[evlatests] L302 and phase spins.

[Pete Whiteis]

Does anyone know which executor script was used?

vdhawan at aoc.nrao.edu wrote:

>		PHASE SPINS on L302
>
>Data of October 27, UT 05:30 to 08:30. X-band.
>
>1. Continuum data at 0.4s integration
>	Short scan ~2min on 3C48, long scan ~15min 3C84, the source pair
>	repeated 4 times plus part of a 5th. 
>
>	Phase spins at:
>
>	ant 14, IF A, C. First 4 long scans spin, 5th does not - 
>			 the L302 has recovered. 
>
>			Short scans do not spin. 
>			B, D IFs never spin.
>
>	ant 16, IFs A, B, C, D spin in the first 4 long scans,
>				not the 5th.
>
>			Short scans do not spin.
>
>
>2. In between each ~20min continuum cycle was a ~20min cycle, 
>	same pattern, line mode, 3s integrations, same 2 sources,
>	5 cycles plus part of a 6th.
>
>	Phase spins at 
>
>	ant 14, IF A, C; on 6th long scan only
>
>	ant 16, IF A, B, C, D. 4th and 6th long scans.
>
>The details above show that the phase spins are absent in the line
>data but present in the continuum data interspersed in time.
>
>In addition to the spins, there are seemingly random offsets in phase 
>when an antenna returns to a source, on most IFs & most scans.
>
>The rate of phase change (fringe frequency or fringe rate)
>was comparable to the natural rate from changing geometry,
>so I thought it may be due to timing errors or mis-application 
>of the geometry model. 
>
>So, I
>
>a. Attempted to extract L302 DDS frequencies from the monitor logs.
>   These are not accurate to the (sub-Hz) level required for
>   this particular goose chase.
>
>b. Commands are not echoed to the log, so no luck checking what
>	the L302 thought it was doing.
>
>c. Finally, I took the geometric delay (TAU_geo) column in the 
>   AIPS CL table, for antenna/source. Differentiated that to get 
>   TAU_dot, the rate of change, in sec/sec. The 'natural' fringe rate 
>   from Earth rotation for an antenna on a given source is FR = 
>   (Tau_dot * F), F=8.4GHz. 
>
>   The FR on a given antenna, (when it spins at all) matches
>   (FR_1 - FR_2) the difference in FR for the 2 sources,
>
>	i.e.,
>
>   The spin is due to the L302 not updating the model for the 
>   (long) scan after it is done with the short scan. The rates
>   on the short scans are so slowly changing that a missed update
>   just shows up as a phase offset with hardly any rate, when that
>   source is re-visited 20 min later.  
>
>   The antenna is correctly pointed at the desired source - it is the 
>   fringe tracking that is not happening for that source.
>
>   If my verbal handwaving is unconvincing, I have plots to wave - 
>   come by or drop an email.
>
>   Some questions:
>
>   How exactly does the L302 gets confused? (Ken S. suggested the FPGA
>   that gets the updates might be confused); 
>
>   Why does it recover without reboot?
>
>   Why does the line data appear normal while the continuum data is hit by
>   L302 not updating? 
>
>   Why is this not seen in baselines runs where many sources are
>   observed in short order? (perhaps a recent breakage?)
>
>   One suggestion is to try a test where a few sources are cycled over, like
>   the test of Oct 27.
>
>Vivek.
>
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>  
>

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