Understanding Active Sensors in Remote Sensing

In the field of remote sensing, sensors are broadly categorized into two main types based on how they acquire data: active sensors and passive sensors.

Active Sensors Explained

An active sensor is a device used in remote sensing that provides its own source of electromagnetic energy to illuminate the target or area being observed. After emitting this energy, the sensor then detects and measures the energy that is reflected or backscattered from the target. This means active sensors do not rely on natural illumination, such as sunlight, to collect data. This capability allows them to collect data at any time, day or night, and in various weather conditions, making them highly versatile for different remote sensing applications.

Passive Sensors Explained

In contrast, a passive sensor detects naturally available electromagnetic energy that is reflected or emitted from the Earth's surface or atmosphere. These sensors depend entirely on external sources of energy, primarily the sun's illumination, to operate. Therefore, passive sensors can only collect data when sufficient natural energy is available, typically during daylight hours and clear sky conditions.

Analyzing Sensor Examples

Let's examine the given options to identify the active sensor:

• Laser scanner: A laser scanner, often used in LIDAR (Light Detection and Ranging) systems, emits pulses of laser light (its own energy source) and measures the time it takes for these pulses to return after reflecting off the target. This direct emission and reception of energy make it a classic example of an active sensor in remote sensing.
• Return Beam Vidicon (RBV): The Return Beam Vidicon camera system was a type of sensor used in early satellite missions (like Landsat 1, 2, and 3). It operated by capturing reflected sunlight from the Earth's surface. As it relied on external sunlight, it is classified as a passive sensor.
• Television camera: A standard television camera collects visible light, which is usually ambient or reflected sunlight. Thus, a television camera functions as a passive sensor because it does not generate its own illumination.
• Electro-optical scanner: This is a broad category of sensors that convert light (optical energy) into electrical signals. While some specialized electro-optical systems can be active (e.g., if combined with an illuminator), typically in remote sensing contexts when described broadly, electro-optical scanners refer to devices like multispectral scanners that capture reflected solar energy. Therefore, without further specification, it is generally considered a passive sensor. The laser scanner is a more specific and clear example of an active sensor.

Based on the definitions and analysis, the laser scanner is a definitive example of an active sensor in remote sensing because it generates its own energy source (laser light) for data acquisition, rather than relying on natural illumination.