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
(2013B)
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 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 ~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 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.

Connect with NRAO

The National Radio Astronomy Observatory and Green Bank Observatory are facilities of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of institutional or national affiliation. Observing time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluation of research proposals on the basis of scientific merit, the capability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media.

The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."

AUI collaborates with the scientific community and research sponsors to plan, build, and operate cutting-edge facilities. We cultivate excellence, deliver value, enhance education, and engage the public.