Table of contents
Introduction
RadioAstron has two modes of synchronization: 1) local synchronization with a H-maser reference and an on-board rubidium reference, and 2) "closed-loop" with synchronization using the H-maser references at Pushchino and Greenbank ground stations.
The H-maser reference was depleted in July 2017. This left the rubidium as the only on-board reference.
Astro Space Center (ASC) reports in RadioAstron Newsletter #33 that the synchronization mode with the on-board reference produced interferometric fringes, but best results were achieved in the closed-loop mode.
Closed-loop mode of synchronization is the default for AO5 and AO6 observations.
The ASC has a dedicated software correlator for RadioAstron experiments. The ASC correlator already implements a delay model that incorporates closed-loop mode.
The DiFX software correlator supports space VLBI ("RA-DiFX"). It too is used for correlating RadioAstron experiments. However, currently RA-DiFX does not support the closed-loop mode (01/2019).
Source code location
This page keeps track of implementing closed-loop support into the existing DiFX RadioAstron source code branch.
The current not yet modified RA-DiFX source code resides in https://svn.atnf.csiro.au/difx/master_tags/DiFX-RA-1.0.0/ and is installed in /cluster/difx/DiFX-RA-1.0.0/, selectable on the cluster with 'dra100'.
The new script for applying closed-loop corrections to DiFX/CALC IM files before the actual correlation run resides in https://svn.atnf.csiro.au/difx/sites/MPIfR/oneoff/raPatchClosedloop.py
Updates
22Nov2018 : brief exchange with J. Anderson (RA-DiFX) on patching closed-loop delays into DiFX CALC model .im files
17Dec2018 : test data set RK18CJ received at Bonn, no documentation
07Jan2018 : first look at raw data and .vex; created v2d and vex.obs; ground-only, no fringes yet in K nor C band; no station clock infos
15Jan2018 : created 'raPatchClosedloop.py' script to apply closed-loop corrections to DiFX/CALC IM prior to running actual DiFX correlation job. Script in DiFX Trunk ./sites/MPIfR/oneoff/
Test data set
Data are on the cluster under /data/rk18cj/ and /Exps2/rk18cj/
Raw data
| /data/rk18cj/rdf/GBTS_20172920*.rdf | RadioAstron raw 4-channel 1-bit data in RDF(?) format |
| /data/rk18cj/ra/GBTS_20172920*.m5b | RadioAstron raw RDF converted to raw Mark5B |
| /data/rk18cj/gb/NRAO+312_005?_RK18CJ_GB_No000? | GBT, 4-thread 1-channel 2-bit VDIF |
| /data/rk18cj/{ys,mc,ir,nt}/* | Yebes and others stations, 1-thread 4-channel 2-bit VDIF |
| /data/rk18cj/tr/* | Torun, unknown format |
Extra files
| README | "This is a RadioAstron experiment raks18cj to test the RadioAstron correlation for the closed-loop (coherent mode) observations." |
| filelist_RA_v2d.txt | spacecraft / ground station clock breaks in undocumented notation |
| rdf/GBTS_2017_292_rk18cj.txt | RDF file 1-bit statistics in 4-channels for all four scans |
| RA_C_COH.TXT | RadioAstron delay polynomials for each scan |
| RA_C_COH_uvw.txt | RadioAstron polynomials for u, v, and w |
Missing information:
- reference FITS visibility data set, to be able to compare ASC results against DiFX
- clocks and rates used for the ground stations in the ASC reference correlation
- EOPs used in the ASC reference correlation
- details on the polynomials; is RA_C_COH.TXT in seconds, does it include ground-to-space delay?
Correlation details
Details of DiFX correlation are below.
| Station | Format | Delay | Rate | Notes |
| Noto @6cm | VDIF/8032 | fs log "gps-fmout/-48.48e-06" fringe: -37.48us | ? | ground fringes |
| Yebes @6cm | VDIF/8032 | fringe: +6.0us | ? | ground fringes |
| Irbene @6cm | VDIF/8032 | fringe: 0.0us (ref) | ? | ground fringes |
| RadioAstron/GBTS @6cm | Mark5B-128-4-1 | ? | ? | none yet |
| GBT @1.3cm | (automatic, multi-threaded VDIF) | fringe: 0.0us (ref) | ? | ground fringes |
| Medicina @1.3cm | VDIF/8032 | ? | ? | none yet |
| Torun @1.3cm | MKIV1_4-256-4-2 | fringe: -13.5us | ? | ground fringes |
Delay polynomials
Produced by DiFX calcif2 from .calc --> .im:
- starts at integer second
- defaults to 5th order, 6 coefficients
- coeffs seem to be given in order of a0, a1, a2, a3, a4, a5 (for p(t) = a0 + a1*t + a2*t^2 + ...)
- .im SCAN 0 NUM POLY: 9 total number of coefficients sets over the scan
- .im SCAN 0 POLY <0..8> MJD: 58045 valid from MJD
- .im SCAN 0 POLY <0..8> SEC: 32400 valid from second-of-day
- .im SRC 0 ANT <n> DELAY (us): coefficients for antenna n
- .im SRC 0 ANT <n> U (m): coefficients for antenna n; similarly for V (m) and W (m)
DiFX calcif2 divides the day (sec 0...86399) into fixed intervals. The intervals start from second 0 of the day. In non-RA DiFX Trunk the length of each interval is 120 seconds. VEX scan start times do not affect the placement of the polynomial intervals. In other words, polynomials are valid in 2-minute intervals, and these are not necessary aligned with the VEX scans.
Provided by ASC in RA_C_COH.txt and RA_C_COH_uvw.txt:
- starts at integer second, valid over a start/stop time region
- .txt start = 19/10/2017 09h00m00s stop = 19/10/2017 09h01m00s
- defaults to "6th order" (.txt: order = 6), 6 coefficients given
- coeffs seem to be given in order of a0, a1, ..., a5 just like in DiFX .im file
- .txt P0 = -5.28201618845368e-001 ... P5 = -1.67944819726498e-019
The current set of files (without documentation) would suggest that if RA-DiFX produces 1-minute intervals as seen in RA_C_COH.txt, then the 6 polynomial coefficients can simply be summed element-wise to those in the .im file.
