ACTIVE SENSORS 
Measure response to a transmitted signal 
SENSOR 
CHARACTERISTICS and TYPICAL APPLICATIONS 
Synthetic Aperture Radar 
Transmits a radar pulse and measures the reflected microwave energy. The 
output is in image format. The instrument is sensitive to differences in 
surface roughness, conductivity and dielectric characteristics. Data sets 
can be acquired at various incidence angles and polarizations. — Spatial 
distribution of ice types, ice dynamics, sea state, ocean circulation, 
veqetation canopy moisture, biomass and morphology. 
Microwave Scatterometer 
Measures the scattering or reflective properties of the surface by measuring 
the reflection and scattering of wave generated by the radar itself. -- Oceanic 
wind speed and direction. 
Lidar Sounder 
Transmits a laser light pulse, and measures the reflected energy as a 
function of time. The use of a tunable laser system allows the instrument to 
probe for specific spectral features. — Atmospheric constituents and 
aerosols at various depths. 
Lidar & Radar Altimeters 
Ranging instruments for precise (cm) measurement of distance. 
- Precise measurement of elevation over land (lidar) and ocean (radar). 
Measurement of tectonic movement. 
Laser Wind Profiler 
Transmits a laser light pulse and measures the Doppler shift of the reflected 
light scattered from aerosol particles in the atmosphere. 
- Direct measurement of wind velocity. 
Table 1b Active Sensing instruments 
Several of these instruments have flown in space for many years, some are recent, and others 
have yet to be flown. In the past, as each instrument was launched on its spacecraft, the remote 
sensing community focussed on attempting to use the characteristics of that particular instrument. 
Our requirement, however, is for measurements of atmospheric and surface phenomena. In many 
cases, with rapidly changing phenomena on the surface or in the atmosphere, frequent 
measurement is a necessity in order to be able to adequately describe the dynamics of what is 
taking place. In the case of optical instruments, cloud cover and long revisit cycles inhibit the 
operational use of the data. It is therefore essential that the data from all sensors be processed in 
ways which allow the straightforward integration of readings from a variety of sensors having 
different spatial characteristics and coverage patterns. In the case of surface measurements this 
requires that we be able to derive surface measurements which characterize the surface material 
independent of the intervening atmosphere. 
Measuring the Surface 
The principal difficulty with respect to measurement of the land surface is illustrated in Figure 3. In 
this figure, the radiation emanating from the source (in this case the sun), arrives at the top of the 
atmosphere (a), passes through the atmosphere and falls on the target (b). At this point, the 
radiation falling on the target is dependent on the characteristics of the source and the properties 
of the atmosphere. The target reflects a portion of this energy, and the ratio of the reflected 
energy to that which illuminated the target is the reflectance. It is the reflectance which 
characterizes the surface material at the target, and it is therefore reflectance which is the physical 
quantity of interest in this case. The amount of energy reflected in the direction of the 
spacecraft is dependent not only on the surface material, but on its elevation and 
orientation relative to the direction of the incoming radiation and the direction of the 
spacecraft view angle. The implication of this is that the elevation and orientation of the surface at 
the target point must be known beforehand. This information can be provided by a Digital 
Elevation Model. The reflected energy (c) travels up through the atmosphere, exiting the top of