The
Automatic Position Reporting System An Overview and Introduction
APRS
by
Arte Booten, N2ZRC
Many of you
have heard about a packet radio program called The Automatic Position
Reporting System, (also called APRS.) It's a system which, unlike
a BBS, node and DX cluster, 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 keyboard-to-keyboard
"chat", has direction-finding capabilities and much
more.
How does
it work? In its simplest form, you transmit a packet containing
your callsign, exact latitude and longitude, information on your
transmitter's power, antenna height, gain and pattern as well
as a brief comment of your choosing, along with some symbols which
make the system work. With this information your station appears
graphically - on a map (or one in a series of many maps) on your
computer's monitor. It'd also appear on the screens of other stations
that are on frequency. Your station would similarly appear on
theirs. Since APRS uses an UNCONNECTED protocol, on-air packets
can be kept to a minimum.
Consider
this: When you connect to your local node, using standard AX.25,
you send a connect request to it. It'll acknowledge that packet,
then send you a connected packet which you must then acknowledge.
This same thing happens with each and every packet you, or the
other station, send.
With APRS
you need send only ONE packet to convey your information. If it's
not received on the first transmission, APRS retransmits it, using
a decaying time delay (that is, the second packet is sent eight
seconds after the first, the third fifteen seconds later, the
fourth half a minute later, the fifth a minute later, the sixth
two minutes later etc. until, after two hours, you're only sending
three packets an hour!) It makes more efficient use of the frequency.
APRS uses
several different kinds of digipeaters in order to propagate beyond
their immediate area. They use aliases such as RELAY, WIDE, TRACE,
ECHO and GATE. There are also variations of WIDE and TRACE known
as WIDEn-n and TRACEn-n. A RELAY station (the default setting)
are usually base stations, and are used to digipeat low-power
portable and mo-
bile stations. They are an essential part of the APRS system as
a whole which is why most versions of APRS default to it.
WIDE digipeaters
retransmit packets addressed either to their specific MYCall or
the generic WIDE to other local VHF stations and WIDEs. Some have
the ability to change that generic WIDE to its own MYCall.
ECHO stations
performs a similar function on HF. GATEs retransmit signals from
HF to VHF. However, they should NEVER be used to retransmit from
VHF to HF. This is because VHF APRS uses 1200-baud signals on
144.39 MHz in most parts of North America. HF APRS uses 10.151
MHz LSB, just inside the upper edge of the 30-meter band, which
is limited to using a maximum of 300-baud.
When setting
up APRS for your location, you'd set your digipath based on the
situation at that QTH and where you want your packets to go. In
using keyboard-to-keyboard communication (the only comms that
use "ACK's") 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, showing real-time weather data at
the touch of a key. The potential for this during SKYWARN situations
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's 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 beam headings
and/or signal strength. Using the antenna gain figures for these
stations, circles are drawn on the map. The "fox" will
usually be located at the converge of these circles. If you have
one of the many "doppler" antenna systems, they can
also be used.
If DX-ing
is your thing, there's a "DX-mode" which also uses the
UI protocol by simply monitoring the DX cluster frequency. As
a new spot is posted, they appear on the map with their callsign,
based on their preprefix. Obviously, since you're not connected
to the cluster, it's not meant as a replacement to your normal
AX.25 program, and you can't SEND messages, but 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 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 a stand-alone
tracker (including radios such as Kenwood's TH-D7a HT and TM-D700
mobile rig) you don't even need a computer. All you need is an
H-T, TNC and a GPS! Think about the possibilities for such a setup
in something like a marathon, walkathon or even for someone shadowing
an important official.
The DOS version
of APRS was written to be able to run on just about any PC clone
from the latest Pentium IV 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 detailed street-level maps. It's lots of fun, has many
ARES/RACES/SKYWARN uses and I'm sure you'll enjoy playing with
it!
Arte
Booten (n2zrc@arrl.net) AEC for Digital Services, NYC ARES/RACES|
|Riverdale, New York [FN30bu] !4052.71N/07354.06WNPHG5370/A=00240|
PGP Key Fingerprint: D73E B889 C630 6F4A F31F 3083 56BD 0AAD 9996
3B03
This
article resides on the Technical Information
and Operations portion of the NYC-ARECS
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