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, must 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 | 25 MB/s (90 GB/hr), or up to 60 MB/s (216 GB/hr) with additional justification |
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 (RSRO). 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 60 seconds. For high frequency observations with short scans, e.g., fast switching (as described in Rapid Phase Calibration and the Atmospheric Phase Interferometer (API)), shorter integration times may be preferable.
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) | |
...equation (4) |
Here N_{chpol} is the sum over all subbands of spectral channels times polarization products:
N_{chpol} = Σ_{sb} 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.
These data rates are challenging for transfer and analysis. Data may either be downloaded via ftp over the Internet, or mailed 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.
Higher time resolutions and data rates are possible in principle but will be considered only through the Resident Shared Risk Observing program.
^{†}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 (also known as the Instrument Configuration Tool) of the Observation Preparation Tool (OPT).