3 SMOS and Aquarius/SAC-D Missions
43
was established as a co-operative ESA-lead mission with contributions from the
French CNES and the Spanish Center for Technological and Industrial Development
(CDTI).
The initial configuration proposed an instrument working at 1.4 GHz with 25
equally spaced antenna elements in each one of its 4.5 m long arms, plus 4 additional receivers in the central hub. In order to fit in the launcher fairing, each arm
was proposed to be folded in five sections in stowed configuration. The instrument
would be installed on a PROTEUS generic platform provided by CNES with an
antenna plane tilted 20–30 ◦ with respect to nadir to guarantee an incidence angle
range within [0 ◦ , 50 ◦ ]. The orbit was proposed to be sun-synchronous with Equator
crossing at 6 AM (ascending) and 6 PM (descending) to minimize the perturbation
on L-band signal (air, vegetation and soil temperature almost identical) and making
the Faraday effect minimum. Raw measuring performances were expected to be:
30 to more than 90 km for ground resolution, 0.8–2 K for radiometric sensitivity,
1–3 days for temporal sampling, depending upon latitude, nature of the target and
location within the instrument FOV.
An important aspect in the SMOS proposal was the need to use novel calibration techniques, combining both on board reference noise sources of known power
level and external constant T B targets, to ensure a high stability of the measurements.
Concerning the operation mode, each 300 ms an image was to be taken, successively
in horizontal and vertical polarization. The huge amount of data generated forced
to propose some pre-processing on board, with averages of 5 images to obtain one
to be formatted and sent to the platform. The resulting equivalent integration time
is 1.5s/polarization, so 2 images are available every 3s. No mention was made by
then of the possibility of full polarization capability. At the moment of preparing the
proposal it was considered that the retrieval of ocean salinity required an independent measurement of sea surface temperature, to be provided through a secondary
frequency. The preliminary analysis indicated that a C-band channel could potentially be useful. However, this option was soon discarded due to the mass and power
limitations imposed by the use of the PROTEUS platform, suited for a minisatellite,
but not allowing simultaneous operations of the two instruments.
The SMOS Phase A development started in 2000, Phase B in 2002, and Phase
C/D in 2003 with a launch expected for 2007, that was later delayed until taking
place in November 2, 2009. A configuration optimization analysis (Waldteufel et al.,
2003) concluded, mainly driven by the more restringing soil moisture requirements
in terms of resolution and coverage, that the number of elements per arm should
be 21 (six on each one of three folding sections, plus three in the hub), the satellite steering angle 30 ◦ , the orbit height around 755 km, the tilt of the antenna plane
close to 33 ◦ , and the spacing between antenna elements 0.875 wavelengths. During
the detailed mission design it appeared that, in spite of the efforts made in successive improvements on the receivers and other components design, the PROTEUS
capability was really at the limit with almost no margin, so it was decided to remove
three of the receivers in the hub. The final number of antenna elements is 69 and
72 receivers, 66 LICEFs and six noise injection radiometers, are connected to them
(McMullan et al., 2008).
43
was established as a co-operative ESA-lead mission with contributions from the
French CNES and the Spanish Center for Technological and Industrial Development
(CDTI).
The initial configuration proposed an instrument working at 1.4 GHz with 25
equally spaced antenna elements in each one of its 4.5 m long arms, plus 4 additional receivers in the central hub. In order to fit in the launcher fairing, each arm
was proposed to be folded in five sections in stowed configuration. The instrument
would be installed on a PROTEUS generic platform provided by CNES with an
antenna plane tilted 20–30 ◦ with respect to nadir to guarantee an incidence angle
range within [0 ◦ , 50 ◦ ]. The orbit was proposed to be sun-synchronous with Equator
crossing at 6 AM (ascending) and 6 PM (descending) to minimize the perturbation
on L-band signal (air, vegetation and soil temperature almost identical) and making
the Faraday effect minimum. Raw measuring performances were expected to be:
30 to more than 90 km for ground resolution, 0.8–2 K for radiometric sensitivity,
1–3 days for temporal sampling, depending upon latitude, nature of the target and
location within the instrument FOV.
An important aspect in the SMOS proposal was the need to use novel calibration techniques, combining both on board reference noise sources of known power
level and external constant T B targets, to ensure a high stability of the measurements.
Concerning the operation mode, each 300 ms an image was to be taken, successively
in horizontal and vertical polarization. The huge amount of data generated forced
to propose some pre-processing on board, with averages of 5 images to obtain one
to be formatted and sent to the platform. The resulting equivalent integration time
is 1.5s/polarization, so 2 images are available every 3s. No mention was made by
then of the possibility of full polarization capability. At the moment of preparing the
proposal it was considered that the retrieval of ocean salinity required an independent measurement of sea surface temperature, to be provided through a secondary
frequency. The preliminary analysis indicated that a C-band channel could potentially be useful. However, this option was soon discarded due to the mass and power
limitations imposed by the use of the PROTEUS platform, suited for a minisatellite,
but not allowing simultaneous operations of the two instruments.
The SMOS Phase A development started in 2000, Phase B in 2002, and Phase
C/D in 2003 with a launch expected for 2007, that was later delayed until taking
place in November 2, 2009. A configuration optimization analysis (Waldteufel et al.,
2003) concluded, mainly driven by the more restringing soil moisture requirements
in terms of resolution and coverage, that the number of elements per arm should
be 21 (six on each one of three folding sections, plus three in the hub), the satellite steering angle 30 ◦ , the orbit height around 755 km, the tilt of the antenna plane
close to 33 ◦ , and the spacing between antenna elements 0.875 wavelengths. During
the detailed mission design it appeared that, in spite of the efforts made in successive improvements on the receivers and other components design, the PROTEUS
capability was really at the limit with almost no margin, so it was decided to remove
three of the receivers in the hub. The final number of antenna elements is 69 and
72 receivers, 66 LICEFs and six noise injection radiometers, are connected to them
(McMullan et al., 2008).