Measurement of wireless communication characteristics in sewer pipes for sewer inspection systems using multiple wireless sensor nodes

Deterioration of sewer pipes is one of the very important problems in Japan. Sewer inspections have been carried out mainly by visual check or wired remote robots with a camera. However, such inspection schemes involve high labor and/or monetary cost. Sewer inspection with boat-type video cameras or unwired robots takes a long time to check the result of the inspection because video data are obtained after the equipment is retrieved from the pipe. To realize low cost, safe and quick inspection of sewer pipes, we have proposed a sewer inspection system using drifting wireless sensor nodes. Water, soil, and the narrow space in the pipe make the long-range wireless radio communication difficult. Therefore, we have to identify suitable radio frequency and antenna configuration based on wireless communication characteristics in sewer pipes. If the frequency is high, the Fresnel zone the needed space for the line of sight is small, but the path loss becomes worse. On the other hand, if the frequency is low, the size of the Fresnel zone is large, but the path loss becomes small. We conducted wireless communication experiments using 920MHz, 2.4GHz and 5GHz band equipment in an experimental underground pipe. The measurement results show that the wireless communication distance of 5GHz IEEE802.11a is about 5m in a 200mm-diameter pipe and it is longer than 920MHz (ARIB STD-T108), 2.4GHz (IEEE802.11g, IEEE802.15.4) and with 10dBm.