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VLBI - techniques p2 (Clocks and timing (All interferometry relies on…
VLBI - techniques p2
Clocks and timing
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To track delay/phase errors to transfer from reference to target, the data recorder time signal must be stable during the cycle time
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Correlator delay model
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Accounts for e.g. Earth rotation, the delay on a single baseline towards a sky target chages with time
To track a source in VBI, we need to mode the decay changes
The "delay model" calculates the delay between two telescopes at a particular time to get fringes in a specific direction
Tota delay = geometry + Earth rotation (+tides etc) + clocks + electronics + troposphere + ionosphere + special and general relativity.
Typically schedule periodic observations of a bright fringe finder source where fringes can be found in correlator
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Phase referencing
Similar to what we have seen for our secondary phase calibrator, just need to be close
Fringe fitting requires observations of a bright, compact source -> e.g. phase cal
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Obtain fringe fitting solutions for phase, delay and rate but applying fringe fitting to calibrator and applying to target
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Closure phase
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Fringe fitting (and self-cal) uses this triangle to solve for the residual phases, rates and delay
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Flux calibration
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Long
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During observations
Combine 1,2,3 noise dioe can be used a flux density reference
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Very big data rates (432 TB per day, 4320TB per session), so discarded basebnd data after correlation (Store and distribute correlated data products)