The design of an automated radiometer traversing system used to measure nocturnal longwave surface radiative budgets in a model urban canyon is described. The system allows two net radiometers to be traversed around the perimeter of a canyon cross section in order to resolve the spatial distribution of net radiation without necessitating extensive arrays of instruments. The system has been used to study the temporal and spatial variations of radiation and near surface air temperature within a model urban canyon and to validate a numerical canyon radiation model. Considerations affecting the placement of the sensors with respect to the canyon surfaces are detailed. The traversing speed is determined using an analysis of sensor response to anticipated spatial changes in radiation across canyon surfaces. Test results confirm the design parameters.