CAP.txt                  CIVIL AIR PATROL and APRS


SAR GRIDS:  APRS can overlay on ANY APRS map the 15x15 minute Search & Rescue
(SAR) grids used by the CAP.  Although this is a national grid system
based on the USA sectional aeronautical charts, it also correlates exactly
with the outlines of the readily available USGS 7.5 minute maps.  All of these
grid squares are well numbered within each sectional chart.  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 displayed 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 temporarily.

TO DISPLAY CAP GRIDS, USE THE MAPS-PLOTS-CAP COMMAND!

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 re-display the
grids.  3) on each new screen re-display the grids and for areas that are not
numbered, move your cursor to the west and re-display again.  This way, the
western numbers will always overwrite with 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 FILES-SAVE to save the latest track history
to file.  Then he should do an FILES-DOS 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 CFIGxxx.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.


GPS UNITS:  I have decyphered the output of the quantity of black box GPS
receivers that were donated to National CAP.  They are Motorola's and can
be switched from the proprietary binary format to NMEA with a simple command.
I wrote the MOTOROLA.BAS program that makes it easy to reset the GPS units
and to send them the NMEA timing requirements.  These devices will make
excellent GPS trackers!


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 if included in a normal HAM registration.  Quantity pricing
of calls in groups of 10 or more is also available.

