396
S. Paloscia
14.4 Summary and Future Prospects
Remote sensing methods are of fundamental importance in the measurement of SMC on large
areas. The most suitable sensors for this purpose are the ones operating in the microwave
region of the electromagnetic spectrum. As a matter of fact these instruments, besides allowing
all-weather, day/night operation, are very sensitive to the moisture content of a soil layer about
5 cm thick even if the terrain is covered by herbaceous crops.
Both active and passive systems showed good performances in such a type of measurements,
although with some basic differences between them. Passive sensors in fact seem to be less
affected by the geometrical features of the surface (i.e. surface roughness and vegetation) than
active sensors and are therefore more accurate and sensitive, but their results are partially
hampered by the poor spatial resolution, at least for an operational use from satellite. On the
contrary, aircraft remote sensing seems to be very promising especially if the new generation of
radiometers is used, such as the PBMR and the ESTAR. Active sensors have very high spatial
resolutions, even from satellite platforms, but their sensitivity to moisture content is more attenuated by structural characteristics of the surface than passive sensors. These considerations
suggest that the integrated use of multiband systems may allow still more precise and reliable
estimates.
As far as passive sensors are concerned, we can say as conclusion that the L-band emission
allows the measurement of the top-layer soil moisture content, while two or more frequency
channels in the higher microwave bands (e.g. 10 GHz and 36 GHz) can be useful for estimating
surface roughness on bare soils. On canopy covered fields, vegetation biomass can be estimated
by using optical data or the X band microwave polarization index.
Multifrequency polarimetric SAR measurements, carried out over composite landscapes of forests and agricultural fields, indicate that SAR data are effective in discriminating among broad
land surface categories. Although the first exciting results obtained with C-band data have been
partially reappraised by later experiments, rather good correlation with soil moisture has been
found on agricultural fields mainly at L-band (co-polar), a correlation which appreciably increases if bare soils are separated from vegetation surfaces. While a fairly good sensitivity
and correlation to the Plant Water Content of some agricultural species has been observed at
L-band especially for the cross-polarized C7HV.
The use of multifrequency, multi polarization remote sensors seems to be a promising approach
both for evaluating the spurious factors which affect the relation between microwave signals
and geophysical parameters and for improving the accuracy of measurements.
A valuable amount of experimental and theoretical research has been carried out worldwide,
aimed at clarifying the physical principles of the measurement and to test and validate sensors
and algorithms. For this purpose satellite data, as well as measurements performed by means of
ground based and airborne sensors, have been used. Further research is in progress to implement
the precision and reliability of the available methods according to users' requirements in different
disciplines.
14.5 References
Autret M, Bernard R, Vidal-Madjar D (1989) Theoretical study of the sensitivity of the
microwave backscattering coefficient to the soil surface parameters. International Journal of
Remote Sensing 10: 171-179
S. Paloscia
14.4 Summary and Future Prospects
Remote sensing methods are of fundamental importance in the measurement of SMC on large
areas. The most suitable sensors for this purpose are the ones operating in the microwave
region of the electromagnetic spectrum. As a matter of fact these instruments, besides allowing
all-weather, day/night operation, are very sensitive to the moisture content of a soil layer about
5 cm thick even if the terrain is covered by herbaceous crops.
Both active and passive systems showed good performances in such a type of measurements,
although with some basic differences between them. Passive sensors in fact seem to be less
affected by the geometrical features of the surface (i.e. surface roughness and vegetation) than
active sensors and are therefore more accurate and sensitive, but their results are partially
hampered by the poor spatial resolution, at least for an operational use from satellite. On the
contrary, aircraft remote sensing seems to be very promising especially if the new generation of
radiometers is used, such as the PBMR and the ESTAR. Active sensors have very high spatial
resolutions, even from satellite platforms, but their sensitivity to moisture content is more attenuated by structural characteristics of the surface than passive sensors. These considerations
suggest that the integrated use of multiband systems may allow still more precise and reliable
estimates.
As far as passive sensors are concerned, we can say as conclusion that the L-band emission
allows the measurement of the top-layer soil moisture content, while two or more frequency
channels in the higher microwave bands (e.g. 10 GHz and 36 GHz) can be useful for estimating
surface roughness on bare soils. On canopy covered fields, vegetation biomass can be estimated
by using optical data or the X band microwave polarization index.
Multifrequency polarimetric SAR measurements, carried out over composite landscapes of forests and agricultural fields, indicate that SAR data are effective in discriminating among broad
land surface categories. Although the first exciting results obtained with C-band data have been
partially reappraised by later experiments, rather good correlation with soil moisture has been
found on agricultural fields mainly at L-band (co-polar), a correlation which appreciably increases if bare soils are separated from vegetation surfaces. While a fairly good sensitivity
and correlation to the Plant Water Content of some agricultural species has been observed at
L-band especially for the cross-polarized C7HV.
The use of multifrequency, multi polarization remote sensors seems to be a promising approach
both for evaluating the spurious factors which affect the relation between microwave signals
and geophysical parameters and for improving the accuracy of measurements.
A valuable amount of experimental and theoretical research has been carried out worldwide,
aimed at clarifying the physical principles of the measurement and to test and validate sensors
and algorithms. For this purpose satellite data, as well as measurements performed by means of
ground based and airborne sensors, have been used. Further research is in progress to implement
the precision and reliability of the available methods according to users' requirements in different
disciplines.
14.5 References
Autret M, Bernard R, Vidal-Madjar D (1989) Theoretical study of the sensitivity of the
microwave backscattering coefficient to the soil surface parameters. International Journal of
Remote Sensing 10: 171-179