Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19--22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200--600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83--137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be ~260 km which corresponds to the bottom side of the mid-latitude ionospheric F layer. ¿ 2000 American Geophysical Union