Ground-Based Sensing in Precision Agriculture
Categorization of Ground-Based Sensors
Ground-based proximal sensors may be described by how they measure (invasive [in situ or ex situ] or non-invasive), the source of their energy (active or passive), how they operate (stationary or mobile), and the inference used in the measurement of the target property (direct or indirect). The proximal sensor is invasive if there is sensor contact during measurement, and it is non-invasive if there is no contact between the sensor and the soil or plant. Measurements with invasive proximal sensors may be made in situ (i.e., the measurements taken directly in the field, without removing the sample) or ex situ (i.e., the sample is removed from the field and analyzed elsewhere, e.g., lab or greenhouse). The other main difference between in situ and ex situ is that in situ is less expensive and less manageable, whereas ex situ is costlier and more manageable. A proximal sensor is active if, for the measurements, it produces its own energy from an artificial source. Many of the ground-based remote sensing platforms have active proximity sensors.
Active and Passive Light Sensors
Active light sensors emit their light source (LEDs or lasers) and measure the reflected light from the soil or crops. Unlike passive sensors, which rely on sunlight, active sensors work in all lighting conditions, including night and cloudy weather. In such systems, variations in daylight have a minimum effect on measured reflectance, thus providing a more accurate and reproducible Normalized Difference Vegetation Index (NDVI) or other vegetation indices (VI) used for crop nutrient status assessment. These sensors are widely used in precision agriculture for nitrogen sensing, crop health monitoring, variable rate fertilization, and disease detection.
Stationary Sensors
Continuous monitoring of a specific point can be achieved through stationary sensor systems that are linked together via a wireless network (Figure 9.1). For example, soil water content sensor networks have been used to monitor soil water over the entire season. Based on this information, the irrigation scheduling can be optimized and rates adjusted. Since these systems are stationary, they are generally used to track micro climatic parameters through time. Sensors measure temperature, humidity, wind speed and direction, and available water.
Mobile Sensors
Mobile proximal sensors measure soil properties and crop conditions while moving, or “on-the-go” or “stop-and-go.” On-the-go plant and soil sensors collect data in real-time while moving (e.g., mounted on a tractor, robot, or vehicle), while stop-and-go sensors require the operator to pause at each sampling point to take a measurement (Figure 9.2). On-the-go sensors offer the advantage of dynamic data collection during fieldwork, while stop-and-go sensors provide potentially more precise measurements when the device is stationary. Like the conventional sampling approach, these sensor systems provide detailed information on specific field locations defined by the user and typically have lower mapping density than on-the-go systems.
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