Central bearing measurement

The last measurement ( 990212 ), suggested that the new version of the empirical correction was working well.

This morning, in deteriorating conditions, Mars was tracked using the SCUBA map16 method, with the empirical corrections left enabled, from :

• (az,el) = (119/31) at 01:29 HST to (130/43) at 02:30

The elevation pointing residuals, del, show little systematics, and, after the initial 7 data, have an rms scatter of 1.5", indicating poor seeing. Despite this, the azimuth residuals clearly show the up and down signature of the defect at about azimuth 125, with a peak-to-peak amplitude here of 8"; 10.3" at the horizon. It is in the sense of the empirical terms under-correcting the effect.

The measured amplitude of the effect decreased to 7" on 990127, but seems to have reverted to a much higher value. The new data are added to the collection .

It is disconcerting that the amplitude seems to have risen so precipitously, and a redetermination of this new amplitude, with the empirical correction disabled, seems urgently necessary.