Satellite solar illluminati

22 Jan 1997 11:52:51 -0800

Hi Greg,

Was following your thread with Ted regarding satellite shadow entry.  I spent
a lot of time on the refractive aspects of this problem, and through a
combination of shadow entry timings, runs of LOWTRAN 7 under various volcanic,
cloud, and seasonal conditions, and adjustment for seasonal atmospheric
variations, I feel that the algorithm I use in SkyMap is pretty good.

As it turns out, atmospheric transmission in the visible band (0.4-0.7
microns) is very sensitive to season, tangent height and recent volcanic
activity.  To simplify the code, SkyMap runs in 2 modes: a spring-summer mode,
and a fall-winter mode.  For spring-summer (determined by time of year and
hemisphere), a 20-km tangent height cutoff is used, where the transmission is
less than 5% for high volcanic extinction.  Atmospheric refraction at that
tangent height is
approximately 0.1 degrees.  For fall-winter, the atmosphere is less dense, so
a lower 15-km tangent height cutoff is used.  Atmospheric refraction at this
altitude increases to roughly 0.2 degrees.  For either mode, when the highest
point of the refracted sun reaches the designated tangent height, I consider
the satellite to be eclipsed.

In older versions of SkyMap, I had assumed that the sun was visible all the
way down to the hard earth (tangent height of 0 km), where light is refracted
by the earth's atmosphere by 1.216 degrees (@ 760 mm Hg, 10 C, 550 nm). 
However, this assumption proved to be too generous with sunlit times.

I am always interested to hear about other's timings of shadow entry (exits
are much harder), particularly when they differ significantly from prediction.
 Sometimes it's just a question of limb geography -- if there's a large storm
system on the limb between the satellite and the sun at orbital sunset,
penumbral and umbral entry may occur earlier than SkyMap predicts.  On the
other hand, if the atmosphere is particularly transparent due to a lack of
recent worldwide volcanism, and the solar tangent point occurs over a
cloud-free ocean, shadow entry can occur later than SkyMap predicts.

If it's not too much trouble, I would be very interested in the details of the
Solar Illumination article in the MEMOIRS OF THE BRITISH ASTRONOMICAL
ASSOCIATION - Artificial Satellites that you referenced.  Thanx in advance!