AIPS HELP file for COHER in 31DEC24
As of Wed Apr 24 17:08:56 2024
COHER: Finds coherence time for selected "BT" or "TB" order uv data
INPUTS
INNAME Input UV data (name)
INCLASS Input UV data (class)
INSEQ 0.0 9999.0 Input UV data (seq. #)
INDISK 0.0 10.0 Input UV data disk #
APARM (1) Max. averaging time (min)
0 => 10*APARM(8)
0 is not recommended
(2) SNR cutoff in coherence
time estimation 0 => 5
(3) Min Vector/Scalar ratio;
0 => 0.8
(4) Prtlev; 0=>reasonable
(5) Step in time in minutes;
0 => infinite
(6) 0 => average frequencies
with delay search
1 => simple vector
averaging
(7) Output type
0 => average coherence
1 => current coherence
times ready for BLING
(8) expected coherence time
(min), max for all basel.
0 => 5min
BADDISK Disk #'s to avoid
TIMERANG Time: start day,hr,min,sec;
stop day,hr,min,sec.
BIF 0.0 100.0 IF number; 0 => 1
BCHAN 0.0 4096.0 Lowest channel number 0=>1
ECHAN 0.0 4096.0 Highest channel number
0 => last available
ANTENNAS Antennas to be selected
BASELINE Baselines with ANTENNAS
See explanation.
SOURCES Source list. See explanation.
FREQID Freq. ID to select. 0 => 1
DOCRT -3.0 132.0 > 0 means use the terminal.
< 0 use the line printer
OUTPRINT Disk file name in which to
save the line printer output.
SUBARRAY 0.0 9999.0 Subarray; 0 => 1.
HELP SECTION
COHER: Task to measure coherence time
NB: data must be sorted to 'BT' or 'TB' order before running.
NOTE: this task does NOT apply flagging or calibration tables
to the input UV data. Run SPLIT first if that operation is
desired.
Adverbs:
INNAME.....Input UV data file (name). Standard defaults.
INCLASS....Input UV data file (class). Standard defaults.
INSEQ......Input UV data file (seq. #). 0 => highest.
INDISK.....Input UV data file disk #. 0 => any.
APARM.....Task enrichment parameters.
(1) Maximum time interval for a source or selected time
interval, in minutes. 0=> 10*APARM(8)
Comparison of vector and scalar average for each source
is terminated having reached a next source or
when time (starting from the beginning of the source or
the beginning of a sequent time interval)
reaches APARM(1). This parameter should be selected
several times more expecting coherence time. If the
output file created for BLING, this interval is
desirable to have as long as possible. It is limited
by the buffer size. APARM(1)<2000*PREAVE, where
PREAVE is the preaverage time.
(2) Max SNR deviation to pass. While calculating
the coherence time, the scalar mean and rms of each
baseline is calculated. If the value of a
visibility exceeds APARM(2) times the RMS, the
visibility is flagged (in buffer; not in the data).
(3) Min Vector/Scalar amplitude ratio. The ratio of
the vector to scalar averaged amplitude is calculated.
The coherence time for each try is
defined as the time interval for which the ratio
falls below APARM(3). 0 => 0.8
(4) Print level:
0 => print tables of coherence times for selected
baselines and antennas for each selected time
interval and source. Print tables of coherence
times averaged for all selected time interval and
sources. Print minimum and mean coherence time
through all antennas
1 => print additional information useful for debugging
(5) Step in time in minutes; 0 => infinite
So if zero, then the only time interval is selected.
Introducing this parameter you can estimate the coherence
time variance versus time.
The less APARM(5) the more data at the time axis you get,
but the more computing time is required.
If output file for BLING is prepared (APARM(7)=1),
APARM(5) is forced to be equall APARM(1). It is allowed
continuously increasing the analysis time.
(6) 0 => provide delay search, averaging frequencies
1 => not provide the search.
Generally speaking it is necessary to search for the
delay to average frequency channels. But if signal/noise
ratio is not high enough, an error in the solution of
the delay can occur. As a result phases of the frequency
average data become random from time to time, and COHER
find a low vector averaging and therefore low coherence
time.
However sometimes delay is known in advance close to
zero. In this case APARM(6)=1 allows to exclude the delay
search providing actual vector averaging at the frequency
axis.
If you are sure that delay is close to zero,
APARM(6)=1 is more preferable because the task ran faster.
(7) Output type.
0 => average coherence. This option gives idea about
the coherence property of the baselines and antennas.
1 => current coherence times ready for BLING.
BLING will use this data (infile) to find solution for
the given variable solution intervals.
(8) Expecting coherence time in minutes. Maximum for all
baselines. 0 => 5min.
The task compares scalar average amplitude with estimated
amplitude by FFT plus least square. The comparison starts
with this time (APARM(8)). Then this interval is splitted
for twice smaller intervals. This process continue while
the maximum possible time is found satisfying condition:
AMPVEC/AMPSCAL = APARM(3). This condition determines the
first coherence interval. The search of the next interval
starts at the end of the previous found interval using
APARM(8) as the initial estimation.
BADDISK...Disk #'s to avoid for scratch files
TIMERANG..Time: start day,hr,min,sec; stop day,hr,min,sec.
0 => all
BIF.......IF number; Only one (the specified) IF can be used;
0 => 1
BCHAN.....Lowest channel number 0 => 1
ECHAN.... Highest channel number, 0 => highest available
ANTENNAS..A list of the antennas to be selected
BASELINE...Baselines between antennas named in ANTENNAS and those
named in BASELINE are selected.. There are four possible
combinations of ANTENNAS and BASELINE:
1. ANTENNAS = 0; BASELINE = 0.
All possible baselines are selected.
2. ANTENNAS <>0; BASELINE = 0.
a)All ANTENNAS > 0
Baselines including an antenna in the ANTENNAS list
are selected;
b)Some ANTENNAS < 0
All baselines NOT including an antenna in the ANTENNAS
list are selected;
3. ANTENNAS = 0; BASELINE <> 0.
a)All BASELINE > 0
Baselines including an antenna in the BASELINE list
are selected;
b)Some BASELINE < 0
All baselines NOT including an antenna in the BASELINE
list are selected;
4. ANTENNAS <> 0; BASELINE <> 0.
a)All ANTENNAS>0 and all BASELINE>0
Baselines between antennas named in ANTENNAS and those
named in BASELINE are selected.
b)Some ANTENNAS<0 .OR. some BASELINE<0
Baselines between antennas named in ANTENNAS and
those named in BASELINE are DE-selected, all others
are selected.
SOURCES....Source list. Blank => all
If the first character of any source names
begins with a "-", all sources EXCEPT those
named will be returned.
The "-" will be ignored in determining the source name.
FREQID.....Freq. ID to select. 0 => 1. Use LISTR (OPTYPE=SCAN)
to find list of FREQIDs of the data
DOCRT......False (<= 0) use the line printer if OUTPRINT = ' '
else write named OUTPRINT file only.
When OUTPRINT is not blank, DOCRT=-2 suppresses the
page-feed character on page headers and DOCRT=-3
suppresses page headers and most other header
information.
True (> 0) use the terminal interactively. The task will
use the actual terminal width as a display limit
unless 72 < DOCRT < width. In that case, the display
limit will be DOCRT characters.
OUTPRINT...Disk file name in which to save the line printer output.
' ' => use scratch and print immediately for interactive
jobs - batch jobs use OUTPRINT = 'PRTFIL:BATCHjjj.nnn'
(jjj= job #, nnn = user #). When OUTPRINT is not blank,
multiple outputs are concatenated, and the file is not
actually printed.
SUBARRAY...Subarray; 0 => 1.
EXPLAIN SECTION
COHER: Task to estimate coherence time
NB: data must be sorted to 'BT' or 'TB' order before running.
Related programs: COHER derived from PHASE
Documentor: Leonid Kogan NRAO/Socorro.
Comments
Coherence time is determined by comparing vector and scalar averaged
amplitudes. When the vector/scalar ratio becomes less than APARM(3)
the coherence time is considered exceeded. A new coherence time is
determined for the following time interval starting with the end of
previously found coherence interval. This process is terminated when
the time reaches the source end or APARM(1). This process begins
again for a new source or new time interval (APARM(5)).
The result is presented as a table of found coherence times for each
source or time interval. The final coherence time is determined as an
average over the selected sources. Complex amplitude for each time is
determined using FFT analysis of the data at the selected frequency
channels and fitting a complex exponent by non linear least square
method. Vector averaging at the time axis is provided by fitting a
complex exponent by non linear least square method. The first approach
for the least square is given by FFT. The fitting of the complex
exponents instead of simple vector averaging allows to estimate
amplitude in the case of non zero value of delay and rate. But such a
method required more computing time. The special methods were used to
minimize the computing time. Now it is equal ~20 sec at SPARC station
IPX for 5 antennas and four 5 minutes intervals. The task ran much
faster if delay search is excluded (APARM(6) = 1).