Time Resolution and Data Rates

by Stephan W. Witz last modified Dec 03, 2014 by Gustaaf Van Moorsel

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 2 seconds

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
(2014A)
Proposal type

Minimum integration time

Maximum data rate
Standard 50 msec
25 MB/s ( ~88 GB/hr)
Shared risk (SRO) 50 msec 60 MB/s (~210 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 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 Nant antennas and integration time Δt, the data rate is:

Data rate = 45 MB/sec × (Nchpol/16384) x Nant × (Nant1)/(27×26) / (Δt/1 sec)
= 160 GB/hr × (Nchpol/16384) x Nant × (Nant1)/(27×26) / (Δt/1 sec)
= 3.7 TB/day × (Nchpol/16384) x Nant × (Nant1)/(27×26) / (Δt/1 sec)
...equation (4)

Here Nchpol is the sum over all subbands of spectral channels times polarization products:

Nchpol = Σsb Nchan,i x Npolprod,i

where Nchan,i is the number of spectral channels in subband i, and Npolprod,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 ~2 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 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.