Off-line baseline fitting
- The so-called ``wrap-dependent phase'': This is an error where the
phase of a source changed significantly depending on whether the source
was observed in the north or south wrap. Very little signature of the
error was noted in the round-trip measurements of both the 13 GHz and
160 MHz reference signals. It was found that the problem
relates to a standing wave in the 160 MHz reference. The changes in the tension
within the LO fibre as it goes through the azimuth wrap was causing
changing attenuation in the fibre. Because the standing wave went through
the azimuth wrap twice (forward and reverse) - unlike the reference
signal - the standing waves contribution to the overall system phase
varied with azimuth.
At its worst, the ``wrap-dependent phase'' could contribute the equivalent of 7mm of excess path to the signal.
A signature of the problem was that the phase error changed sign when going from 3cm to 12mm observing. Careful measurements also showed the phase error scaled with frequency between 6 and 3cm, but its magnitude was about half the frequency-scaled value at 12mm, and that the error seemed inconsistent with frequency-scaling at 3mm. The measurements were all consistent with the error scaling with the contribution of the 160 MHz reference signal to the downconversion process.
The solution to the problem has been to search for and remove the causes of the standing waves, and to instigate changes in the optical path to attenuate any standing waves that are generated.
- Implicitly, CAOBS assumes that the displacement between the focus points
on the different antennas is constant (for a given reconfiguration). Changing
azimuth and elevation does not vary the distance between the focus points.
Put another way, CAOBS implicitly assumes that the antennas are mechanically
identical in the way the focus point moves with azimuth and elevation. Although
this is good to first order, at some point this this approximation will
break down.
Although this issue has not been investigated in depth, there appears to be an effect on CA01 which is equivalent of up to 1mm of excess path being introduced as a function of elevation. This is quite reproducible and detectable when the wrap-dependent phase is low and the atmospheric seeing is reasonable. There also appears to be a similar signature on CA05, but of a quarter the amplitude. The nature of the error suggests that it is a mechanical difference between the antennas. Because this error is readily predictable, it can be corrected in the on-line or off-line software.
An off-line Miriad-based baseline fitting program, blfit
,
was written to assist in exploring these issues. Blfit
has
a number of strengths over CABSLN
.
- It handles sets of observations at different frequencies or at different times. It solves in a fashion so that phase wrapping at higher frequencies is avoided. It handles the two polarisations as two separate sets of data.
- It uses the seeing monitor data and a Kolmogorov model to estimate the rms contribution of the atmosphere to the phase. This allows data to be correctly weighted in a solution process.
- It allows a number of plotting options to explore the residuals from the baseline fit.
- It includes a model of the instrumental phase contrbuted by CA01 and CA05, and can correct for this.
- It includes a simple model of the signature of the wrap-dependent phase, and can solve for a first-order term to eliminate the wrap-dependent phase.
Using blfit
The code, executable and standard Miriad help file forblfit
in directory $OPER/blfit
. To use blfit
to solve for
the the wrap-dependent phase term, you will need to merge the dial azimuth
into the datasets used by blfit
. Example scripts are present
in the $OPER/blfit
.
GetAz.csh Retrieved dial azimuth from MoniCA. The dates in this script will need to be monified. Load.csh Atlod the data, split and bandpass calibrate the data. The RPFITS file and the name of the bandpass calibrator need to be set. DoFit.csh Merge in the dial azimuth, and run blfit.
Output from blfit
blfit produces output as belowFrequency range: 4.17 - 5.43 GHz Scans selected: 78 Ant=1 Iter=1 Normalise rms=0.72 Ant=2 Iter=1 Normalise rms=0.70 Ant=3 Iter=1 Normalise rms=0.65 Ant=4 Iter=1 Normalise rms=0.00 Ant=5 Iter=1 Normalise rms=0.84 Ant=6 Iter=1 Normalise rms=0.50 Ant X (mm) Y (mm) Z (mm) --- -------------- --------------- --------------- 1 -0.063 +/- 0.442 -0.704 +/- 0.395 -0.722 +/- 1.624 -3.027 +/- 0.859 2 0.183 +/- 0.357 -0.515 +/- 0.319 0.107 +/- 1.340 -1.751 +/- 0.693 3 0.628 +/- 0.309 0.103 +/- 0.276 0.969 +/- 1.136 -2.373 +/- 0.600 4 0.000 +/- 0.006 0.000 +/- 0.004 0.000 +/- 0.015 0.000 +/- 0.010 5 -0.099 +/- 0.170 -0.389 +/- 0.151 1.520 +/- 0.623 2.099 +/- 0.330 6 15.163 +/- 2.094 -11.261 +/- 1.872 -21.086 +/- 7.699 -2.273 +/- 4.064It steps through increasingly large frequency spreads in its solution process, until the spread takes in all the frequencies of the available data. At each step, it list lines such as
Ant=1 Iter=1 Normalise rms=0.72This gives some information about the solution for each antenna. In this line, for CA01, it required just one iteration to find a solution, with the residual being 0.72 compared to the expected residual. The expected rms residual is estimated from the seeing monitor measurements, and so does rely on the accuracy of the Kolmogorov model in predicting the change of seeing with baseline. One expects the normalised residual to be near 1, but values from 0.5 to 1.5 are not unreasonable. The normalised residual for CA06 is usually on the low side, because the Kolmogorov model tends to be pessimistic at long baselines.
The solution is given as three or four numbers - the fourth number being the wrap-dependent phase term. Each has an associated 1-sigma error. The three coordinates are in an local x-y-z system corresponding to east, north and local vertical. The solution is the baseline error in millimeters.
At the end of the process, blfit
prints out the eventual
solution in two formats - that used by Miriad and that used by the
ATCA on-line system.
The Miriad format is nanosec in a local equatorial system, and
is the numbers that Miriad task uvedit
uses to
correct the data. These are the numbers that are saved in the
dantpos.
yymmdd file.
Miriad baseline convention: Ant X (nsec) Y (nsec) Z (nsec) --- -------- -------- -------- 1 0.07700 -0.05059 -0.00443 2 0.07941 -0.04967 -0.00475 3 0.08195 -0.04802 -0.00368 4 0.08065 -0.05048 -0.00369 5 0.08372 -0.05077 -0.00687 6 0.00000 0.00000 0.00000 CABSLN convention: Ant X (mm) Y (mm) Z (mm) --- -------- -------- -------- CA01 12.215 -24.774 1.329 CA02 12.976 -24.900 1.423 CA03 13.884 -24.860 1.103 CA04 13.174 -25.300 1.107 CA05 13.923 -25.842 2.059 CA06 0.000 0.000 0.000The ATCA on-line solver,
CABSLN
presents solutions in two
ways: what I will call the CABSLN.TXT
and CABSLN.DAT
formats. blfit
presents in the CABSLN.TXT
format.
The CABSLN.DAT
form is that stored in the file
station.errors
and that used by the on-line system.
To convert from CABSLN.TXT
and CABSLN.DAT
,
use
olformat.pl blfit.log station.errorswhere
blfit.log
is the output log file of blfit
,
and station.errors
is the relevant station errors file to the
observation.
Using non-standard continuum frequencies
To solve for the wrap-dependent phase,blfit
needs to know what
part of the LO downconversion frequency is produced by the 160 MHz
reference. At centimetre wavelengths, the 13 GHz reference is not used, and
the 160 MHz reference contributes the entire LO. This is not the case
at 12mm and 3mm. The way to determine the contribution of the 160 MHz
reference is to subtract the frequency generated from the 13 GHz reference
from the observing frequency. Use LO_CHAIN
to determine the
frequency generated by the 13 GHz reference.
For example, when observing at 18496 and 19520 MHz (the standard 12mm continuum
frequencies), LO_CHAIN
indicates the 13 GHz reference
contributes a tone at 28358 MHz. This is the Wiltron frequency less 160 MHz,
and then multiplied by two.
So the contribution of the 160 MHz reference
is
18496-28358 19520-28358Note these contributions are negative - which is fine. At 3mm, the 13 GHz reference contributes 8 times the (Wiltron frequency - 160 MHz). As well as at the centimetre frequencies,
blfit
can work out the contribution of the 160 MHz reference when using the
12mm and 3mm systems when using the standard continuum set-ups.
Note this assumes the current LO algorithm.
If you use 3mm and 12mm systems at frequencies other than standard
continuum ones,
to let blfit
know the contribution of the 160 MHz reference,
you need to set a uv variable, f160
with this value. Use
puthd in=set1.uv/f160 value=-9.862This sets the contribution of the 160 MHz reference as -9.862 GHz.
Original: Bob Sault (27-Feb-2006)
Modified: Bob Sault (16-May-2006)