[evlatests] Large Bz error?

[Craig Walker]

See below.

Rich Moeser wrote:
> Vivek Dhawan wrote:
>> Looking under the lamp post for the usual things:
>>
>> EOP looks correct - I compared the numbers in the
>> WIDAR data header to the USNO file here:
>>
>> http://gemini.gsfc.nasa.gov/solve_save/usno_finals.erp
>>
>> I next tried to compare the XYZ antenna positions in the
>> AIPS header (via SDM and CASA) to the VLA pad positions
>> in the VLBA database which purport to be in ITRF:
>>
>> $SCHED/catalogs/locations.dat
>>
>> There is 20-30cm of difference in all Y & Z axes,
>> between a pad's location in the WIDAR header and its
>> location in the SCHED/USNO database. Its not just
>> a longitude shift since Z is involved. The error is
>> (to within ~1cm) independent of which pad I compare.
>>
>> It is also not clear whether including the continental
>> drift term (12.73 years since the USNO solution epoch)
>> is useful or not - X is better with the correction, but
>> the discrepancy is about 30cm in at least 2 axes with
>> or without the drift put in.
>>
>> Where do those WIDAR positions come from?
>>
>>   
> 
> The station positions in the SDM are derived using the geodetic location 
> constants x=-1601185.4, y=-5041977.5, and z=3554875.9 and adding the 
> station offsets x, y, and z from the parameters database.

Here is the entry for the phase center based on geodetic VLBI from the 
SCHED catalog.  I suspect that the above coordinates are from some other 
source, although they are good to order a few tens of cm.

         DBCODE=' Y'  DBNAME='VLA'  AXISTYPE='altz'  AXISOFF=  0.0000
     X= -1601185.2028 Y= -5041977.1611 Z=  3554875.7011
     DXDT= -0.01380 DYDT= -0.00030 DZDT= -0.00820 EPOCH=50449

These coordinates were derived with the help of a geodetic observation 
that included the antenna on N8.  Also, the VLA coordinates for Pie Town 
with the Pie Town link were useful as they could be related to the 
geodetic position for Pie Town and firm up the rotation between the VLA 
system and the ITRF.  The observations on which all this was based were 
in something like 1994 so they really should be repeated.  Also note 
that the plate tectonic motions are enough to build up errors that would 
be significant for VLBI in the nearly 13 years since the reference epoch 
(the rates, in m/yr, are based on the nearby VLBA stations).  I guess 
Vivek did a comparison in detail.  In case anyone else wants to look, 
SCHED calculates and prints the positions for the date of observation.

Using the phased array position from SCHED would likely be more accurate 
than the ones currently in the software, but it probably would not make 
much difference for internal VLA observations.

Recall that the last time I tried to put VLA coordinates into the ITRF,
I had to make a rotation that was about right to correspond to a one 
waveguide cycle timing offset.

Sometime we should do another geodetic run with two well-separated EVLA 
antennas and the VLBA to bring everything into the same system.

Cheers,

Craig




> 
> --Rich
> 
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     R. Craig Walker            Array Operations Center
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