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### Why the GPS receivers are installed on the surface?
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### Why the GPS receivers are installed on the surface?
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GPS antenna, cable and receiver are normally characterized as a single
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entity. It is very convenient to keep an antenna cable length of ~40m,
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in such a way that the ensemble can be dismounted and sent to
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calibration by another lab (calibration by METAS) or another lab can
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bring a traveling GPS to our installation (PTB calibration).
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## Fiber length calibration
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## Fiber length calibration
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### Why haven't you used fiber reflectometry to measure fiber lengths?
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### Why haven't you used fiber reflectometry to measure fiber lengths?
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We avoided using reflectometry to calibrate our delays for the following
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We avoided using reflectometry to calibrate our delays for the following
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consideration:
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considerations:
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1. Commercial reflectometers are known to be accurate at the ~10ns
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1. Commercial reflectometers are known to be accurate at the ~10ns
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level, whereas we know that our internal delay chain is stable at
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level, whereas we know that our internal delay chain is stable at
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