42
G. Lagerloef and J. Font
power of the scene is also measured using at least one real aperture radiometer connected to one of the antennas. The relatively high spatial resolution (less than 50 km)
and the short revisit time (1–3 days) imposed by soil moisture science objectives,
are more feasible at this low frequency with such a new approach than with a classical pushbroom technique. Le Vine et al. (2000) were able to generate an SSS
map using the Electronically Steered Thinned Array Radiometer (ESTAR), the first
one-dimensional synthetic aperture radiometer flown on an aircraft.
Early in the 1990s ESA started preparing the specifications for a polarimetric
two-dimensional synthetic aperture radiometer, improving the ESTAR design. The
result was MIRAS (Microwave Imaging Radiometer with Aperture Synthesis), with
a large number of antennas along a Y-shaped 3 arms structure (Martín-Neira and
Goutoule, 1997). The optimum sampling strategy of the spatial frequency plane is
on a hexagonal grid, instead of the rectangular one commonly used in signal or
image processing (Camps, 1996). This strategy allows an increase of the maximum
antenna separation without suffering from aliasing effects in the image reconstruction process, or alternatively for the same antenna spacing enlarging the alias-free
field-of-view (AF-FOV) as compared to rectangular sampling. For a given number
of elements, the array structure that provides the largest spatial frequency coverage
(best angular resolution) is a Y structure.
The two-dimensional MIRAS interferometer allows to measure T B at large incidences, for two polarisations. Moreover, the instrument records instantaneously a
whole scene; as the satellite moves, a given point within the 2D FOV is observed
from different view angles. One then obtains a series of independent measurements, which allows retrieving surface parameters with much improved accuracy.
The concept is fully scalable and allows achieving very fine spatial resolution without moving parts. A first MIRAS feasibility study was carried out by France in
1992–1996 and the development of receivers (LICEF, Lightweight Cost-Effective
Front-end) was started in 1995 by Spain. In 1998 EADS-CASA Espacio took the
lead of the technology development through the MIRAS Demonstrator Pilot Project.
The first measurements of the MIRAS prototype led to the finding of the Corbella
equation (Corbella et al., 2004) that introduced a fundamental modification in the
visibility equation used in radioastronomy.
In summer 1998 ESA launched the first call for Earth Explorer Opportunity
Missions within its new Living Planet program. Taking advantage of the recent technological developments, a large group of land and ocean researchers, together with
microwave technologists, submitted a SMOS (Soil Moisture and Ocean Salinity)
proposal (Kerr et al., 2001; Font et al., 2004). The previous year a mission using
MIRAS had been proposed to the French Space Agency (CNES) under the name
of RAMSES (Radiométrie Appliquée à la Mesure de la Salinité et de l’Eau dans le
Sol), but although being initially selected its implementation was finally discarded.
ESA considered SMOS was a risky proposal, due to its new technological concept, never flown before on a satellite, but the maturity and the innovative character
of this concept, as well as the timeliness and relevance of the proposed objectives
for Earth observation qualified the proposal for being selected in May 1999 as the
second Earth Explorer Opportunity Mission out of 27 submitted proposals. SMOS
G. Lagerloef and J. Font
power of the scene is also measured using at least one real aperture radiometer connected to one of the antennas. The relatively high spatial resolution (less than 50 km)
and the short revisit time (1–3 days) imposed by soil moisture science objectives,
are more feasible at this low frequency with such a new approach than with a classical pushbroom technique. Le Vine et al. (2000) were able to generate an SSS
map using the Electronically Steered Thinned Array Radiometer (ESTAR), the first
one-dimensional synthetic aperture radiometer flown on an aircraft.
Early in the 1990s ESA started preparing the specifications for a polarimetric
two-dimensional synthetic aperture radiometer, improving the ESTAR design. The
result was MIRAS (Microwave Imaging Radiometer with Aperture Synthesis), with
a large number of antennas along a Y-shaped 3 arms structure (Martín-Neira and
Goutoule, 1997). The optimum sampling strategy of the spatial frequency plane is
on a hexagonal grid, instead of the rectangular one commonly used in signal or
image processing (Camps, 1996). This strategy allows an increase of the maximum
antenna separation without suffering from aliasing effects in the image reconstruction process, or alternatively for the same antenna spacing enlarging the alias-free
field-of-view (AF-FOV) as compared to rectangular sampling. For a given number
of elements, the array structure that provides the largest spatial frequency coverage
(best angular resolution) is a Y structure.
The two-dimensional MIRAS interferometer allows to measure T B at large incidences, for two polarisations. Moreover, the instrument records instantaneously a
whole scene; as the satellite moves, a given point within the 2D FOV is observed
from different view angles. One then obtains a series of independent measurements, which allows retrieving surface parameters with much improved accuracy.
The concept is fully scalable and allows achieving very fine spatial resolution without moving parts. A first MIRAS feasibility study was carried out by France in
1992–1996 and the development of receivers (LICEF, Lightweight Cost-Effective
Front-end) was started in 1995 by Spain. In 1998 EADS-CASA Espacio took the
lead of the technology development through the MIRAS Demonstrator Pilot Project.
The first measurements of the MIRAS prototype led to the finding of the Corbella
equation (Corbella et al., 2004) that introduced a fundamental modification in the
visibility equation used in radioastronomy.
In summer 1998 ESA launched the first call for Earth Explorer Opportunity
Missions within its new Living Planet program. Taking advantage of the recent technological developments, a large group of land and ocean researchers, together with
microwave technologists, submitted a SMOS (Soil Moisture and Ocean Salinity)
proposal (Kerr et al., 2001; Font et al., 2004). The previous year a mission using
MIRAS had been proposed to the French Space Agency (CNES) under the name
of RAMSES (Radiométrie Appliquée à la Mesure de la Salinité et de l’Eau dans le
Sol), but although being initially selected its implementation was finally discarded.
ESA considered SMOS was a risky proposal, due to its new technological concept, never flown before on a satellite, but the maturity and the innovative character
of this concept, as well as the timeliness and relevance of the proposed objectives
for Earth observation qualified the proposal for being selected in May 1999 as the
second Earth Explorer Opportunity Mission out of 27 submitted proposals. SMOS