Satellite data

Aerial photographs and satellite images now allow us to visualize landscapes in entirely new ways. We can see vast areas all at once. We can construct images that measure diverse properties of landscapes, and we can compare images taken at different times to discover changes.

Multispectral imagery consists of image data selectively acquired in multiple spectral bands throughout the visible and infrared (IR) portions of the electromagnetic spectrum. Panchromatic imagery covers the entire visible spectrum. Hyperspectral sensing represents the special type of image acquisition in many (often hundreds) very narrow, contiguous spectral bands.

Besides the passive sensing systems mentioned above, an increasing amount of valuable environmental and resource information is being derived from active sensing systems like radar and lidar. radar is an active sensor operating in the microwave portion of the electromagnetic spectrum. Lidar sensors use pulses of laser light to illuminate the terrain.

Satellite sensor can also be divided according to the spatial resolution, which is a measure of the smallest separation (angular or linear) between two objects that can be resolved by the remote sensing system. We typically describe a sensor's spatial resolution as a pixel size, e.g. 10 x 10 m or 30 x 30 m. In order to detect a feature, the spatial resolution of the sensor system should be less than one-half the size of the feature.

There are small resolution satellites like NOAA (pixel size about 1 km, meteorological satellites), medium resolution satellites (pixel size 10 - 100 m, e.g. Landsat, Aster, IRS, ALOS) and high or very high resolution (VHR) satellites (pixel size less than 10 m, e.g. Quickbird, Worldview, Ikonos, Kompsat).

Some remote sensing systems record the electromagnetic radioation with more precision than others. This sensitivity of a remote sensing detector to differences in signal strength is called the radiometric resolution or bit depth. For example Landsat TM sensors recorded data in 8-bits (values from 0 to 255), Quickbird and Ikonos record values in 11 bits (values from 0 to 2047). Several new sensor systems have 12-bit radiometric resolution (values ranging from 0 to 4095). Radiometric resolution can have a significant impact on our ability to measure the properties of scene objects.

The temporal resolution (or revisit time) of a remote sensing system generally refers to how often the sensor records imagery of a particular area. Obtaining imagery at a high temporal resolution is very important for many applications. Revisit time of satellites ranges from 1 day (Formosat, Orbview) to 46 days (Alos).