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.