Passive remote sensors depend for imaging on the sun’s illumination of the earth’s surface. These sensors record solar energy upon their interaction with objects on the earth. For this reason, they are also referred to as optical sensors. But their reliance on the sun’s illumination and earth surface reflectance is problematic in that the sun does not directly illuminate at night and that reflectance is hindered during unfavorable weather conditions (e.g., cloud cover). This is often a serious limiting factor in the application of passive optical sensors for agricultural management, especially so in agricultural regions where rainfed irrigation of crops is the norm. Excessive cloud cover during the cropping season prevents optical sensors from obtaining useful images at important crop growth stages. As we see later, this disadvantage can be reduced when the satellite constellation (on which the sensor is) is able to revisit the same spot on earth at a high frequency. The Sentinels-2 are equiped with passive sensors.
Active sensors overcome, to a very large extent, the challenges of passive sensors in obtaining images at night or in bad weather conditions. These sensors are designed to carry their own source of energy, instead of relying on that of the Sun. Like passive sensors, they record the reflected energy upon interaction with objects on the earth’s surface. Most active sensors emit radiation with long wavelengths, which can penetrate through clouds, haze and dust. This mode of operation makes active sensors less susceptible to unfavorable weather conditions and can therefore acquire images at all times and under varying conditions. In terms of agricultural management, Synthetic Aperture Radar (SAR) systems are the most prominent active sensors. This is because useful SAR images can be obtained at key crop growth stages where passive systems fail. SAR data, therefore, constitute an important source for mapping agricultural systems, especially in rainfed-dominated areas where cloud cover may persist during the cropping season. It must be noted however that SAR data processing is more complex than that of optical data. More information about SAR data and their application in agriculture can be found here. The Sentinels-1 are equiped with active sensors.