[mmaimcal]photonic calibration

[bshillue]

A few comments based on the recent thread which was forwarded to me. I
have almost no understanding of calibration techniques but can answer
straightforward questions.

1. The budget number $4.9M for photonic phase cal is wrong.  The real
cost has never been estimated.  A full up system which is phase locked
could be very expensive and a simpler system could be a lot less
expensive than that.

2. If the frequency is swept, the output power (amplitude) of the
photonic calibrator will vary a lot more than 1%.  Maybe it could be as
high as 100% over 10 or 20 GHz for  waveguide device, much lower but
still significant for a broadband lens-antenna.  The ouput power will
also change with time at a fixed frequency due to polarization effects
in the fiber.  The time scale of the changes depends on environmental
factors not well understood without a field test.  Careful calibration
and polarization control could be used to mitigate these variations.

3. The types of tests that are do-able at the ATF are limited.  The only
calibrator is the one on the holography tower so that is a bit different
from a subreflector-mounted source.  The frequency is not sweepable, but
can be stepped from about 70-110 GHz.

4. Practically speaking, there is no manpower set aside for photonic
phase cal, no specifications, no system design, no schedule, and the
budget is fictitious for lack of a better word.

Bill Shillue