Pointing model update dec06


The pointing errors for 2004,2005,2006 (.ps) (.pdf):
The 1az and sin(za) fits to the pointing errors (.ps) (.pdf):

Intro  top

    Pointing model 15 was installed on 14may04. Since then we have had some azimuth and tiedown encoder failures. These were on: (note that this list may not be complete)         A tiedown encoder failure requires us to reset the encoder position of the jack. This is done with a tape measure and it is probably not accurate to more than .1 inches. If the jack position changes (since the model was made) because of the resetting of the encoder, you will see a 1 azimuth term in the pointing errors (with the maximum or minimum aligned with the tiedown azimuth). A .1 " error in the tiedown height would give a 5 arcsecond 1 azimuth error (.1/(192'*12)/1.76 tdInchPerPlatformInch).

    An azimuth encoder failure requires a resetting of the azimuth encoder position using a mechanical pointer/index mounted next to the encoder. The resetting can probably not be done  to better than .05  inches (the radius is about 60 feet). This gives an error of 5 asecs at 20 degs za (or 2.5 asecs at 10 degrees za). The error on the sky changes as the sine of the zenith angle.

    A third source of pointing error is the UT1-UTC approximation that is used in pointing. This error was about 1 asec jun06 to nov06 and then climbed up to about 2.7 Asec by dec06 (see ut1-utc updates).

    The more recent pointing errors showed a 1 azimuth term in the za pointing errors. To investigate this the following was done:

    The data used came for the x102 calibration runs. Receivers with frequency greater than 1 GHz were used.   Any data with low tiedown tensions (less than 5 kips) as well as data taken 9am to 6pm where not included.

The Measurements   top

The first set of plots show the pointing errors for 2004,2005,2006 (.ps) (.pdf):

    The data after 14nov06 was used to fit a 1 az term and a sin(za) term in azimuth. The error in the UT1-UTC  conversion was removed before the fits (a .167 second time second or 2.5 arc seconds). The plots show the 1az and sin(za) fits (.ps) (.pdf):

    The fits were done separately for azimuth and za errors. A tilt of the platform should give the same amplitude and phase for az and za errors (with a 90 degree phase shift). I tried fitting the az,za errors simultaneously. The results did not differ a whole lot from the separate fits.

Conclusions  top

processing: x101/061207/chkpnt.pro

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