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| Special
Issues at Low Frequencies: |
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| Ionosphere |
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| • | Gravity effects
on observations |
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| – | Due to first and
second derivative of TEC |
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| – | Very dependent
on geometry: maximum when line of sight is |
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| along wave
fronts, small when perpendicular to wavefronts, or //B. |
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| – | Typical image
motion +/- 1 arcmin at 169 MHz, typical timescale: |
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| 30 mn. |
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| • | At least 10x in worst conditions (+ some
image distorsion) |
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| • | 10x less at
nightime. |
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| – | Focusing may
occur at low frequencies, exceptionnal cases up to |
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| 50 MHz at low
elevation. |
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