26 Pierre-Yves Le Traon
results recently obtained with TIP and ERS-1I2 altimeter data before discussing
real-time aspects.
2.2.2 Measurement principle
The principle of altimetry measurement is simple (although the system is complex). The altimeter measures the range from the satellite to the ocean surf ace by
determining the time taken by the radar pulse to travel from the satellite to the
ocean surface. U sing a precise orbitography system, we can determine the position
of the satellite relative to a reference ellipsoid. Combining these two measurements
yields an estimation of the sea level relative to a reference ellipsoid. This estimation comprises the geoid (an equipotential of the earth gravity field to which a
motionless ocean would exactly conform) and the ocean dynamic topography. The
geoid can vary as much as 100 m for only 1 m for the dynamic topography (the
parameter of interest here).
Altimetry also provides measurements of significant waveheight (the radar pulse
is first reflected by crests and then by troughs) and wind speed (the power of the
retumed signal depends on wind speed). These parameters are also used to estimate electromagnetic bias correction (interaction of the electromagnetic wave with
the ocean surf ace ).
The satellite repeats exactly the same ground track every cycle. For TIP and
Jason-l, this cycle is 10 days, for ERS-1I2 multi-disciplinary phase and for ENVISAT it is 35 days, and for GEOSAT and GFO 17 days. Every cycle, it thus
observes the same geoid signal (which does not vary over time) and the dynamic
topography (which varies over time).
2.2.3 Content of altimetric measurement
Geostrophy
Assuming the geoid is known (we will discuss this particular problem later on),
satellite altimetry provides measurements of ocean surface dynamic topography,
which is directly related to ocean currents through the geostrophic approximation.
Simple scaling of the Navier Stokes equations shows that for space scales larger
than a few tens of km and time scales longer than a few days (except for a thin
layer at the surf ace which is directly driven by winds-the so-called Ekman layer),
the circulation is in geostrophic equilibrium (e.g. Pedlosky, 1979). This means that
the pressure force is balanced by the Coriolis force. This yields equations 1 and 2:
f = 2Qsin9
(1)
(2)
where Q is the angular rotation ofthe earth, e is latitude and P the pressure.
results recently obtained with TIP and ERS-1I2 altimeter data before discussing
real-time aspects.
2.2.2 Measurement principle
The principle of altimetry measurement is simple (although the system is complex). The altimeter measures the range from the satellite to the ocean surf ace by
determining the time taken by the radar pulse to travel from the satellite to the
ocean surface. U sing a precise orbitography system, we can determine the position
of the satellite relative to a reference ellipsoid. Combining these two measurements
yields an estimation of the sea level relative to a reference ellipsoid. This estimation comprises the geoid (an equipotential of the earth gravity field to which a
motionless ocean would exactly conform) and the ocean dynamic topography. The
geoid can vary as much as 100 m for only 1 m for the dynamic topography (the
parameter of interest here).
Altimetry also provides measurements of significant waveheight (the radar pulse
is first reflected by crests and then by troughs) and wind speed (the power of the
retumed signal depends on wind speed). These parameters are also used to estimate electromagnetic bias correction (interaction of the electromagnetic wave with
the ocean surf ace ).
The satellite repeats exactly the same ground track every cycle. For TIP and
Jason-l, this cycle is 10 days, for ERS-1I2 multi-disciplinary phase and for ENVISAT it is 35 days, and for GEOSAT and GFO 17 days. Every cycle, it thus
observes the same geoid signal (which does not vary over time) and the dynamic
topography (which varies over time).
2.2.3 Content of altimetric measurement
Geostrophy
Assuming the geoid is known (we will discuss this particular problem later on),
satellite altimetry provides measurements of ocean surface dynamic topography,
which is directly related to ocean currents through the geostrophic approximation.
Simple scaling of the Navier Stokes equations shows that for space scales larger
than a few tens of km and time scales longer than a few days (except for a thin
layer at the surf ace which is directly driven by winds-the so-called Ekman layer),
the circulation is in geostrophic equilibrium (e.g. Pedlosky, 1979). This means that
the pressure force is balanced by the Coriolis force. This yields equations 1 and 2:
f = 2Qsin9
(1)
(2)
where Q is the angular rotation ofthe earth, e is latitude and P the pressure.