Nomiii.ll SbV.vth = 1000 km

PI : Boiesiyhl P2: Nadir P3: Lowei <ilony tuck b.ickwjtd

P-t: U|>|)«i cornel P5: U|>|jei cornel Nairow sw.nli

Fig. 13 SMOS Field of View and its Projection onto the Earth's Surface (Barre et al. 2008)

Fig. 13 SMOS Field of View and its Projection onto the Earth's Surface (Barre et al. 2008)

For ocean salinity, the entire earth shall be covered in no longer than 7 days. The narrow swath of SMOS is used to satisfy this requirement. In Fig. 13, the two inner vertical lines represent the narrow swath of about 600 km.

The nominal measurement mode of MIRAS is characterised by an earth-fixed attitude with a constant forward tilt-angle of 32.5 deg. between the instrument bore-sight and the local nadir in the flight direction. A yaw-steering angular motion around the local nadir is implemented to compensate for earth rotation effects of about 4 deg. on the ground-trace of the MIRAS images. This measurement-mode attitude and image geometry results in the instrument FOV on ground as illustrated by Fig. 13.

The external calibration modes used to calibrate the MIRAS instrument by pointing to known celestial targets, mainly deep space, are implemented by the satellite by executing slew manoeuvres in the orbital plane in two attitude sub-modes:

• inertial attitude, where the instrument boresight is controlled and pointed in a constant inertial direction

• earth-fixed attitude, where the instrument boresight is controlled and pointed in a constant pitch (or tilt) angle defined in the local orbital reference frame.

A particular case of interest is when the satellite is oriented and maintained in the zenith direction allowing the payload to image the deep sky while keeping the earth outside the main lobe of the antenna. Both external calibration modes allow calibration of the instrument against given celestial targets for a duration of up to 30 minutes with a pointing stability of better than 0.3 deg. and a pointing knowledge accuracy of less than 1 deg. The complete duration of the external calibration modes, including slews and returning to nominal measurement attitude, is less than 1 orbital period of 100 minutes. Slews have a typical duration of 24 minutes.

In addition to the MIRAS-specific modes described above, the PROTEUS platform features standard attitude modes such as Orbit Correction Manoeuvres (OCM) and Safe Hold Mode (SHM). OCM modes with two or four thrusters (OCM-2/OCM-4) are used to maintain the altitude (in-plane manoeuvres) and inclination (out-of-plane manoeuvres) of the SMOS orbit throughout the mission lifetime. Safe Hold Mode (SHM) is initiated to ensure safety and survivability of the satellite in case of anomalies or FDIR (Failure Detection, Isolation and Recovery). In SHM, also entered just after separation from the launcher, the base of the PROTEUS platform is pointed towards the sun ensuring a known and stable thermal environment and for provision of electrical power via optimum solar array orientation to the sun with battery charging.

An example of a typical brightness temperature measurement along one swath is presented in Fig. 14. It consists of a series of consecutive snapshots (as per Fig. 5) and for each snapshot, which corresponds to a period of 1.2 s, the MIRAS instrument measures the complex visibility function of the observed scene which is subsequently converted to a calibrated brightness temperature map in the Level 1 processor.

Fig. 14 SMOS Sea Brightness Temperature Image of Western Europe

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