The SKIM Mission

Scientific objectives: surface currents, waves & sea ice drift

Time series of typical ocean conditions showing surface current speed (left) and time series of eastward current component (right), provided by the MITgcm 1.5 km resolution model, AVISO altimetry portal, CMEMS operational 1/12° global ocean model. The in situ data from the North Pacific Ocean location (Cronin et al., 2019) and equator (Bourles et al., 2008) are at 15 and 11 m depth, and show the importance of near-inertial motions and diurnal variability that are weakly captured in models and absent in altimetry.

ESA

The figure above shows two example timeseries of in-situ surface current measurements (in blue) performed in the North Pacific (station PAPA) and the Equatorial Atlantic (PIRATA buoy array), together with estimates obtained using satellite altimetry (in green) and state-of-the-art numerical models (in black and red). Clearly, a very large share of the variability visible in the in-situ observations is not accounted for by any of the other estimation methods.

To obtain accurate estimates of the Total Surface Current Vector over the world ocean, it is clear that a new measurement technique has to be implemented.

The main objective of the SKIM mission is to bridge this gap using satellite microwave radar Doppler measurements of the Total Surface Current Vector, (i.e. including the geostrophic component, but also the other components such as the wave-associated Stokes drift and wind-associated Ekman drift, tidal currents, Near-Inertial Oscillations, etc...), with a near-global (82°S →82°N) coverage and subweekly (4.5 days max.) repeat period.

Measurement Concept

Sketch of the interaction between microwave radiation and a wave modulated sea surface. The backscattered radiation is modulated both in intensity and frequency (Doppler frequency shift). The intensity modulation is used in SKIM to retrieve the sea surface wave directional spectrum, and the Doppler frequency modulation is used to retrieve the Geophysical Doppler (GD) frequency shift. The intrinsic motion of the surface waves induces a Wave Doppler (WD) frequency shift. The WD is estimated using the directional spectrum, and removed from the GD to yield the line-of-sight projection of the TSCV. Figure taken from (Marié et al, 2020).

EGU

The figure above now shows a sketch of the interaction between microwave radiation and a sea surface modulated by the motion of a surface wave: the backscattered radiation is modulated by the surface wave, both in intensity and frequency (Doppler shift).

The amplitude modulation is exploited to retrieve the sea surface waves directional spectrum (as by the SWIM instrument onboard the CFOSat mission, see Hauser et al, 2021), and the Doppler frequency modulation, after subtraction of the Non-Geophysical (NG) Doppler due to the platform motion, gives access to the Geophysical Doppler (GD) frequency shift.

The GD frequency shift contains a large component due to the intrinsic motion of the surface waves, the Wave Doppler (WD) frequency shift.

In the SKIM concept, this WD is estimated using the sea surface waves directional spectrum and subtracted from the GD, to finally give access to the line-of-sight projection of the TSCV.

SKIM sea surface wave spectrum and TSCV retrieval concept

To obtain measurements with the necessary azimuthal diversity, the SKaR transmits its microwave pulses from six microwave horns mounted on a rotating turntable, which are sent towards the sea surface using an offset parabolic reflector.

Schematic of the SKaR sampling pattern illustrated over the Gulf Stream region, using 6 beams and a rotating plate rotation rate of 6.25 rpm. The blue points mark the Nadir-beam footprints, the red and yellow points mark the footprints of the two 6° incidence beams, and the other points mark the footprints of the three 12° incidence beams. The 6° swath is roughly 150 km wide, while the 12° swath is roughly 300 km wide.

ESA

The resulting sampling pattern is illustrated in the figure above. One can see that by optimizing the succession of the horns in the macrocycle, the rotation rate of the rotating plate and the translation motion of the platform finally allows the SKaR to populate its swath with a quite high density of footprints.

The figure on the left represents the interaction between a SKaR microwave pulse and the sea surface at one instantaneous Field-of-View (FoV). The colour shading represents the intensity of the radiation backscattered from the sea surface. The SKaR implements a time-of-flight resolution scheme in the range direction and a SAR resolution scheme in the azimuth direction. The SKaR sends 1024 pulses towards each such Field-of-View (FoV) at 32 kHz, then switches to a different beam to illuminate a different FoV. During a full macrocycle, the six beams are successively illuminated, in a little under 226 ms.

When a full 360° azimuthal coverage has been achieved, the range-resolved intensity modulation segments acquired over the individual cycles are combined together to produce a sea surface wave directional spectrum, as shown on the right in the figure. This step is a direct heritage of the SWIM instrument onboard CFOSat.

The directional spectrum is also used to estimate the wave contribution to the observed Geophysical Doppler frequency shift. This contribution can then be subtracted to the individual Geophysical Doppler measurements obtained at the different footprints. At this stage, one obtains samples of the line-of-sight projection of the TSCV over the swath. This L2b_U product (depicted on the left in the figure above) can be used by expert users for fine-grained analysis of specific TSCV features. In most cases, however, mapped estimates of the TSCV will be more directly relevant. For these use cases, L2c maps of the cartesian components of the TSCV will be provided. The right panel in the above figure above shows the coverage achieved by SKIM over 1 day.