Doppler tracking works again - with restrictions

A Doppler tracking timeline

Prior to the July 2007 Modcomp retirement, while testing the transition system, we discovered that small frequency shifts caused phase jumps on crossed baselines (VLA - EVLA). Since such shifts in frequency are a given when using online Doppler tracking, we urged observers not to use Doppler tracking unless they were only interested in VLA - VLA or EVLA - EVLA baselines.

Some time after the Modcomps were retired, we discovered that, when Doppler tracking, the phase did not remain constant when moving from one source to another - nearby - source. The small frequency change caused by moving from source to nearby source caused a large phase shift, which would make it impossible to copy calibrator phase solutions to a nearby source. This is explained in more detail here. As a result, we had to discourage all use of Doppler tracking, regardless of which antennas were used.

This particular problem was fixed in the executor on 9 April 2008 at 0900 MDT. From that moment on, we are back in the situation in which we support Doppler tracking for VLA - VLA or EVLA - EVLA baselines. It is still not advised to use this for crossed baselines, since the phase jump problem is still with us, and will be so until the last VLA antenna has been upgraded.

Modified on Tuesday, 29-Jan-2013 13:57:48 MST by Gustaaf van Moorsel