Correlator

1. Correlators in KJCC

KJCC(Korea-Japan Correction Center) gathers the raw VLBI observation data from each sites of KVN, VERA/JVN and CVN, and performs the correlation process with two VLBI correlators.

First one, Daejeon Correlator, is one of the fastest VLBI correlator in the world, and is used for processing the KaVA and EAVN observations mainly. It is capable to correlate the data streams of max. 8Gbps for max. 16 stations in one pass, to produce the correlated output of 8192 spectral points for each sub-bands. The number of spectral points are reduced to 128 for continuum, 512 for line observation after correlation process by channel integration.

Second one, DiFX (Distributed FX), is the world-famous software correlator, and is used for processing the KVN observations. It provides quite flexible correlation modes. You can request the accumulation time and the frequency resolution appropriate for your science purpose.

For the final correlation output, the default accumulation time is 1.6384 seconds for Daejeon correlator, or 0.8192 seconds for DiFX correlator. The final frequency resolutions are 16MHz/128 for continuum observations, and 16MHz/512 for line observations in default.

2. Correlation Processing

2.1 Correlation mode and Integration time

KJCC is currently able to support the following correlation modes.

Table 2. Correlation Modes

Obs. Mode	Total Data Rate	Bandwidth/sub-band	# of sub-bands	Minimum Accum. Time	# of Freq. Channels/sub-band
C5	1024 Mbps	16 MHz	16	1.6384 sec	8192
C4	1024 Mbps	32 MHz	8	0.8192 sec	8192
C3	1024 Mbps	64 MHz	4	0.4096 sec	8192
C2	1024 Mbps	128 MHz	2	0.2048 sec	8192
C1	1024 Mbps	256 MHz	1	0.1024 sec	8192
W1	2048 Mbps	512 MHz	1	0.0512 sec	8192
W2	4096 Mbps	512 MHz	2	0.0512 sec	8192
W4	8192 Mbps	512 MHz	4	0.0512 sec	8192

2.2 CODA/FITS

KJCC will basically support following frequency channel for preparing FITS file.

• Basic output channel of correlator : 8192 frequency channel
• Continuum : 128 frequency channel (64 channels integrated in post-correlation)
• Spectral line : 512 frequency channel (16 channels integrated in post-correlation)

2.3 FITS delivery

Correlations will be done using either the DiFX or the Daejeon correlator. KJCC will deliver FITS file to PI by using FTP server or mobile disk.

• When correlation is finished, the FITS file will be prepared through the post-processing, and then KJCC will announce the correlation processing completion to PI by e-mail. In the e-mail, PI will be able to get the FITS file by temporary URL link.
• PI should download the FITS file as soon as possible, and check the FITS file using his/her preferred analysis tool. And then PI should give his response to KJCC with "Success"(data quality is good) or "Fail"(download fail, bad FITS file or data quality is bad etc). Especially in case of "Fail" opinion, please send error message to KJCC, then your quick response for FITS will be helpful for KJCC to solve the problem as soon as possible.
• KJCC would like to receive PI's response within 2 weeks after announcing e-mail. If there is no response within 4 weeks, KJCC determined that PI is regarded as his response as "Success".
  • In case of "Fail", according to the fail type, KJCC will conduct the URL check, file reconstruction or re-correlation, and then announcement will be sent again to PI via e-mail.
  • In case of "Success", correlation processing for that observation will be closed(at that time, download link in temporary URL will be unavailable), and the tapes or disk module will be included to release pool for recycle.
• The FITS file provided to PI will be stored at observation data archive separately. PI SHOULD analyze, perform the research, and publish the paper within some period(in general, 18-months). After 18-months, the FITS file stored in archive system will be opened to the public who need to do his/her research using that FITS according to the procedures.

2.4 Archiving policy

KJCC will organize the archiving policy for observation data, CODA and FITS file as below.

• Observation data : 2 weeks after FITS release (usually, about 2 months after observation), and then it will be released.
• CODA : if correlated data will be used for astrometry or geodesy, it will be permanently stored at CODA server.
  Otherwise, the correlated raw data and CODA file system will be deleted after receiving the response from PI.
• FITS : it will be permanently archived at Archiving server.

2.5 Correlation expected time

Correlation processing will be about 1 week for preparing 1st version of FITS, after the data arrival of last station. However, KJCC will do their best to make correlation as fast as possible to deliver FITS file to PI.