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A technical summary of the MWAX fringe stopping implementation will be published soon as part of the forthcoming MWAX paper, a detailed description of the design and validation will be published shortly thereafter.
A (very) brief overview: when a fringe-stopping observation is scheduled, the scheduling software generates an additional data table in the metafits file sent to the correlator, containing periodically updated Az/Al values for each tile, reflecting small changes in pointing direction required during the observation period. These updated pointing directions are calculated at 4-second intervals using the astropy library and picked up where the real-time part of the fringe stopping system begins, during UDP voltage capture. For each Az/Al entry, and for each tile, a path length difference is calculated relative to the array centre, and converted to a time delay. The time delay is interpolated between the 4s reference points at a 5ms granularity, corresponding to the FFT block size in the correlator. The delay is split into a whole-sample and fractional-sample component, the latter of which corresponds to a delay shorter than the sampling period of the coarse-channelised voltage data. The whole-sample component is used to shift actual voltage samples in the data forward or back by an integer number of samples. The fractional component is stored in a metadata table in the voltage capture file, and applied by the correlator after the F-stage, in the frequency domain, as a phase rotation.
The fundamental unit of time for fringe-stopping is the sample, which at a sample rate of 1.28MHz, corresponds to about 0.7 microseconds. Delays are typically in the range of +/- 20 whole samples. The fractional component is represented in the metadata table with 32-bit floats, which we found to be sufficient - targeting a precision on the order of microsample resolution. 16-bit signed integers targeting millisample resolution were found to be insufficiently precise for fringe stopping.
Diagram: Comparison of geometric corrections process for using Birli and fringe-stopping
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