256
A. Rango, A.E. Walker and B.E. Goodison
improve resolution at about 0.8 cm wavelength from the existing 25 km to a projected
8 km. It will give investigators currently experienced with the 25 km capabilities the
chance to compare the improved resolution advantages over study areas previously
used.
11.4.2 Improved Algorithms in the Passive Microwave
It is generally agreed that no single algorithm will produce representative values on
a global basis due to spatial variations in land cover, terrain and snow cover characteristics. Hence, a regional approach to snow cover algorithm development has been
adopted by several research groups (e.g., Goodison and Walker, 1995; Solberg et al.,
1998).
In 1992, a strategy for future snow cover algorithm development was outlined by
the passive microwave research community at an international workshop - "Passive
Microwave Remote Sensing of Land-Atmosphere Interactions". This strategy is
outlined in Choudhury et al. (1995). The priorities identified at this meeting included:
(i) the need for signature research using ground-based microwave radiometry to
understand the influence of vegetation cover and physical variations in snowpack
structure; (ii) the development of theoretical models to characterize microwave
interactions with snowpack properties; (iii) the incorporation ofland cover infonnation (e.g., type and density) into snow cover retrieval algorithms; (iv) the identification of target areas with good in-situ snow cover measurement for algorithm development and validation; and (v) investigation of "mixed pixels", characterized by high
spatial variations in snow distributions and/or land use, and the associated variations
in microwave emission that contribute to the brightness temperature measured for the
pixel. Since that meeting, research has focussed on these priority areas with a common goal of developing algorithms that will produce consistently representative
infonnation on snow cover parameters over as much of the globe as possible. Examples of recent passive microwave snow cover research developments in relation to the
above priorities include Chang et al. (1997), Matzler (1994), Woo et al. (1995),
Foster et al. (1996), De Seve et al. (1997), Sun et al. (1997), and Kurvonen and
Hallikainen (1997).
11.4.3 Outlook for Radar Applications
The application of current satellite active microwave sensors for snow cover infonnation retrieval is generally limited to wet snow detection and mapping (Rott, 1993),
mainly due to the single frequency (typically C-band) and single transmit/receive
polarization characteristics of these sensors. Thus, the outlook for improved snow
cover infonnation retrieval from active microwave sensors has focussed on multifrequency and multi-polarization systems (Rott, 1993).
In 1994, a multi-frequency and multi-polarization SAR system (SIR-CIX-SAR) was
flown on two NASA Space Shuttle missions, providing research data sets for investigating the potential of an advanced SAR system for retrieval of snow cover infonnation. The SIR-C (Spaceborne Imaging Radar-C) operated at L- and C-bands, and the
X-SAR (X-Band Synthetic Aperture Radar) operated at X-band, thus providing
A. Rango, A.E. Walker and B.E. Goodison
improve resolution at about 0.8 cm wavelength from the existing 25 km to a projected
8 km. It will give investigators currently experienced with the 25 km capabilities the
chance to compare the improved resolution advantages over study areas previously
used.
11.4.2 Improved Algorithms in the Passive Microwave
It is generally agreed that no single algorithm will produce representative values on
a global basis due to spatial variations in land cover, terrain and snow cover characteristics. Hence, a regional approach to snow cover algorithm development has been
adopted by several research groups (e.g., Goodison and Walker, 1995; Solberg et al.,
1998).
In 1992, a strategy for future snow cover algorithm development was outlined by
the passive microwave research community at an international workshop - "Passive
Microwave Remote Sensing of Land-Atmosphere Interactions". This strategy is
outlined in Choudhury et al. (1995). The priorities identified at this meeting included:
(i) the need for signature research using ground-based microwave radiometry to
understand the influence of vegetation cover and physical variations in snowpack
structure; (ii) the development of theoretical models to characterize microwave
interactions with snowpack properties; (iii) the incorporation ofland cover infonnation (e.g., type and density) into snow cover retrieval algorithms; (iv) the identification of target areas with good in-situ snow cover measurement for algorithm development and validation; and (v) investigation of "mixed pixels", characterized by high
spatial variations in snow distributions and/or land use, and the associated variations
in microwave emission that contribute to the brightness temperature measured for the
pixel. Since that meeting, research has focussed on these priority areas with a common goal of developing algorithms that will produce consistently representative
infonnation on snow cover parameters over as much of the globe as possible. Examples of recent passive microwave snow cover research developments in relation to the
above priorities include Chang et al. (1997), Matzler (1994), Woo et al. (1995),
Foster et al. (1996), De Seve et al. (1997), Sun et al. (1997), and Kurvonen and
Hallikainen (1997).
11.4.3 Outlook for Radar Applications
The application of current satellite active microwave sensors for snow cover infonnation retrieval is generally limited to wet snow detection and mapping (Rott, 1993),
mainly due to the single frequency (typically C-band) and single transmit/receive
polarization characteristics of these sensors. Thus, the outlook for improved snow
cover infonnation retrieval from active microwave sensors has focussed on multifrequency and multi-polarization systems (Rott, 1993).
In 1994, a multi-frequency and multi-polarization SAR system (SIR-CIX-SAR) was
flown on two NASA Space Shuttle missions, providing research data sets for investigating the potential of an advanced SAR system for retrieval of snow cover infonnation. The SIR-C (Spaceborne Imaging Radar-C) operated at L- and C-bands, and the
X-SAR (X-Band Synthetic Aperture Radar) operated at X-band, thus providing