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E.T. Engman
9.5 Future Microwave Remote Sensing of Soil Moisture
The previous sections have discussed the basis of microwave remote sensing for
soil moisture and presented a discussion of various target-sensor interactions. As
promising as microwave remote sensing for soil moisture appears to be, the future
for using microwave data for operational use is somewhat uncertain. For the next
few years, researchers will be limited by the lack of suitable data. Currently the
ERS-l&2 SAR from the European Space Agency, the JERS-l SAR from the Japanese and the Canadian RADARSAT are the only operational active microwave
satellite sensors with frequencies suitable for soil moisture. Although these instruments should be valuable data sources for extending our knowledge of SAR for
measuring soil moisture, to date very little data have been available for this purpose. Fortunately there have been some intensive and science-driven aircraft experiments conducted in the last several years and these data are beginning to become available to the scientific community. These should be invaluable for providing sample data for developing and testing algorithms as well as answering
some of the target-sensor questions.
Looking ahead to when there may be more microwave sensors on orbiting platforms, one confronts the basic differences between passive and active instruments
and the intended use of the data. Comparing the instruments simplistically, the
active sensors have the capability to provide high spatial resolution data (on the
order of tens of meters) but their sensitivity to soil moisture may be confused more
by roughness, topographic features, and vegetation than the passive systems. On
the other had, the passive systems, although less sensitive to target features, can
provide spatial resolutions only on the order of tens of kilometers. One then must
consider how the data will be used. Meteorological and climate models currently
use computational cells on the order of 10-100 km which may be well within the
capacity of future passive systems. However, if one is interested in more detailed
hydrologic process studies and partial area hydrology, the passive data would
appear to be of little use. It is in this context that the active systems appear promising. For example, existing and planned SAR'S can provide at least 20-30 m resolution over a swath width of 100 km. RADARS AT has the capability of a scanning
mode (SCANSAR) to cover a much wider swath (300-500 km) at a reduced resolution (250 m) as well as a high precision mode with a spatial resolution approaching 8 m. There are a number of soil moisture missions on the drawing
boards or in the proposal stage that would provide a major boost for the applications of soil moisture in hydrology. These include two proposals based on passive
microwave aperture synthesis, the NASA HYDROST AR and the CNES/ESA
SMOS and the NASA-JPL LIGHTSAR with a polarametric L-band sensor. More
certain is the AMSR passive microwave (C-band) instrument that will fly on the
Japanese ADEOS II and the EOS-PM and the CMIS instrument that will be on
future US weather satellites.
Research Needed. There are going to be more and more orbiting microwave sensors suitable for measuring soil moisture in both the near and distant future. However, before the hydrologic community makes good use of t.h.ese data there are a
number of research questions that must be addressed. Perhaps the foremost re-
E.T. Engman
9.5 Future Microwave Remote Sensing of Soil Moisture
The previous sections have discussed the basis of microwave remote sensing for
soil moisture and presented a discussion of various target-sensor interactions. As
promising as microwave remote sensing for soil moisture appears to be, the future
for using microwave data for operational use is somewhat uncertain. For the next
few years, researchers will be limited by the lack of suitable data. Currently the
ERS-l&2 SAR from the European Space Agency, the JERS-l SAR from the Japanese and the Canadian RADARSAT are the only operational active microwave
satellite sensors with frequencies suitable for soil moisture. Although these instruments should be valuable data sources for extending our knowledge of SAR for
measuring soil moisture, to date very little data have been available for this purpose. Fortunately there have been some intensive and science-driven aircraft experiments conducted in the last several years and these data are beginning to become available to the scientific community. These should be invaluable for providing sample data for developing and testing algorithms as well as answering
some of the target-sensor questions.
Looking ahead to when there may be more microwave sensors on orbiting platforms, one confronts the basic differences between passive and active instruments
and the intended use of the data. Comparing the instruments simplistically, the
active sensors have the capability to provide high spatial resolution data (on the
order of tens of meters) but their sensitivity to soil moisture may be confused more
by roughness, topographic features, and vegetation than the passive systems. On
the other had, the passive systems, although less sensitive to target features, can
provide spatial resolutions only on the order of tens of kilometers. One then must
consider how the data will be used. Meteorological and climate models currently
use computational cells on the order of 10-100 km which may be well within the
capacity of future passive systems. However, if one is interested in more detailed
hydrologic process studies and partial area hydrology, the passive data would
appear to be of little use. It is in this context that the active systems appear promising. For example, existing and planned SAR'S can provide at least 20-30 m resolution over a swath width of 100 km. RADARS AT has the capability of a scanning
mode (SCANSAR) to cover a much wider swath (300-500 km) at a reduced resolution (250 m) as well as a high precision mode with a spatial resolution approaching 8 m. There are a number of soil moisture missions on the drawing
boards or in the proposal stage that would provide a major boost for the applications of soil moisture in hydrology. These include two proposals based on passive
microwave aperture synthesis, the NASA HYDROST AR and the CNES/ESA
SMOS and the NASA-JPL LIGHTSAR with a polarametric L-band sensor. More
certain is the AMSR passive microwave (C-band) instrument that will fly on the
Japanese ADEOS II and the EOS-PM and the CMIS instrument that will be on
future US weather satellites.
Research Needed. There are going to be more and more orbiting microwave sensors suitable for measuring soil moisture in both the near and distant future. However, before the hydrologic community makes good use of t.h.ese data there are a
number of research questions that must be addressed. Perhaps the foremost re-