156
IAN ROBINSON
3.
Some satellite measurements used for operational
models
3.1
Sea surface height anomaly from altimeters
3.1.1
The principles of altimetry over the ocean
A satellite altimeter is a nadir-viewing radar which emits regular pulses
and records the travel time, the magnitude and the shape of each return
signal after reflection from the earth s surface. The travel time is the
essential altimetric measurement, leading to a determination of the ocean
surface topography at length scales longer than about 100 km. Ocean
surface topography contains information about ocean dynamical and
geophysical phenomena. If the travel time can be measured to a precision of
6×10
-11 s then, knowing the speed of light, the distance can be calculated to a
resolution of 1 cm. Corrections have to be made to allow for the changed
speed of light through the ionosphere and the atmosphere, and for delays
associated with reflection from a rough sea surface (Chelton et al., 2001). It
is generally agreed that for these corrections to approach the target accuracy
of 1 cm a dual frequency altimeter must be used (to determine the
ionospheric refraction), and a three channel microwave radiometer is needed
to sound the water vapour in the atmosphere.
The altimeter is not an imaging sensor. Viewing only the nadir point
below the satellite, it simply records measurements of distance between the
satellite and the sea surface along the ground track. As discussed in 2.2, the
spatial and temporal sampling characteristics therefore depend entirely on
the exact orbit repeat cycle of the satellite. This was chosen to be about 10
days for the TOPEX/Poseidon (T/P) and Jason altimeters which fly on
platforms dedicated to the altimetric mission, although for other altimeters it
has ranged between 3 days, 17 days and 35 days. The longer the revisit
interval the finer the spatial sampling grid.. Typically, ocean topography
data are interpolated onto a geographical grid and composited over the
period of an exact repeat cycle, to produce images which are comparable
with global SST or ocean chlorophyll composite images although produced
in a completely different way.
By itself, knowing the distance, R alt between the ocean surface and a
satellite is of limited value. Figure 5 shows what else needs to be defined or
measured for this to yield an oceanographically useful property. First of all,
when the height of the satellite, H sat , is known relative to a reference level,
then the height, h, of the sea above the reference level can be determined.
The reference level is a regular ellipsoid-shaped surface defined within a
frame of reference fixed in the rotating earth. It is chosen to match
approximately the shape of the earth at sea level, and provides a convenient
datum from which to measure all other heights.
“
”
,
IAN ROBINSON
3.
Some satellite measurements used for operational
models
3.1
Sea surface height anomaly from altimeters
3.1.1
The principles of altimetry over the ocean
A satellite altimeter is a nadir-viewing radar which emits regular pulses
and records the travel time, the magnitude and the shape of each return
signal after reflection from the earth s surface. The travel time is the
essential altimetric measurement, leading to a determination of the ocean
surface topography at length scales longer than about 100 km. Ocean
surface topography contains information about ocean dynamical and
geophysical phenomena. If the travel time can be measured to a precision of
6×10
-11 s then, knowing the speed of light, the distance can be calculated to a
resolution of 1 cm. Corrections have to be made to allow for the changed
speed of light through the ionosphere and the atmosphere, and for delays
associated with reflection from a rough sea surface (Chelton et al., 2001). It
is generally agreed that for these corrections to approach the target accuracy
of 1 cm a dual frequency altimeter must be used (to determine the
ionospheric refraction), and a three channel microwave radiometer is needed
to sound the water vapour in the atmosphere.
The altimeter is not an imaging sensor. Viewing only the nadir point
below the satellite, it simply records measurements of distance between the
satellite and the sea surface along the ground track. As discussed in 2.2, the
spatial and temporal sampling characteristics therefore depend entirely on
the exact orbit repeat cycle of the satellite. This was chosen to be about 10
days for the TOPEX/Poseidon (T/P) and Jason altimeters which fly on
platforms dedicated to the altimetric mission, although for other altimeters it
has ranged between 3 days, 17 days and 35 days. The longer the revisit
interval the finer the spatial sampling grid.. Typically, ocean topography
data are interpolated onto a geographical grid and composited over the
period of an exact repeat cycle, to produce images which are comparable
with global SST or ocean chlorophyll composite images although produced
in a completely different way.
By itself, knowing the distance, R alt between the ocean surface and a
satellite is of limited value. Figure 5 shows what else needs to be defined or
measured for this to yield an oceanographically useful property. First of all,
when the height of the satellite, H sat , is known relative to a reference level,
then the height, h, of the sea above the reference level can be determined.
The reference level is a regular ellipsoid-shaped surface defined within a
frame of reference fixed in the rotating earth. It is chosen to match
approximately the shape of the earth at sea level, and provides a convenient
datum from which to measure all other heights.
“
”
,