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Antenna Characterization Spring 2004

• Antenna Characterization Spring 2004
  • Scope of Work Package
  • Current Status and Priorities
  • Surface Efficiency/Beam Shape Measurements and Improvements
  • Continued Pointing/Focus Improvements
  • Miscellaneous Things To Do

Scope of Work Package

The purpose of this work package is to co-ordinate and/or perform the work necessary to characterize and improve the performance of the antenna within the framework of the existing Antenna Manager implementation. This therefore includes items such as:

• Improved techniques for executing and analyzing pointing/focus observations
• Development and execution of techniques to characterize other antenna performance metrics (e.g. half-power tracking, surface efficiency)
• Maintenance and improvement of the pointing and focus tracking models
• Acquisition of pointing/focus data to correlate with antenna temperature data and other engineering information
• Co-ordination of OOF and traditional holography work

This work package is also responsible for co-ordinating incremental upgrades to the existing antenna manager and ancillary software. The work package is not responsible for items such as the following:

• Major re-designs of the control software (this falls under the system design workpackage)
• Delivery of antenna instrumentation (although it might make use of such instrumentation).

Current Status and Priorities

As of now (15th January 2004) we have released model3, the thermally neutral pointing/focus model with systematic errors removed. However, the corresponding dynamic corrections are turned off by default, until some final issues have been resolved. RTFM is usable and close to complete, but is not in general use. We have started making traditional holography measurements of the surface; the results of work to date need to be brought together and summarized. The AntennaInstrumentation group, lead by Kim, have started work on inclinometers as the next major source of pointing improvement. We have two 24 hour periods of commissioning time scheduled in the second half of January, the possibility of a small amount in early February, and the possibility of two or three 24 hour periods in the second half of February.

As far as the Antenna Characterization work package is concerned, I believe our priorities need to be:

• To start work immediately on surface efficiency/beam shape measurements and improvements
• To complete the work necessary to get RTFM and the dynamic corrections in production use
• To work on the list of Miscellaneous Things to Do as time allows

Each of these is discussed in more detail below.

Surface Efficiency/Beam Shape Measurements and Improvements

I'm starting to move some of this to OOFSurfaceMeasurementsSpring2004.

I've started a page on EfficiencyMeasurementsSpring2004.

As far as I am concerned, this is largely uncharted waters. We have some Q-band efficiency measurements (see PTCS/PN/31 - GBT Efficiency at 43GHz) which imply:

• The aperture efficiency has a maximum at elevation , which corresponds to an rss surface error mm.
• The aperture efficiency falls off symmetrically at higher and lower elevations as: .
• The aperture efficiency falls off dramatically during daytime.

We also have initial traditional holography results which imply that at an elevation of around 40 degrees, the surface has a large-scale, saddle-shaped error with a peak-to-peak deviation of around 1mm. Although I haven't revisited this in detail, I don't recall seeing such results in the OOF holography. Finally, the difference between two maps made with the FEM corrections on/off appears to correspond well to the FEM corrections applied for the "FEM on" maps.

In broad terms, I believe our strategy should be as follows:

• Develop fast routines (i.e. spider scans) which will allow obervers to monitor the aperture efficiency/beamshape as conveniently as they currently use jack scans to monitor pointing/focus.
• Use the time available at the end of January/early February to perform any more investigatory observations required.
• Use the time in the second half of February for a major campaign to:
  • improve the surface at the rigging angle in the absence of thermal gradients,
  • characterize the variations of surface error with gravity, and potentially try and improve this (e.g. by tweaking the FEM corrections),
  • characterize the variations of aperture efficiency with temperature.
• Digest these results in March.
• Potentially, have one further campaign in April.
• At what point do we try and measure/improve the subreflector focus-tracking curves - i.e. improve the subreflector collimation?
• If we want any high-frequency measurements to ponder over the summer, we should make sure we have performed these by the end of April (before the high-frequency weather runs out).
• I believe we should not even consider attempting any sort of correction of the surface as a function of temperature gradients this winter. However, we should consider whether there are any high-frequency obervations required, or whether we can do this with traditional holography at 12GHz over the summer.

Assuming everyone agrees to the above, we need to decide the specific details and order of measurements required to achieve this. Here is a miscellany of things which I think we need to do to prepare for the above:

• one of Dana, Jim or myself has to work closely with Ron and Fred to integrate the holography measurements/analysis into this plan.
• we should complete development of Carl Hieles' "spider scan" technique, and associated python analysis routines, as the fast scheme for characterizing aperture efficiency/beamshape. Where are his memos?
• We need to tie down once and for all continuum calibration in python, so that we can use jack scans (or spider scans) reliably for aperture efficiency measurements.
• we need to try and confirm a.s.a.p. that the OOF results are consistent with the traditional holography results; otherwise we presumably don't trust the OOF results.
• we need to try and get some OOF data at Q-band.
• we need to decide whether to switch the holography correlator to wide-band mode, to allow traditional holography on satellites.
• plans for commissioning in January
• plans for commissioning in February - Claire to visit?

Continued Pointing/Focus Improvements

This only refers to current "astronomical" work, not anything related to inclinometers, additional temp sensors, etc. I believe Kim may well want more pointing/focus data once additional (air or structural) sensors are installed.

• release RTFM (and follow up on a host of minor issues).
• resolve Dana's problem with reducing "dynamic-correction-on" data.
• use 29th December data to validate dynamic corrections.
• decide when to release dynamic corrections.
• agree and have implemented graceful degradations.
• agree and have implemented dynamic corrections display (traffic light plus strip chart).
• decide what if anything we want to do to port Dana/Kim/Jim's routines to python, and/or have them supported by the software development division.

Miscellaneous Things To Do

• write up astromer wiki documentation!
• anaylze the pointing hysteresis data
• update the refraction model

-- RichardPrestage - 15 Jan 2004