Software Correlators for VLBI
Walter Brisken
May 14, 2007
U.S. VLBI meeting
Why software correlators?
Rapid Development
● Core software available for free
● DiFX written by grad student Adam Deller in < 1 year
●Flexibility
● Limitations are not built in
● Arbitrary number of inputs, spectral channels,
integration time
● Special processing modes can be implemented
● e.g., pulsar binning
● Can support multiple file formats
●Scalability
● Can add more processing power to increase performance
● CPU power is fully usable for any number of inputs
●
DiFX architecture
Diagram by Adam Deller
DiFX for the VLBA
Immediate goal: integrate
DiFX into VLBA operations
●Use existing path for
generation of correlator jobs
●Use same delay model
(Calc 9.1)
●Add to DiFX direct Mark5
access
●Add operator tools that
interact via multicasts
●Convert output to FITS-IDI
●
A prototype cluster
Use Mark5s as a cluster for a software correlator
●Upgrade motherboards to dual-dual core XEON CPUs
● Cost ~= $15,000
●18 Mark5s should correlate 10 stations at 128 Mbps in
real-time
●Software correlation does not interfere with Mark5
playback to the hardware correlator
●
Note 1 : for 10 stations about 2 Gflop compute power per
Msample/sec is needed.
Note 2 : Computing costs about ¼ storage cost for the VLBA's
typical usage pattern.
First Fringes!
●
●
●
Phase
First with old version
Found some VLBA
specific bugs
New version now working
Amplitude
●
10 Antennas on existing
Mark5 units sustained 16
Mbps equivalent speed
DiFX Multicast System (draft plan)
●
●
Emitted by several software components:
–
DiFX
–
Fringe Tool
–
Queue Manager
Messages contain information on:
–
Progress
–
Warnings
–
Data quality
●
Simplicity through decoupling
●
Driven by EVLA's multicast success
Fringe Tool
●
●
Used to provide real-time feedback of data
quality to operators
Data Products:
–
Baseline amplitudes/phases/delays
–
Auto-correlation spectra
●
Results to be Multicast to operator display
●
Clock search on-the-fly possible
Optimistic Time-line
●
Project Start : Feb 23, 2007
●
First fringes goal : Apr 1, 2007
–
●
Actual : Apr 23, 2007
First correlation off Mark5A : Jul 1, 2007
–
Actual : May 1, 2007
●
Modules-to-FITS in 1 day : Jun 1, 2007
●
Usable by operations : Sep 1, 2007
●
Routine correlation : Nov 1, 2007
Open issues
●
Playback of incomplete modules
–
●
Is this even possible using XLRRead()?
Can this be scaled to 4 Gbit/sec on 10 stations?
–
Will it be affordable?