[daip] Possible POSSM bug

[Eric Greisen]

Walter Brisken wrote:
> Hi DAIP,
> 
> Craig and I stumbled upon a likely bug in POSSM last week.  We were 
> plotting some VLBA data taken in 8 contiguous 32 MHz sub-bands produced 
> by the new digital equipment.  SPFLG suggested that the phases matched 
> perfectly at the sub-band edges, as one would expect.  In POSSM, phase 
> jumps at each sub-band boundary were seen.
> 
> It turns out that the parameter SHIFT was unintentionally set to 0,1. 
> Changing it to 0,0 produced the results we expected.  Craig correctly 
> later stated that the shift's effect on phase should not care about the 
> location of the sub-band edges.  I further tested this and for small 
> shifts (SHIFT=0,0.01) do see continuous phase as a function of frequency. 
> I suspect either there is an error in the handling of delay or there is 
> some numerical precision issue.
> 
> Test data can be found on parallax, user id 2179, disk 4, catalog number 
> 81 (TD010.DIFX.6).

POSSM in its wisdom decided that - for phase shifting but not other 
things - that all IFs have the same frequency.  Fixed by tomorrow.

Eric Greisen