Models of the electrical current collected by a planar aperture ion sensor generally assume that the 2 pi steradian field of view is unobstructed. This is not always achieved. This report shows the development of a model for the measured current based on a simple approximation for the obstructions that are placed in the field of view of any planar aperture ion sensor. The details of the model given herein are related to the ion sensors onboard two Air Force satellites, where the sensors have been recessed below the level of the spacecraft skin. However, the model can easily be adapted to many other cases of obstructions in the field of view. For collection of data when the ambient ion thermal speed is negligible compared to the relative bulk flow (ram) speed, the obstructions are only important when the flow vector from the ambient plasma to the plasma sensor intersects the obstructions. For the collection of data when the ion thermal speed is comparable to the relative bulk flow speed, the obstructions affect the amount of current collected by the sensors at all orientations of the spacecraft to the bulk flow direction. (Author).

Models of the electrical current collected by a planar aperture ion sensor generally assume that the 2 pi steradian field of view is unobstructed. This is not always achieved. This report shows the development of a model for the measured current based on a simple approximation for the obstructions that are placed in the field of view of any planar aperture ion sensor. The details of the model given herein are related to the ion sensors onboard two Air Force satellites, where the sensors have been recessed below the level of the spacecraft skin. However, the model can easily be adapted to many other cases of obstructions in the field of view. For collection of data when the ambient ion thermal speed is negligible compared to the relative bulk flow (ram) speed, the obstructions are only important when the flow vector from the ambient plasma to the plasma sensor intersects the obstructions. For the collection of data when the ion thermal speed is comparable to the relative bulk flow speed, the obstructions affect the amount of current collected by the sensors at all orientations of the spacecraft to the bulk flow direction. (Author).

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