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'.
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 (Irbene/Yebes/Noto)
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" | ? | none yet |
| Yebes @6cm | VDIF/8032 | ? | ? | none yet |
| Irbene @6cm | VDIF/8032 | ? | ? | none yet |
| RadioAstron @6cm | Mark5B-128-4-1 | ? | ? | none yet |
| GBT @1.3cm | (automatic) | ? | ? | none yet |
| Medicina @1.3cm | VDIF/8032 | ? | ? | none yet |
| Torun @1.3cm | --unknown-- | fs log, no gps-fmout | ? | none yet |
