Time Resolution and Data Rates
The default integration times for the various array configurations are as follows:
Configuration | Observing Bands | Default integration time |
---|---|---|
D, C |
L S C |
5 seconds |
D, C |
X Ku K Ka Q |
3 seconds |
B | all | 3 seconds |
A | all | 1 second |
Observations with the 3-bit (wideband) samplers 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:
Minimum Integration Times and Maximum Data Rates (2013A) |
||
---|---|---|
Proposal type |
Minimum integration time |
Maximum data rate |
Standard | 1 sec |
20 MB/s ( ~70 GB/hr) |
Shared risk | 50 msec | 60 MB/s (~210 GB/hr) |
The maximum recommended integration time for any EVLA 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) x 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) x 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} x 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 & 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 ~3 seconds for standard observations, and 0.8 seconds for shared risk observations.
These data rates are challenging for data transfer, as well as data analysis. Currently data may either be downloaded via ftp within the Science Operations Centers, or mailed on hard drives for those not in the same building as the archive. The Archive Access Tool (https://science.nrao.edu/facilities/evla/data-archive/evla) allows some level of frequency averaging to decrease data set sizes before ftp, for users whose science permits; 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 program.