An Optimized Method to Determine Exposure due to GSM Base Stations Applied in the City of Salzburg

In the last years exposure levels next to GSM base stations were becoming an increasing concern to the population. In several projects recently performed, e.g. Bergqvist et al 2001, exposure levels next to such installations were already determined, however the assessment by so - called spot measurements was restricted to rather small areas. These measurements are usually done in areas as single houses, flats or even rooms. Therefore, such evaluations can give information on exposure in the investigated area, but are not adequate to draw conclusions on the electromagnetic field levels in the complete environment of a base station. Due to several reasons like juridical requirements in some countries it becomes very often necessary to evaluate exposure at all the places where exposure may potentially exceed exposure limits set by the national or local administrations. It is extremely time consuming to evaluate exposure conditions around a typical base station by measurements only. On the other hand exposure assessment by computersimulation or calculations alone leads to considerable uncertainty budgets, therefore both, exposure simulation and exposure measurement has been applied in the case of the "Salzburg study". One of the major goals of this study was to get a picture of the NIR (Non Ionizing Radiation) exposure in the vicinity of GSM base stations, some being rather complex. For that purpose a specialised and highly sophisticated 3D computer simulation has been applied as well as specially developed measuring devices and measuring method. This method was applied in the vicinity of 13 base stations in the city of Salzburg. The first step to be done is to collect all necessary technical data of the respective base station like antenna pattern and gains, input power, frequencies and number of carriers with BCCHs (Broadcast Channel) and carriers with TCHs (Traffic Chancel) only. The base station needs to be examined visually and the topography of the environment needs to be documented. The relevant information of all buildings in an area of about 200 m around the base station also needs to be known in order to perform a reliable simulation of the field distribution around the respective base station. In the frame of this project the software "Quick_Plan" developed by the Italian company Teleinformatica e Sistemi s.r.l. was used to perform the simulations. Based on the results of these simulations that give adequate information of the outdoor field distribution around a base station, the areas inside the buildings where the highest field levels are to be expected, can easily be identified. These areas usually consist of large parts of houses, therefore, in most cases it is not possible to exactly identify the rooms with the highest field levels. Therefore, it is imperative to perform some preliminary, evaluative measurements ("prescan") at the respective places inside the buildings using broadband field meters during the second step of the procedure. It has to be taken into account that immissions from other sources, e.g. broadcast stations or radar installations may influence the results of the measurements. Having identified the room with the highest field levels, the positions of the maximum level of the carriers with BCCHs of the considered base station are to be identified by a second "prescan". The second "prescan" is performed using a spectrum analyser and a log periodic antenna. Because the measuring engineer is forced to hold the antenna in his hands, the field distribution is influenced by this procedure. Consequently, it is not possible to rely on the absolute field levels obtained during this second "prescan". Therefore the levels of the carriers with BCCHs are determined at the obtained positions by using the method Add3D (Haider et al 2002) to finally measure the exposure levels of the considered base station. The contribution of the carriers with TCHs is also considered by calculation assuming that all slots of the respective carriers are emitting with the maximum possible power .