README.CAP             CIVIL AIR PATROL and APRS

Version 4.04 added the 15x15 minute Search and Rescue (SAR) grids for use by
the Civial Air Patrol (CAP).  This was improved in version 4.04C to include
the numbers of all grids and the display of the aeronautical sectional
charts on which they are based.  In making room for the additional data,
bugs were introduced that were corrected in version 4.05.  Version 5.00 gave
users the capability to modify the default beacon decay rate.  See the section 
on Beacon rate below.

SAR GRIDS: Since the numbers of all grids are consistent and well defined for
all sectional charts, the only problem is when two charts overlap in the same
area.  This is resolved in the CAP definitions that the western most map
will always take precedence.  APRS accomplishes this ordering by making sure
that all map data in the CAPGRID.DAT file are listed so that the more
westerly map of an overlap pair is later in the list.  Also, the ALBUGUERQUE
map must be the first one (APRS uses that to know if the file has been
loaded).  Except for the overlap sections, most of the charts are listed
alphabetically.

The numbering plan used by APRS is determined by the exact location of the
cursor.  If the cursor is in an overlap area, the proper grid numbers will
be seen.  If you are just to the side of the overlap area, then APRS will
use the numbering scheme that applies to the exact grid found at the cursor.
This may place the "wrong" numbers in the adjacent overlap area.

There are several ways to determine if you are in an overlap area and if you
are getting the correct numbers.  1) zoom up to where you can see the
sectional chart boundaries (yellow).  Any overlap areas whould be obvious.
2) be sure that your cursor is in the overlap area and hit Z.  3) on each
new screen hit Z; and for areas that are not numbered, move your cursor to
the west and hit Z again.  This way, the western numbers will always overwrite
the correct numbers.

Adding the numbering system and sectionals to APRS was easy and only took
two days.  Recovering from the memory problems took a whole weekend and
4 more days!  I hope this effort will pay off in the usefullness of APRS
in Search and Rescue.

TRACK HISTORIES:  Back at the SAR headquarters, complete track histories can
be processed offline from the main APRS Communications computer.  Periodically
the main APRS computer should do a ^C to save the latest track history to file.
Then he should do an ALT-D to shell to DOS and copy the latest track history
file from the HSTS sub directory onto a floppy disk.  This disk can then be
taken to another computer for analysis and the APRS computer can EXIT back
into APRS without loosing anything.  APRS maintains a 2k comm buffer, so even
if the packet channel is continuing to operate at full capacity, you have a
total time of at least 40 seconds before you begin to loose data.  APRS will
automatically do a save to disk whenever 199 positions have been received.
After all saves, memory is cleared except for the last position of all
stations.

BEACON PERIOD:  At the request of the Arizona section, I have added the ability
for the user to set his maximum beacon period to a few hours vice the normal
default of 15 minutes.  This would reduce the number of UI frames on their
shared packet channel.  To do this, the user must modify the Decay time using
a text editor on the CFIG406.xxx file found in their root directory and
change the value to something other than 750.  I do not feel that this is
necessary or advisable. 
 
  1)  It defeats the real-time objective of APRS to maintain knowledge of
  the activity of all stations on the net.
 
  2)  The channel time used up by in-active APRS stations is less than 0.2%
  each.  Ten such stations would use only 2% of channel capacity.

  3)  Each station can simply use the CONTROLS-XMTR-OFF command to silence
  APRS (it will still respond to incomming messages)

The argument from Arizona was not necessarily the channel loading, but the
accumulation of lots of APRS UI frames in all TNC buffers on frequency that
were not in use.  The practice there, was to accumulate MAIL-FOR beacons from
the BBS system so that a user comming home would find MAIL BEACONS in his
buffer without having to leave his PC on all day.  The simple solution here,
is for those users to simply set APRS in the LCALLS list of their TNC's and
set BUDLIST OFF.  This way they will ignore all APRS packets.


REGISTRATION:  Since most CAP communications personnel are also radio
amateurs and will probably want to use APRS for both HAM and CAP applications,
each additional call sign registration per individual has been discounted
to only $9 each.

