The Automatic Position Reporting System
An Overview and Introduction
by Arte Booten  N2ZRC (packet: N2ZRC@KB2VLX.#BRONX.NY.USA.NA)
                      (E-mail: N2ZRC@gnn.com)

	Many of you have heard discussions over the repeater about a packet
radio program called The Automatic Position Reporting System, (also called
APRS.)  It's a system which, unlike PBBS's, nodes and DX clusters, uses an
unconnected protocol to transmit your exact position, a symbol denoting
the type of station you're running and a brief comment about it.  It also
uses direct keyboard-to-keyboard "chatting," has direction-finding capa-
bilities and much more.

	How does it work?  In its most simplistic form you transmit a packet
which contains your call-sign, exact latitude and longitude, information
on your transmitter's power, your antenna's height, gain and pattern as
well as a brief comment of your choosing along with some symbols necessary
to make the system work.  With this information your station appears gra-
phically on a map (actually, one in a series of many maps) on the monitor
as would other stations that are on frequency.  Since this is an UNCONNEC-
TED protocol, on-air packets can be kept to a minimum.

	Consider this: When you connect to a local station using standard
AX.25, you send a connect request to that station, they acknowledge that
packet, send you a connected packet which you must then acknowledge.  The
same thing happens with EVERY packet you, or the other station, sends.  With APRS you only send ONE packet to convey your information.  If it's
not received on the first transmission, APRS retransmits this information
using a decaying time delay (that is, the second packet is sent twenty
seconds after the first, the third forty seconds later, the fourth a min-
ute later, the fifth two minutes later etc. until, after a day, you're
only sending six packets a day!)  This makes more efficient use of the
frequency.

	APRS uses four different kinds of digipeaters, which use the aliases
RELAY, WIDE, ECHO and GATE.  RELAY stations (the default setting) are base
stations used to digipeat low-power portable and mobile stations.  WIDE
stations will digipeat packets addressed either to their specific callsign
or the generic WIDE to other VHF stations and WIDEs.  An ECHO performs a
similar function on HF.  A GATE will digipeat either from VHF to HF, HF to
VHF or HF to HF.  When setting up APRS for your location you'll set your
digipeater path based on the situation at that QTH and where you want your
information to go.  For keyboard-to-keyboard communications (which are the
only comms in which "ACK's" are used) you can also set alternate digipeater
paths.  Not only does this direct your message via the shortest possible
route, but it also reduces QRM.

	The program also interfaces with popular weather stations such as
those made by Davis and Peet Brothers, thus allowing for real-time weather
data which is available at the touch of a key.  The potential for this du-
ring a SKYWARN situation is obvious.  You'll get wind speed and direction,
temperature, rainfall amounts by the hour and 24-hour period and, in some
cases, barometric readings.  Such weather data can also be entered manually
if a station has the information but not the hardware.

	There is also a Direction-Finding mode which can be used by stations
with either a beam or omni antenna!  When the "fox" transmits, stations
can call, by voice (on another frequency!) or keyboard their beam headings
and/or signal strength.  Using the antenna gain figures for these sta-
tions, circles are drawn on the map.  The "fox" will usually be located
where these circles converge.  If you have one of the many "doppler" an-
tenna systems they can also be used.

	If DX-ing is your thing, there's also a "DX-mode" which also uses the
UI protocol by simply monitoring the DX cluster frequency.  As new spots
are posted, they appear on the map with their callsign.  Their location is
based on the callsign prefix of the spot.  Obviously, since you're not
connected to the cluster, this isn't meant as a replacement to your normal
AX.25 program, and you can't SEND messages, you can receive them (the
program will flag yours and display them when asked.)  It's just another
tool for your county- or country-hunting efforts.

	If, like me, you have a Global Positioning System (GPS) receiver with
NMEA-0183 output this, too, can be utilized with amazing results!  Your
mobile or portable position can be regularly updated.  Using such a
"stand-alone tracker" you don't even need a computer.  All you'd need is
an H-T, TNC and GPS!  Think about the possibilities for such a setup in
something like a marathon, walkathon or even for someone shadowing an im-
portant official.

	APRS was written to be able to run on just about any PC compatible
computer from the latest Pentium Pro down to a lowly 8086.  Heck, I know
several people that use it with a Hewlett-Packard HP-200 palmtop!  Maps
are available from a large-scale map of the whole world to extremely de-
tailed street-level maps.  There's even a mail-reflector about it to which
you can subscribe.  It's lots of fun, has many potential ARES / RACES /
SKYWARN uses and I'm sure you'll enjoy playing with it!