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Occultation Movie
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QuickTime Movie: Occultation Concept
Here we show an animation of a setting GPS occultation observed by a
low earth orbiter (LEO). The LEO will track all GPS satellites above its
local horizontal (red links), sampling once every 10 sec, for onboard
navigation and for later precise orbit determination. When a GPS satellite
falls below the local horizontal, the receiver switches to a 1 sec sample
rate for more detailed ionospheric sensing (green link). When the signal
reaches the top of the neutral atmosphere at ~100 km altitude, represented
by the light blue curve, the receiver switches to a high sampling rate
(50-100 samples/sec) to capture a detailed profile of atmospheric
refractivity (white link). In the lowest part of the atmosphere the receiver
will perform "open loop" sampling to ensure capture of the full profile down
to the surface, even after the tracking loops have lost lock. Because of
significant bending in the lower atmosphere, the signal can be received by
the LEO well beyond the point where pure straight line contact would be lost.
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