NavList:

Hi Bill Ritchie,

Many thanks for your help and quick response to my query.  We found that the issue is not directly related to the Astron program but reflects the slight differences between precomputed Altitude Correction tables (Almanac yellow pages) and the more precise calculation of Astron.  The error of measurements was enhanced since the measurements were taken near the winter solstice and at high latitudes in above 55deg  in cold below 0C weather where at low Ho the refraction errors and turbulent air are larger.   I think the old rule of not trusting winter sun Ho below 10deg holds true and my preference is minimum Ho of 15deg unless you have a steady hand and eye.

Regards

Barry Messer