Time Resolution and Data Rates
The default integration times for the various array configurations and frequency bands are as follows:
Configurations | Observing Bands | Default integration time |
---|---|---|
A, B, C, D | P | 2 seconds |
A | L S C X Ku K Ka Q | 2 seconds |
B | L S C X Ku K Ka Q | 3 seconds |
C, D | X Ku K Ka Q | 3 seconds |
C, D | L S C | 5 seconds |
Observations with the 3-bit (wideband) samplers, when applicable, should use these integration times. Observations with the 8-bit samplers may use shorter integration times, but these must be requested and justified explicitly in the proposal, and obey the following restrictions:
Proposal type |
Minimum integration time | Maximum data rate |
---|---|---|
General Observing (GO) and Shared Risk Observing (SRO) |
50 msec | up to 60 MB/s (216 GB/hr) |
Resident Shared Risk Observing (RSRO) | < 50 msec | > 60 MB/s (216 GB/hr) |
Note that integration times as short as 5 msec and data rates as high as 300 MB/s can be supported for some observing, though any such observing is considered Resident Shared Risk Observing. For these short integration times and high data rates there will be limits on bandwidth and/or number of antennas involved in the observation. Those desiring to utilize such short integration times and high data rates should consult with NRAO staff.
The maximum recommended integration time for any VLA observing is 10 seconds.
Observers should bear in mind the data rate of the VLA when planning their observations. For N_{ant} antennas and integration time Δt, the data rate^{†} is:
Data rate | ~ 45 MB/sec × (N_{chpol}/16384) × N_{ant} × (N_{ant} − 1)/(27×26) / (Δt/1 sec) |
~ 160 GB/hr × (N_{chpol}/16384) x N_{ant} × (N_{ant} − 1)/(27×26) / (Δt/1 sec) | |
~ 3.7 TB/day × (N_{chpol}/16384) × N_{ant} × (N_{ant} − 1)/(27×26) / (Δt/1 sec) |
Here N_{chpol} is the sum over all subbands of spectral channels times polarization products:
N_{chpol} = Σ_{i} N_{chan,i} × N_{polprod,i} |
where N_{chan,i } is the number of spectral channels in subband i, and N_{polprod,i} is the number of polarization products for subband i (1 for single polarization [RR or LL], 2 for dual polarization [RR and LL], 4 for full polarization products [RR, RL, LR, LL]). This formula, combined with the maximum data rates given above, imply that observations using the maximum number of channels currently available (16384) will be limited to minimum integration times of ~2 seconds for standard observations, and 0.8 seconds for shared risk observations.
We note that frequency averaging in the correlator will reduce the total number of channels. Therefore, the data rate and the data volume will be reduced by the same channel averaging factor. See the Chromatic Aberration section for more details on the frequency averaging in the correlator and to assess its impact on your science.
These data rates are challenging for transfer and analysis. Data may either be downloaded via ftp over the Internet, or shipped on hard drives for large data sets or for those with slow Internet connections (please review the data shipping policy). For users whose science permits, the Archive Access Tool allows some level of frequency averaging in order to decrease data set sizes before ftp; note that the full spectral resolution will be retained in the NRAO archive for all observations.
^{†}Note: The data rate formula given above does not account for the auto-correlations delivered by WIDAR. Precise data rate values can be obtained through the use of the General Observing Setup Tool (GOST), or the Resource Catalog Tool (RCT) portion of the Observation Preparation Tool (OPT).