[evlatests] ea25 pointing

[Jon Thunborg]

Mike Romero and the antenna mechanics performed an inspection on this 
bearing and could not find any signs of degradation.  There were no 
metal particles in the grease and the runout looked smooth.  The 
horizontal (up and down) runout did change slightly with azimuth angle.  
This could be because of the difference in the stiffness of the yoke 
structure with respect to azimuth angle.  We will measure a good antenna 
and see if we get similar results.

On 10/29/2018 10:08 AM, Ken Sowinski wrote:
> Since it underwent a bearing replacement in 2011 ea25 has exhibited
> pointing errors which are both not modeled by the usual pointing model
> and are hard to explain.  This note documents one example of what we
> see in the hope of inspiring an explanation or repair.
>
> This example is drawn from the pointing run of October 26, 2018;
> other pointing runs could be examined to find similar examples if
> that is useful.
>
>  Az    El    Azerr Elerr (errors in arc-min)
> 248.4 109.8 -0.09  0.24
> 248.2 109.5 -0.26 -0.37
> 248.1 109.2 -0.47 -0.91
>
> 334.5  11.2 -0.13 -0.59
> 286.5  13.6 -0.05 -1.15
> 293.2  36.6  0.04  0.20
> 313.0  78.2 -0.03 -0.27
>
> The first example is especially striking because the large change in
> elevation error occurs when neither Az nor El change by even one
> degree.  The second example covers a larger range of Az and El, but
> still has a change in El error which is large and sudden.
>
> An examination of the post-fit residuals to the pointing model fit,
> shows that since the bearing replacement the pointing behavior has
> deteriorated, and is limited to azimuths between 240 and 330 degrees.
> This limited range makes it very difficult to explain the poor
> pointing by software errors, FRM faults, loose mechanical structure,
> or hysteresis.  There is nothing left to invoke except for a defective
> bearing.  What ever the mechanism, if must be able to explain erratic
> poinitng changes greater than an arc-minute occurring on azimuth
> scales of a degree or less.
>
> If a measurement of the bearing run out error confirms this suspicion,
> then the only effective remedy is another bearing replacement. None
> of our usual solutions to this kind of problem: referenced pointing,
> modeling the error profile, or measuring it with a tiltmeter, can
> correct for errors that change so fast in both time and azimuth.
>
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