivity reaching 0.000377 lx, and a wide dynamic range (WDR) of 600 M:143. The sensor used comprises two diodes with different spectral properties. One of them registers visible light with infrared (in the range of 400 to 1100 nm), while the other is responsible for registering infrared light (between 500 and 1100 nm). Thanks to this solution, we can use the results in different ways. Using the formula provided by the manufacturer allows us to obtain spectral characteristics similar to the human eye. The presence of a compensation diode makes a difference to the sensor used in the SQM device, so the results obtained in the measurements may be slightly different.
To measure additional environmental parameters, the X-NUCLEO-IKS01A2 development board from STMicroelectronics was used, which is connected to the STM32 microcontroller via the I2C interface44. This card allows the recording of a number of parameters, however, in the built device it is only responsible for reading the temperature and humidity of the environment. This results from the need to limit the size of the message packets sent, while improving the operating range and reducing the power consumption of the device.
After all components were selected, tested and integrated, the final connection and programming process was completed. The base of the device, i.e. the B-L072Z-LRWAN development board, was connected to the X-NUCLEO-IKS01A2 environmental sensor board, using Arduino connectors. Using standard wires, a TSL2591 light sensor was added by connecting the corresponding pins I2C (SCL and SDA), power supply (VIN), sensor ground (GND) and the X-NUCLEO-IKS01A2 board .
All components used were placed in a standard external box with dimensions of 8.0 × 5.4 × 15.8 cm. In its lower part, an opening has been made for an external antenna, while in the upper part, a specifically selected opening has been cut, protected by a window, through which the measurements are made by the light sensor (Fig. 2).
Following the above steps, an automatic device was built to record the light intensity in the lower troposphere, i.e. to measure the pollution of the night sky by artificial light coming from the Earth’s surface. The selected technical parameters of the device are shown in Table 1.
System Operation Flowchart
After building the device and writing the control software, construction of the entire measurement system began. Each of the measuring instruments is finally connected to the communication gateway using LoRa technology. A MultiTech communication gateway with a LoRaWAN module was used as the access device. To successfully connect the gateway to the measuring device, it was necessary to configure the communication gateway software. For this purpose, the information about the unique device number (Dev EUI) and the application key and its number (App EUI and App Key) were used. Once the unit is configured, it is possible to send data to the communication gateway and read it using NodeRED, a programming tool where the data is redirected to a selected server, which stores all the results of measurement. Figure 3 shows a schematic representation of the built measurement system.
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