Multispectral Band Options

Key to Map

The new satellite systems can be conveniently divided into passive optical systems and active Synthetic Aperture Radar (SAR) systems. SAR systems actively transmit a microwave energy pulse to the earth, then record the amount of energy that returns to the satellite after it interacts with the earth's surface. One very important advantage of the SAR systems is that microwave systems are not dependent on daylight and can penetrate all precipitation, dust, or haze, which enables the collection of data under any atmospheric condition.

Optical systems divide the light reflected (or emitted for long-wave infrared) from the earth's surface into discrete intervals or bands. For instance, the Space Imaging System collects four-meter ground sample distance (GSD) in four spectral bands: visible blue, green, red, and near infrared. If different features in the image have different in "color" (i.e., different values in each of the four bands), then it is possible to identify and characterize the presence of water, cultural features, vegetation, and many other significant phenomenon of the earth's surface. The figure above shows how a land-use map that depicts part of Camp Lejeune, North Carolina, can be created using a multispectral airborne system to approximate the capabilities of the new satellite systems. However, the difference in color between features is sometimes relatively subtle, which makes discriminating between objects difficult.

One solution to this problem is the use of hyperspectral systems that divide the spectrum into hundreds of fine bands. The basic assumption is that fine differences in color can now be used to identify and characterize more features, which is particularly attractive in the case of mineral exploration. However, hyperspectral capabilities do not come without cost; in order to gather more spectral data, it is often necessary to integrate over a larger GSD. For instance, when the Space Imaging System uses only one broad band, it is able to provide one-meter GSD. For four-color, it is restricted to a larger four-meter GSD. The proposed hyperspectral NEMO system is restricted to 30-meter GSD.