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¥The 3 minute timescale corresponds to the timescale over which wind
characteristics (speed or direction) change.
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¥Measurements shown encompassed a variety of antenna mostions (FS,
OTF, sidereal tracking, etc.).
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¥Measurement time periods were (L1,L2,L3,L4) = (250,360,25,est) hours
and (est,360,25,est) hours for VertexRSI and AEC.
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¥The daily temperature variation of the fork arms and the
corresponding path length variation Delta(L2) are correlated. These temperataure-induced
variations can be approximated with good accuracy by sine--functions. This
holds also for the ambient air temperature, at least as measured at the VLA
site during most of the time of the tests. When adopting a sine--function
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L$_{2}$(t) = L$_{\rm o}$ +
$\Delta$L$_2\sin(\omega\,t)$,
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with $\omega$ =
2\,$\pi$/24\,h and t = time, the path length variation $\Delta$\,L$_{2}$ at
a 3, 10, and 30 minute time interval can be derived by differentiation of
the function L$_{2}$(t). The corresponding result is shown by solid lines,
which gives an explanation of the double peaked form of the measured
variations. The minima of
$\Delta$\,L$_{2}$ occur around noon and midnight where the temperature
changes of the ambient air and the steel of the fork are smallest.
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