[evla-sw-discuss] VLA archive records after the Modcomps
Ken Sowinski
ksowinsk at aoc.nrao.edu
Wed Aug 3 16:11:27 EDT 2005
VLA Computing Memo #xxx
Changes to the VLA Archive Record for the Interim VLA
August 3, 2005
K. Sowinski
In the era after the Modcomps and prior to WIDAR the VLA will continue
to produce data in a format very close to the traditional VLA archive
record as described in VLA Computing Memo #188. This memo sets out
differences in format and semantics of the interim EVLA archive record
from the traditional VLA archive record. Data from the WIDAR
correlator will be represented by some expression of the Science Data
Model. The goal is to leave the existing archive record format and
contents as untouched as possible to relieve the burden of change upon
existing programs which read these records. Some information will no
longer be provided, but space in the record will remain and be filled
with zeros or blanks as appropriate. Other items may change slightly
in meaning; these will be documented. I don't expect to add anything
new to the archive record.
Each archive record is a self contained logical record that fully
describes and contains the result of a single VLA integration. Each
such record is independent of all others. Sequential records with
the same start time comprise a 'scan'. If multiple subarrays are in
use archive records for each subarray are interleaved. Even a subarry
of a single antenna will produce an archive record, but it will have null
pointers to the visibility data. The record is divided into four parts:
a Record Control Area (RCA) which contains bookkeeping information, time
and offsets to the other sections, a Subarray Data Area (SDA) which
contains information describing the antenna independent parts of the
record, Antenna Data Areas (ADA) for each antenna in the subarray which
provide the antenna specific information, and depending upon correlator
mode up to four Correlator Data Areas (CDA) contain the visibility data.
The results in the CDA can be interpreted only if the number of antennas
in the subarray and the order of the ADAs is known. A description of
the formatting of the visibility data, as well as all the other details
ignored here are available in VLA Computing Memo #188.
The archive record will still be constructed in network (big-endian)
order so the usual byte shuffling will still be necessary on Intel
machines. I am tempted to continue writing floating point numbers
in Modcomp rather than IEEE format so that programs I have written to
digest the records will not have to be changed. The argument against
this is 1) the need to write IEEE to Modcomp conversion functions, and
2) the fact that it is about time isn't it? I am open to arguments, or
merely pleading, in either direction.
All the times that are described as IAT will likely be UTC if we
continue to run the system on UTC as we have so far. If UTC is
the time of the future we should consider modifying the existing system
to run on UTC to ease the transition and to make comparisons easier
when testing the new system. If I understand it rightly it requires
only changing a value in a system file and a modifiable parameter in the
"IAT" clock. Alternatively all the UTCs can be converted to IAT so
that any analysis programs that depend on time being expressed in IAT
will still do the right thing.
The visibility data is expected to be processed as we are used to with
the current system. For VLA antennas the both the T_sys and round trip
phase corrections will be applied; for EVLA antennas we may apply a
backend T_sys correction and will apply a phase correction based on
the path length measurement in the L352 when we figure out how it
works and learn whether the changes are fast and large enough to warrant
a correction.
The source of the data presented in the header records described below
can be divided into four classes:
0. Known to or calculable by the archive record creator when the record
is constructed,
1. The executor, or other components of the system, know it and can
present it to DCAF as needed,
2. The information will be encoded in the script and passed, untouched
to DCAF,
3. The information is not explicitly present so will have to be
calculated from known quantities or teased out of CALC.
It is straightforward to determine the source of items in the first
three classes and class 2 will be discussed in more detail in a
separate document. The items in class 3 deserve more explanation.
I will provide that here and leave the complete list for the end of
this memo.
SD.PDA 32 3 APPARANT RA AND DEC AT OBSERVATION
My understanding is that in the approach taken by CALC this is
irrelevant. We will have to document the what this item means
for planetary observations, for both the old and the new systems.
I expect that this will be reported as zero or as the position
of epoch.
SD.SKY 56 3 SKY FREQUENCY AT BAND CENTRE IN GHZ
The executor knows and will provide the signed sum of the LOs and
the backend bandwidth. If the net sideband is known this is easily
calculated.
SD.SRF 84 3 CURRENT SURFACE REFRACTIVITY (N-1)
SD.ZAP 86 3 ESTIMATED ZENITH ATMOSPHERIC PHASE PATH (NSEC
The are estimated by CALC and included in the pointing angle and
total delay. So far as I know these have never been used except
to be compared to independent calculations as a check. If we can
get them out of CALC they will be provided, else they will be
reported as zero.
SD.TRI 88 3 SIN AND COS OF EL, AZ AND ETA
These are the array-center angles. The executor knows them only for
individual antennas. Eric says that sin(El) is used in AIPS; the
only other use I know of is to attach the Az and El at the time of
a pointing offset determination to the pointing measurement for PEEK.
To be consistent with current usage it must be the geometric Az and El
with no pointing model or refraction corrections included. It is
probably sufficient to report the pointing angles for the innermost
of the antennas in the subarray. Parallactic angle will not be
provided.
AD.U 28 3 U
AD.V 30 3 V
AD.W 32 3 AND W AT CENTER OF INTEGRATION FOR SPECIFIED E
These need to be calculated. Eric says that they are needed in AIPS.
If CALC will not provide this, then we will need the source position
at epoch, an accurate LST for the center of the integration interval
and antenna positions. In any case, this must be U and V suitable
to make a map in J2000 coordinates.
AD.BL 34 Apparant BX, By and BZ (nsec)
In the current implementation we report the nominal antenna locations
as given in the BASELINE file corrected for local differential earth
tides and the IERS estimate of the position of the pole. The
calculation of apparant source position includes all the usual effects
(precession, nutation, aberrations, light bending) but not these two.
U and V are then calculated using the corrected antenna positions and
source positions and accounts for all the relevant effects. The
approach taken by CALC is quite different. I don't understand it well
enough to describe it with any certainty, but I expect that we will
only be able to record nominal antenna positions and source position
of epoch expressed in the equator and equinox of J2000. The
astrometry community will have to tell us how important this is and
what ought to be recorded. The simplest approach is to record the time
of observation, the nominal antenna positions, the source position of
epoch and say that CALC version x.y was used to calculate the delay.
* FIRST THE RECORD CONTROL AREA (RCA)
Name Offset Class Description
==== ====== ===== ===========
RA.LRL 0 0 LOGICAL RECORD LENGTH IN WORDS
RA.FMT 2 0 FORMAT TYPE (THIS IS FORMAT 1)
RA.REV 3 0 REVISION LEVEL (-1 WILL DO FOR NOW)
RA.MJD 4 1 DATE (MJAD)
RA.INT 6 1 IAT TIME IN INTERRUPT COUNTS SINCE MIDNIGHT
RA.CP 8 0 CONTROL PROGRAM ID
RA.SDP 12 0 POINTER TO SUBARRAY DATA AREA (SDA)
RA.ADP 14 0 POINTER TO ANTENNA DATA AREA (ADA)
RA.ADL 16 0 LENGTH OF ADA IN WORDS
RA.NAN 17 1 NUMBER OF ANTENNAS IN THIS SUBARRAY
RA.C1P 18 0 POINTER TO FIRST CORRELATOR DATA AREA (CDA)
RA.C1L 20 1 NUMBER OF CORRELATORS (CH*BL)
RA.C2P 22 0 POINTER TO SECOND CDA
RA.C2L 24 1 NUMBER OF CORRELATORS
RA.C3P 26 0 POINTER TO THIRD CDA
RA.C3L 28 1 NUMBER OF CORRELATORS
RA.C4P 30 0 POINTER TO FOURTH CDA
RA.C4L 32 1 NUMBER OF CORRELATORS
RA.RAT 34 0 RATIO OF BLOCK SIZE TO 'ANSI' RECORD SIZE
RA.SUB 35 1 ACTIVE SUBARRAY MASK (BIT15 == SUB1, ETC)
RA.LEN 36 SIZE OF RCA
* THE SD.XXX ARE IN THE SUBARRAY DATA AREA
SD.ID 0 1 SUBARRAY ID
SD.SOU 1 1 SOURCE NAME (ASCII)
SD.QUA 9 2 SOURCE QUALIFIER (INTEGER)
SD.CFG 10 2 ARRAY CONFIGURATION (ASCII)
SD.PID 11 2 OBSERVING PROGRAM ID (ASCII)
SD.OID 14 2 OBSERVER'S DEC-10 NUMBER (INTEGER)
SD.MOD 15 2 OBSERVING MODE AND SUBMODE (ASCII, MORE OR LE
SD.ASB 17 ARRAY STATUS BYTE
SD.NCH 18 1 NUMBER OF COMPLEX CORRELATORS/BASELINE FOR EAC
SD.AVG 19 1 INTEGRATION TIME (WG CYCLES)
SD.STO 20 1 LST STOP TIME (TURNS OR RADIAMS?)
SD.STA 22 1 LST START TIME "
SD.POS 24 1 RA AND DEC AT EPOCH
SD.PDA 32 3 APPARANT RA AND DEC AT OBSERVATION
SD.SLO 40 1 SIGNED SUM OF LOS (A-D) IN GHZ
SD.SKY 56 3 SKY FREQUENCY AT BAND CENTRE IN GHZ
SD.IAT 72 1 IAT AT END OF INTEGRATION INTERVAL (RADIANS)
SD.LST 76 1 LST " "
SD.GTI 80 1 IAT FOR GEOMETRY CALCULATIONS (RADIANS)
SD.SRF 84 3 CURRENT SURFACE REFRACTIVITY (N-1)
SD.ZAP 86 3 ESTIMATED ZENITH ATMOSPHERIC PHASE PATH (NSEC
SD.TRI 88 3 SIN AND COS OF EL, AZ AND ETA
SD.BW 100 1 BACKEND BANDWIDTH
SD.FEF 101 1 FRONT END FILTER (VLA antennas only)
SD.REC 102 1 RECIRCULATOR CONTROL CODE
SD.ZSP 103 2 ZERO SPACING FLUX
SD.UVL 105 UV LIMITS SUPPLIED FOR ANTSOL
SD.ACB 109 0,1 ARRAY CONTROL BITS
SD.WEA 111 1 FIVE NUMBERS OF WEATHER INFO
SD.VEL 121 1 VELOCITIES FOR IF A-D (KM/SEC)
SD.LRF 137 1 LINE REST FREQUENCIES FOR IF A-D (MHZ)
SD.VSC 153 1 VELOCITY SYSTEM CODE FOR IF A-D (ASCII)
SD.CRM 157 1 CORRELATOR MODE (ASCII)
SD.APO 159 1 AP OPTIONS FOR IF A-D (ASCII)
SD.EPY 161 0,1 EPOCH YEAR (INTEGER)
SD.SEL 162 1 DATA SELECTION PARAMETERS FOR FOUR IFS
SD.CHS 166 0 CHANNEL SEPARATION CODES FOR 4 SPECTRA
SD.LEN 170
* AD.XXX ARE ALL IN THE ANTENNA DATA AREA
AD.ID 0 1 ANTENNA ID AND DCS ADDRESS (ONE PER BYTE)
AD.CTL 1 0,1 ANTENNA CONTROL BITS
AD.IFS 3 1 IF STATUS (FLAGGING INFORMATION)
AD.NOM 4 1 NOMINAL SENSITIVITY IF A-D (UNITLESS)
AD.PDE 12 1 PECULIAR DELAY (NSEC)
AD.PPH 20 1 PECULIAR PHASE (TIRNS B+0)
AD.DEL 24 1 TOTAL DELAY (NSEC)
AD.U 28 3 U
AD.V 30 3 V
AD.W 32 3 AND W AT CENTER OF INTEGRATION FOR SPECIFIED E
AD.BL 34 1 APPARANT BX, BY AND BZ (NSEC)
AD.BA 46 1 BA (K-TERM)
AD.TSF 48 1 FE Tsys -- VLA antennas only
AD.TSB 56 1 Be Tsys
AD.IFC 64 0,1 OR OF I.CNTL FROM FOUR IFS
AD.LEN 68
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