National Radio
Astronomy Observatory

4.4.3.3 GSPLINE solutions

At high radio frequencies, where tropospheric phase fluctuates rapidly, it is often the case that there is insufficient signal-to-noise ratio to obtain robust ’G’ or ’T’ solutions on timescales short enough to track the variation. In this case it is desirable to solve for a best-fit functional form for each antenna using the ’GSPLINE’ solver. This fits a time-series of cubic B-splines to the phase and/or amplitude of the calibrator visibilities.

The combine parameter (§ 4.4.1.5) can be used to combine data across spectral windows, scans, and fields. Note that if you want to use combine=’field’, then all fields used to obtain a ’GSPLINE’ amplitude solution must have models with accurate relative flux densities. Use of incorrect relative flux densities will introduce spurious variations in the ’GSPLINE’ amplitude solution.

The ’GSPLINE’ solver requires a number of unique additional parameters, compared to ordinary ’G’ and ’T’ solving. The sub-parameters are:

The duration of each spline segment is controlled by splinetime. The actual splinetime will be adjusted such that an integral number of equal-length spline segments will fit within the overall range of data.

Phase splines require that cycle ambiguities be resolved prior to the fit; this operation is controlled by npointaver and phasewrap. The npointaver parameter controls how many contiguous points in the time-series are used to predict the cycle ambiguity of the next point in the time-series, and phasewrap sets the threshold phase jump (in degrees) that would indicate a cycle slip. Large values of npointaver improve the SNR of the cycle estimate, but tend to frustrate ambiguity detection if the phase rates are large. The phasewrap parameter may be adjusted to influence when cycles are detected. Generally speaking, large values (> 180^∘) are useful when SNR is high and phase rates are low. Smaller values for phasewrap can force cycle slip detection when low SNR conspires to obscure the jump, but the algorithm becomes significantly less robust. More robust algorithms for phase-tracking are under development (including fringe-fitting).

For example, to solve for ’GSPLINE’ phase and amplitudes, with splines of duration 600 seconds,

ALERT’: The ’GSPLINE’ solutions can not yet be used in fluxscale. You should do at least some ’G’ amplitude solutions to establish the flux scale, then do ’GSPLINE’ in phase before or after to fix up the short timescale variations. Note that the “phase tracking” algorithm in ’GSPLINE’ needs some improvement.

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