A mobile agricultural sprayer, comprising: a frame; a tank configured to carrying a substance to be sprayed; a spraying mechanism that sprays the substance; likely drift detector logic that receives sprayer location information (314, 346) indicative of a geographic location of the sprayer, a wind sensor signal indicative of a value of a sensed wind variable and field characteristic data indicative of a geographical characteristic of a field, and generates an overspray likely signal when a likely overspray condition is identified, based on the sprayer location information (314, 346), the value of the sensed wind variable and the geographical characteristic of the field; and an overspray detection system that generates control signals to deploy a mobile sensor, that senses a spray variable indicative of a presence of the substance, to a sensor location corresponding to the likely overspray condition, and that receives an overspray detected signal indicative of the mobile sensor sensing the presence of the substance at the sensor location, and the likely drift detector logic is configured to receive, as the wind sensor signal, a wind speed signal indicative of wind speed, and a wind direction signal indicative of wind direction and to generate the overspray likely signal based on the wind speed and wind direction, and the likely drift detector logic is configured to receive, as the field characteristic data, boundary information indicative of a geographic location of boundaries of the field, and to generate the overspray likely signal based on the geographic location of the boundaries of the field, characterized in that the mobile sensor comprises: an unmanned aerial vehicle (124, 125, 126); and a substance sensor mounted to the unmanned aerial vehicle (124, 125, 126) to sense the spray variable. a frame; a tank configured to carrying a substance to be sprayed; a spraying mechanism that sprays the substance; likely drift detector logic that receives sprayer location information (314, 346) indicative of a geographic location of the sprayer, a wind sensor signal indicative of a value of a sensed wind variable and field characteristic data indicative of a geographical characteristic of a field, and generates an overspray likely signal when a likely overspray condition is identified, based on the sprayer location information (314, 346), the value of the sensed wind variable and the geographical characteristic of the field; and an overspray detection system that generates control signals to deploy a mobile sensor, that senses a spray variable indicative of a presence of the substance, to a sensor location corresponding to the likely overspray condition, and that receives an overspray detected signal indicative of the mobile sensor sensing the presence of the substance at the sensor location, and the likely drift detector logic is configured to receive, as the wind sensor signal, a wind speed signal indicative of wind speed, and a wind direction signal indicative of wind direction and to generate the overspray likely signal based on the wind speed and wind direction, and the likely drift detector logic is configured to receive, as the field characteristic data, boundary information indicative of a geographic location of boundaries of the field, and to generate the overspray likely signal based on the geographic location of the boundaries of the field, characterized in that the mobile sensor comprises: an unmanned aerial vehicle (124, 125, 126); and a substance sensor mounted to the unmanned aerial vehicle (124, 125, 126) to sense the spray variable. an unmanned aerial vehicle (124, 125, 126); and a substance sensor mounted to the unmanned aerial vehicle (124, 125, 126) to sense the spray variable.