Note on the phase/pathlength stability requirement for the receivers:

S.Guilloteau 31 May 2001

Problem:
	Because of sensitivity reasons, the basic phase calibration strategy for ALMA will use two different frequencies, one (called "C") for calibration and one (called "O") for observation. Hence its accuracy will rely on the relative stability of the phase and pathlength difference between the two "C" and "O" frequency channels.

	To determine the phase at the observing frequency, several parameters will play a role:
	1) the short-term reproducibility of the PHASE of either "C" or "O" when the LO system is commuted between "C" and "O" and back again.
	2) the medium-term stability of the PHASE difference between "C" and "O"
	3) the medium-term stability of the PATHLENGTH difference between "C" and "O"
	4) the atmospheric pathlength stability and/or
	5) the accuracy of WVR pathlength prediction 

An ideal instrument would require the errors to be dominated by only non-controlable term,
i.e. the atmosphere. Fast switching and WVR can give an rms pathlength error of about 20 microns, although this is not easy. The specification in the hardware for the WVR have been to set a stability requirement corresponding to a rms pathlength of 10 microns. 

I suggest that we put a similar specification for the short-term reproducibility of the PHASE
of the local oscillator system when switched back and forth between two receivers. To convert
such a pathlength specification into a phase specification requires to know the frequency.
In practice, it is the calibration frequency "C" which should be use for that, about 90 GHz.
(We can ignore the case when "O" is at a lower frequency than "C", because the 10 microns spec
is derived from the highest possible value of "O"). 10 microns correspond to about 1 degree
of phase at 90 GHz. 

The medium-term stability must be such that, after removal of a drift on the calibration timescale, the residual be less than 10 microns of pathlength also. The relevant timescale is that of observations of a phase calibrator at both "C" and "O" frequencies. Its shortest
acceptable value is 0.5 to 1 hour.

Note that there is another technical solution, namely to have two LO loops working in parallel,
one controlling of calibration frequency "C", the other controlling the observing frequency
"O". This obviously is a more costly approach, but may be a fallback solution if the repeatability specification of 10 microns cannot be met.

In summary:
	Phase repeatability of the LO system: 1 degree (frequency /100 GHz) (i.e. 10 microns
	pathlength error)