3 SMOS and Aquarius/SAC-D Missions
51
for resolving the basin scale SSS field. This will filter much of the eddy and frontal
scales, yet provide much greater detail than is derived from historical data (World
Ocean Atlas 2005), as illustrated by Lagerloef et al. (2008). The three Aquarius
microwave radiometers will measure microwave brightness temperature in vertical
and horizontal polarizations (T BH and T BV respectively), as well as polarimetric
channels to correct for the Faraday rotation of the signal as it passes through the
ionosphere (Yueh, 2000). These sensors are aligned with an offset 2.5 m aperture
antenna reflector to generate the 3 fixed beams at incidence angles of 28.7, 37.8 and
45.6 ◦ relative to the ocean surface and form the 3 distinct footprints aligned across
the swath (more technical details in Le Vine et al., 2007). The Aquarius microwave
radiometers have very demanding requirements for low noise and calibration
stability, and will be the most accurate ever developed for Earth remote sensing.
The satellite will be placed in a sun-synchronous polar orbit crossing the equator northward (ascending) at 6 PM. The sensor will be viewing away from the sun
to avoid solar contamination of the science measurement. The 7-day repeat orbit
track spacing at the equator is equal to the swath width. This ensures that the sampling pattern gives total area coverage (no significant swath gaps) and sufficient
repeat observations to allow the errors to be reduced by monthly averages. The
primary Aquarius microwave sensor combines an L-band microwave radiometer
of unprecedented accuracy with an integrated L-band radar to provide a measurement correction for surface roughness, which as noted above, is the significant error
source.
The CONAE Microwave Radiometer (MWR) will make complementary measurements of rain, wind and sea ice with 23.8 and 36.5 GHz channels in an
overlapping swath pattern (Lagerloef et al., 2008). The MWR data will be used
by the Aquarius data processing for rain and sea ice flags, and as supplementary
rain and surface wind speed correction algorithms. The New InfraRed Scanner
Technology (NIRST) camera is a narrow swath imager intended to detect forest
fires and other thermal events on land. It can be tilted to observe preferred targets,
and on occasion will be used to map SST within one Aquarius footprint.
3.5.2 Key Science Requirements
The principal scientific requirement is to make global SSS measurements over the
open oceans with 150 km spatial resolution, and to achieve a measurement error less
than 0.2 (pss) on a 30 day time scale, taking into account all sensor and geophysical
random errors and biases. For comparison, the Global Ocean Data Assimilation
Experiment (GODAE) requirement is one sample every 10 days/200 km 2 and SSS
error of 0.1. Presently, the Global Ocean Observing System (GOOS) provides about
40% of this global sampling (exclusive of the sea ice covered regions and continental
shelf) with in-situ observations (principally Argo). The requirement applies to the
open ocean, sufficiently far from land or ice boundaries so that the warmer land and
ice brightness temperatures, as compared to the ocean surface, do not contaminate
the radiometric measurement. Generally this boundary zone is about 2–3 times the
footprint diameter. The baseline mission is designed to operate for up to 3 years,
with potential extended durations for 2 years or longer.
51
for resolving the basin scale SSS field. This will filter much of the eddy and frontal
scales, yet provide much greater detail than is derived from historical data (World
Ocean Atlas 2005), as illustrated by Lagerloef et al. (2008). The three Aquarius
microwave radiometers will measure microwave brightness temperature in vertical
and horizontal polarizations (T BH and T BV respectively), as well as polarimetric
channels to correct for the Faraday rotation of the signal as it passes through the
ionosphere (Yueh, 2000). These sensors are aligned with an offset 2.5 m aperture
antenna reflector to generate the 3 fixed beams at incidence angles of 28.7, 37.8 and
45.6 ◦ relative to the ocean surface and form the 3 distinct footprints aligned across
the swath (more technical details in Le Vine et al., 2007). The Aquarius microwave
radiometers have very demanding requirements for low noise and calibration
stability, and will be the most accurate ever developed for Earth remote sensing.
The satellite will be placed in a sun-synchronous polar orbit crossing the equator northward (ascending) at 6 PM. The sensor will be viewing away from the sun
to avoid solar contamination of the science measurement. The 7-day repeat orbit
track spacing at the equator is equal to the swath width. This ensures that the sampling pattern gives total area coverage (no significant swath gaps) and sufficient
repeat observations to allow the errors to be reduced by monthly averages. The
primary Aquarius microwave sensor combines an L-band microwave radiometer
of unprecedented accuracy with an integrated L-band radar to provide a measurement correction for surface roughness, which as noted above, is the significant error
source.
The CONAE Microwave Radiometer (MWR) will make complementary measurements of rain, wind and sea ice with 23.8 and 36.5 GHz channels in an
overlapping swath pattern (Lagerloef et al., 2008). The MWR data will be used
by the Aquarius data processing for rain and sea ice flags, and as supplementary
rain and surface wind speed correction algorithms. The New InfraRed Scanner
Technology (NIRST) camera is a narrow swath imager intended to detect forest
fires and other thermal events on land. It can be tilted to observe preferred targets,
and on occasion will be used to map SST within one Aquarius footprint.
3.5.2 Key Science Requirements
The principal scientific requirement is to make global SSS measurements over the
open oceans with 150 km spatial resolution, and to achieve a measurement error less
than 0.2 (pss) on a 30 day time scale, taking into account all sensor and geophysical
random errors and biases. For comparison, the Global Ocean Data Assimilation
Experiment (GODAE) requirement is one sample every 10 days/200 km 2 and SSS
error of 0.1. Presently, the Global Ocean Observing System (GOOS) provides about
40% of this global sampling (exclusive of the sea ice covered regions and continental
shelf) with in-situ observations (principally Argo). The requirement applies to the
open ocean, sufficiently far from land or ice boundaries so that the warmer land and
ice brightness temperatures, as compared to the ocean surface, do not contaminate
the radiometric measurement. Generally this boundary zone is about 2–3 times the
footprint diameter. The baseline mission is designed to operate for up to 3 years,
with potential extended durations for 2 years or longer.