The objective of this work is to provide the performance-based guidance of multiple mobile sensing agents onboard air vehicles that are used for the state reconstruction associated with gaseous sources. A source is assumed to move freely in the spatial domain of interest and multiple sensing air vehicles (SAV) are employed to assess the effects of the moving source. A model-based estimation scheme is implemented in order to primarily estimate the effects (concentration) of the moving source with a secondary goal of estimating the proximity of the moving source. The proposed guidance scheme provides the spatial relocation of the SAVs based solely on the performance of the estimation scheme. Extensive numerical studies for both stationary and mobile sources are considered to provide a convincing argument on the performance-based guidance of mobile sensors.