Although there are many types of man-made satellites, they can be broadly divided into three categories: communication, navigation, and remote-sensing. We will discuss the first two in separate posts. Instead, we focus here on the remote-sensing satellites. The market for remote-sensing satellites is ever evolving and so it's important to understand the various types of sensors currently in use.

Remote-sensing satellite can be equipped with either a passive or an active sensor. A passive sensor detects a specific natural radiation reflected by the land, sea, or from the atmosphere. Detection of reflected sunlight is a common example of passive sensor technology. On the other hand, active sensor generates its own energy to illuminate the target and measures the radiation that is reflected back. Although, passive sensors can detect microwaves, the majority of active sensors operate in microwave spectrum. Below is a non-exhaustive list of different examples of passive and active sensors:

Passive sensors

Radiometer - A radiometer detects and quantifies the power of electromagnetic radiation. Different radiometers detect different portion of the electromagnetic spectrum, such as visible, infrared, or even microwave.

Imaging radiometer - Generates 2D images by scanning the target and using multiple detectors.

Hyperspectral radiometer - A sensor that can simultaneously and accurately detect very narrow bands of visible and infrared (near and mid) spectrum. This allows for data/image discrimination amongst hundreds of observed targets simultaneously.

Spectrometer - Generally, spectrometers can disperse the observed electromagnetic radiation into bands using gratings or prism. Once dispersed, individual bands can be analyzed further.

Spectroradiometer - Can measure the strength of radiation in multiple bands usually to detect specific geophysical parameters.

Accelerometer - A sensor that can detect and calculate changes in speed linearly or rotationally.

Sounder - Used for measurements of multiple atmospheric parameters vertically, such as temperature, pressure, etc.

Active sensors

Radar - The most original form of sensor technology. It emits microwaves from an antenna at the target(s). Depending on the duration and the magnitude of the signal received, an image of the target can be produced.

LIDAR - Short for Light Detection and Ranging. As the name implies, this type of sensor identifies target by emitting and detecting light.

Laser altimeter - A specific kind of LIDAR that measures topography.

Scatterometer - Measures high-frequency microwave that is back-scattered from the surface.

Ranging instruments - Uses a pair of identical microwave device on two different platforms to emit and receive signals to each other allowing for the accurate measurement of the distance between them.

Sounder - Somewhat similar to passive sounder sensor, this device emits microwave to measure vertical atmosphere parameters, such as temperature, humidity, and cloud composition.