The content here reflects a full regularly scheduled release of products 
for Phase 2 CASM/APT in HST Cycle 28 completed 2020 April 17.

A description of the changes is summarized in RELEASE_NOTES.TXT .
Details of the procedure followed for defining and implementing
the changes are to be found in the file 
working_material/mar20_construction_notes.txt .

Note that compared to the previous release, the files that
have been changed are:  

	* all new files in constraint-sets/ (7 of these)
	* archival of obsolete constraint-sets/ material to constraint-sets/archive/
	* all files in casm-control/ have been updated to point to each of the new constraint sets..

Although this delivery is a regularly scheduled Phase 2 release, 
both Phase 2 and Phase 1 constraint-sets files are delivered,
with all of them containing a new orbit model.  This orbit model
is a substantial departure from deliveries in recent years and 
is the result of a more careful analytic technique to factor in 
nuanced relationships between FDF, SPSS, and Spike models.  
This new orbit model also increases the pad between Operations 
(SPSS/Spike) and APT by 30-seconds to accommodate future 
possible changes to SPSS activity separations and durations 
which may result from deteriorating Gyro 3 performance.  These
changes have the net effect of reducing the average time
available for users to construct science observations by 
approximately an equivalent amount compared with previous 
releases.  On the up-side, the new orbit model balances 
the total pad amount between more parameters which should 
result in a lower incidence of false=negatives for targets
in and near CVZ opportunities.  

On the down-side, the increased pad means that users can 
expect that the span of Visit-Planner suitable intervals 
for packed visibility usage will shrink by an average of 
about 13%.  Approximately 5% of visits suitable under 
previous CASM releases will become unschedulable and 
require exposure time cuts.  

The above-described orbit-model adjustment is not the 
only change impacting orbit packing in this Cycle 28 
APT release:  adjustments to the length of guide-star 
acquisition and reacquisition durations are increased 
substantially to accommodate deteriorating performance
of Gyro 3.  Those changes will have a further 50-90
second reduction in the amount of science which can be
used in a given orbital visibility.  All combined, 
exposure time cuts as large as 50-120 seconds mayl be 
necessary to restore suitability for visits that are 
newly unschedulable in this Cycle 28 Phase 2 APT release.