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ITU-R RS.1449

January 1, 2000

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Description / Abstract:


Spaceborne microwave science sensors make use of very narrow beam antennas to obtain such information as surface temperature, moisture content, sea state and others. Certain passive sensors make use of the 18.6-18.8 GHz band. This same band is shared with GSO satellite systems operating in the FSS. There is potential for excessive interference into the passive sensors from these GSO satellites. This would be dominated by energy scattered from terrestrial targets into the receiving antennas of these passive sensors. The level of interference is affected by: the individual GSO pfd; number of simultaneous GSO systems; methods of frequency reuse within the FSS; the reflectivity of the terrain as characterized by the terrain scattering coefficient; and the mode of operation of the spaceborne sensor. In the United States of America, passive sensors have a primary allocation in the 18.6-18.8 GHz band. The United States of America limits the FSS pfd to −101 dB(W/(m2 · 200 MHz)). Internationally, passive sensors have a primary allocation in Region 2 and secondary allocations in Regions 1 and 3. RR Table S21.4 limits FSS pfd to –92 dB(W/(m2 · 200 MHz)) at low elevation angles up to –82 dB(W/(m2 · 200 MHz)) and higher elevation angles.

Consequently, there is a potential of pfd from the FSS from –101 dB(W/(m2 · 200 MHz)) to –82 dB(W/(m2 · 200 MHz)) globally. Since the spaceborne passive sensors would be exposed only intermittently to scattered energy from FSS coverage areas, it is of interest to determine the rate of occurrence of excessive interference events. Recommendation ITU-R RS.1029 states that in shared frequency bands (except in the absorption bands), the interference levels given above (−155 dB(W/100 MHz) for 18.6-18.8 GHz) can be exceeded for less than 5% of all measurement cells within a sensor's service area in the case where the loss occurs randomly, and for less than 1% of measurement cells in the case where the loss occurs systematically at the same locations.

The objective of this work is to identify the areas of excessive interference for the different levels of potential FSS pfd. The criteria from Recommendation ITU-R RS.1029 indicate the appropriate metric for interference depends on the particular sensor application and the nature of the interference that occurs. Herein, we report estimates for both conditional events (rate of occurrence given the spaceborne sensor is within a FSS coverage area) and unconditional events (rate of occurrence globally).

A secondary goal is to evaluate potential interference mitigation techniques and to describe a potential method of mitigating interference by avoiding geometry where the sensor might be pointed directly, or nearly so, into a specular reflection. It has been suggested that additional mitigation could be achieved by restricting the sensor scan range from ±70° to ±35°. Both methods were evaluated for a specific case of 4, 8, and 16 GSO systems in the FSS simultaneously serving a coverage area described with 24 spot beams.

The approach was to make use of Monte-Carlo simulations where the interference into a spaceborne sensor is estimated as it orbits the Earth. The motion of the GSO satellites, the Earth, and the spaceborne sensor are all accounted for. Since the scattering of energy from the Earth's surface is a random phenomenon (due to independent fading effects and terrain variability), this variability is also included in the simulation. At each simulation time instant, the interference from all 16 satellites was accounted for including the weighting by the directivity and angular offset of the individual spot beams.