TO:  All GCN Sites
RE:  The New Error_limit and Delta_T_limit filters
DT:  09 Jan 00

INTRODUCTION:
The GCN system now has the ability to apply two new filtering functions
to the notices that are distributed to each site.  The two new functions
are based on size of the location uncertainty and on the time delay between
the burst and when the Notice is available.  These two filter functions
are independent of the current set of filter functions.  That means
that sites can elect to receive notices of burst locations that are, say,
VISIBLE to their location AND have a location uncertainty less than X.X degrees
AND are available in less than Y.Y hours after the burst.

ACTION ITEM:
The current set of GCN sites have had their initial default values
for the error-limit set to 360.1 degrees and the delta_time_limit
set to 999.9 hours (ie, 42 days).  The default values are such that
they have no effect in filtering (ie, no notices will be eliminated).
After reviewing the material below, those sites wishing to change
these values (presumably to smaller values) should send their requests
to Scott Barthelmy (scott@lheamail.gsfc.nasa.gov).

ERROR_LIMIT FILTER:
The GCN system introduced a couple of filter functions a few months ago
based on the size of the uncertainty in the burst location (ERROR_1d and
ERROR_2m for 1-degree and 2-arcmin diameter error circles, respectively).
But these were exclusionary of the regular ALL, VISIBLE and NIGHT filters.
Now the filtering based on the size of the error circle & visibility filtering
has been made independent.  Plus, it has been made completely adjustable
on a site-by-site basis (sites are no longer constrainted to the discrete
values of 1deg or 2arcmin).

The error limit filter is useful in that it can match the site's telescope FOV
with the size of the error boxes received.  For a given source s/c-instrument
supplying burst location notices, the range of the uncertainties can be such
that some will be smaller than the telescope FOV and some will be larger
than the FOV.  If the site does not want to be bothered by notices
that are larger than their FOV (say 12 arcmin), then they can set their
error_limit filter at 0.20 degrees.

Sites can filter notices based on error size AND either ALL-or-VISIBLE-or-
NIGHT-or any of the custom-site-filters.

DELTA_TIME_LIMIT FILTER:
Since the non-BATSE-based burst locations (RXTE, BeppoSAX, COMPTEL, ALEXIS, etc)
are not generated in real-time, they can be hours to even a few days after
the burst.  If sites do not wish to receive notices after a certain delta_T,
they can adjust delta_time_limit accordingly.  This new filtering function
is also applied to the notices independent of the other filtering criteria.

BACKGROUND ON GCN FILTER FUNCTIONS:
Prior to the addition of these two filter functions, the GCN system
had the ability to filter notices based on:
  1) source type (eg s/c-instrument) and sub-type (eg, initial, final, etc)
  2) traditional location/time based (eg, ALL, VISIBLE, NIGHT)
     or
     any site-custom filter based on various combinations of parameters.
     (which included Dec-bands, time windows, sun-angles, etc)
The filters in catagories 1 and 2 are independent of each other.
[It should be noted that there is an intensity filter for the BATSE-based
notices as well, which is independent of the catagory 1 & 2 criteria.
This BATSE-intensity filter was an attempt from the early days to "select"
BATSE notices since the BATSE-error is correlated with BATSE-intensity.
This BATSE-intensity filter is still operational and is independent of this
newly added error-limit filter.]

Now there are two more independent catagories:
  3) error circle size,
  4) delta time.

The details of these new filter functions, the previous filter functions,
and how all the functions interact (or not) for a given site's configuration
can be found at  http://gcn.gsfc.nasa.gov/gcn/tech_describe.html#tc25.