A model to calculate what a remote sensor 'sees' of an urban surface

Whilst the measurement of radiation emissions from a surface is relatively straightforward, correct interpretation and proper utilization of the information requires that the surface area 'seen' be known accurately. This becomes non-trivial when the target is an urban surface, due to its complex three-dimensional form and the different thermal, radiative and moisture properties of its myriad surface facets. The geometric structure creates shade patterns in combination with the solar beam and obscures portions of the surface from the sensor, depending on where it is pointing and its field-of-view (FOV). A model to calculate these surface-sensor-sun relations (SUM) is described. SUM is tested against field and scale model observations, and theoretical calculations, and found to perform well. It can predict the surface area 'seen' by a sensor of known FOV pointing in any direction when placed at any point in space above a specified urban surface structure. Moreover, SUM can predict the view factors of the roof, wall and ground facets 'seen' and whether they are sunlit or shaded at any location and time of day. SUM can be used to determine the optimal placement and orientation of remote sensors to study urban radiation emissions; if the facet temperatures are known or modelled it can calculate the average temperature of the system, and it can determine the directional variation of temperature (anisotropy) due to any particular surface-sensor-sun geometric combination. The present surface geometry used in SUM is relatively simple, but there is scope to make it increasingly realistic.