Measurements which are geocoded in this way are then entered into a Measurement Database, 
where they are integrated with information from other sources. Such a database can be thought of 
as a generic description of a region of the planetary system in space and time. Its contents 
describe that region. The final step in the information delivery system is to extract information from 
the database in support of resource and environmental management decisions. This is shown in 
Figure 2 by the many "Information Extraction & Decision Support" elements at the bottom of the 
delivery system. This final step in the process is very application dependent. The same physical 
measurements have different meanings and are interpreted in different contexts depending on 
the discipline of application. For example, the infrared reflectance profile over a forest has a 
different meaning and significance to a forester than it does to an exploration geologist. Thus we 
see that information drawn from the database is treated and interpreted differently by different 
application disciplines. 
Technological Considerations 
Sensors 
The process of acquiring information begins with the sensing instrument in the spacecraft. The 
function of these instruments is to acquire the basic measurements which the system requires. 
They can be divided into two broad categories: passive and active . Passive sensors rely on the 
sun to provide the illumination energy which is reflected off the surface or the atmosphere and 
measured by the instrument. Active sensors provide their own illumination energy. Tables 1a and 
1 b show a list of the major categories of sensing instruments together with a brief description of 
the characteristics and typical applications of each instrument. 
PASSIVE SENSORS 
Measure reflected solar energy and/or energy emitted by earth 
SENSOR 
CHARACTERISTICS and TYPICAL APPLICATIONS 
Spectroradiometer 
A class of instruments which measure the radiant flux entering the instrument 
in a number of very narrow spectral bands. Many types of these instruments 
exist, using a variety of different techniques for achieving the desired 
spectral measurement characteristics. -- Substance identification. 
Multispectral Scanner 
An imaging radiometer which makes spectrally rather course measurements 
(typically 80 - 100 nanometer spectral resolution) in the form of a number of 
separate "images" at different wavelengths. Thus for each point on the image 
a crude "spectrum" is obtained. - Identification of broad classes of surface 
material and general indication of condition. 
Imaging Spectrometer 
A similar instrument to the multispectral scanner, except that its spectral 
resolution is much finer (typically 10 nanometers or less), which allows it to 
acquire detailed spectral responses from each point on the surface. 
-- Identification of specific surface materials and condition. 
Atmospheric Sounder 
Imaging Instruments which operate in a number of very narrow wavelength 
bands in the infrared and microwave areas of the spectrum where the 
atmosphere is known to absorb radiation. Operation in this way enables 
measurement of radiance from different levels in the atmosphere. -- Temp., 
pressure, and H2O profiles, Total ozone content. 
Imaging Microwave 
Radiometer 
An imaging instrument which measures and maps microwave energy radiated 
(emitted) from the earth in a number of spectral bands. -- Surface temp., 
moisture, precip. Atmospheric temp, and H2O vapour profiles. 
Limb Sounder 
A spectral radiometer which is directed at the limb of the planet, thus 
measuring the characteristics of the atmosphere at various heights above 
the surface. - Atmospheric profiles of H2O vapor, O3, CH4, CO2, HNO3, etc 
Table 1a Passive Sensing Instruments