Overview of Satellite Oceanography Techniques 231 Passive Active Techniques and Choice of Frequencies

There are two main types of satellite techniques to observe the ocean1. Passive techniques measure the natural radiation emitted from the sea or from reflected solar radiation. Active or radar techniques send a signal and measure the signal received after its reflection at the sea surface. In both cases, the propagation of the signal through the atmosphere, the emission from the atmosphere itself must be taken into account to isolate the sea surface signal. The intensity and frequency distribution of the radiation that is emitted or reflected from the ocean surface allows the inference of its properties. The polarization of the radiation is also often used in microwave remote sensing.

1 Gravimetry satellites (e.g. GRACE, GOCE) which measure the earth gravity field and its variations do not enter into these two categories.

Satellite systems operate at different frequencies depending on the signal to be derived. Visible (400-700 nm) and infra-red (0.7-20 ^m) frequencies are used for ocean colour and SST measurements. Passive (radiometry) microwave systems (130 cm) are used for SST in cloud situations, wind, sea ice and sea surface salinity retrievals. Radars operate in the microwave bands and provide measurements of sea surface height, wind speed and direction, wave spectra, sea ice cover and types and surface roughness. Radar pulses are emitted obliquely (15°-60°) (SAR, scatterom-eter) or vertically (altimetry).

The choice of frequencies is limited by other usages (e.g. radio, cellular phones, military and civilian radars, satellite communications). Those are particularly important at microwave frequencies in the range 1-10 GHz which puts strong pressures on the frequencies used for earth remote sensing. The atmosphere also greatly affects the transmission of radiation between the ocean surface and the satellite sensors. The presence of fixed concentrations of atmospheric gases (e.g. O2, CO2, O3) and of water vapor means that only a limited number of windows exist in the visible, infra-red and microwave for ocean remote sensing. Even at these frequencies, the propagation effects through the atmosphere must be taken into account and corrected for. Propagation effects through the ionosphere must also be taken into account. Clouds are a strong limitation of visible and infrared measurements.

There are also technological constraints for the choice of frequencies. The resolution of a given sensor is generally related to the ratio between the observed wavelength (A) and the antenna diameter (D). For antenna diameters of a few meters, typical resolution around 1 GHz (wavelength of 30 cm) is about 100 km while at 30 GHz (wavelength of 1 cm), resolution is about 10 km. Radar altimeters use pulse limited techniques (that are much less sensitive to mispointing errors). Their footprint size is related to the pulse duration and is much smaller than for a beam limited sensor. Synthetic Aperture Radar uses the motion of the satellite to generate very long antenna (e.g. 20 km for ASAR) and thus to provide very high resolution measurements (up to a few meters).

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