Remote Sensing

Overview

In general, remote sensing means measurement of physical values from a distance. Earth observation is one type of remote sensing. Observation systems installed on balloons, airplanes, satellites, the space shuttle or the future space station, measure physical properties of the earth surfaces, sea surfaces, and clouds. An appropriate platform needs to be selected along with a proper range of the observation wavelengths (spectrum), which depends on the regional range, the cycle, the period and the object of the observation.

Observation Systems

Various observation systems have been designed to help fulfill various observation requirements. The following items are usually specified for observation requirements:

Requirements:


	Observation target

	Observation width

	Observation cycle

	Observation period

	Spatial resolution

	Data format

Observation Target

Observation TargetEither a passive optical instrument or microwave radiometer, or an active synthetic aperture radar (SAR) is used, depending on the target of observation. A proper instrument needs to be selected, in accordance with the spectral characteristics of the object to be viewed. The observation instrument should have sensitivity in a specific wavelength.

Observation Width

Observation width is the width of an image obtained in a single observation. Although it is most ideal to have a wide observation width and a high spatial resolution at the same time, these two qualities are mutually incompatible. That is to say that when the detector size is constant, the spatial resolution decreases as the observation width increases; and if a good spatial resolution is needed, the observation width must be sacrificed.

Observation Cycle (Recurrence Cycle)

One of important factors for an earth observation satellite is an observation cycle, how many days a polar orbiting satellite takes to return to the same location over the earth. The recurrence cycle is determined by the altitude and the inclination of the orbit. The shorter the cycle , the better. ASTER has a pointing mechanism which changes the direction of each telescope in order to ease the limitation of the observation range caused by the recurrence cycle. Geosynchronous satellites observe the same region at any point in time.

Observation Period

The satellite can continue working until its sensors or functional lifetime comes to an end.

Spatial Resolution

Spatial resolution is an area on the ground corresponding to the view of one pixel of the detector. Although small substances on the ground can be better identified with a high spatial resolution, the observation width has to be sacrificed, and it takes more time to process the image.

Data Format

Data format is the format of the image data.Digital data and photographs are used among others. ASTER uses digital data that is suitable for computer processing.