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README.DUM
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1993-05-04
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USING APRS WITH DUMB TERMINALS
Most of the features of APRS can be used even if a station is not running
the APRS software. This might often be the case from lightweight portable
packet stations. Frequently the need for the map display is not out in the
field where the portable packet stations are, but at the headquarters, or net
control point. The following paragraphs suggest ways to use dumb terminals
or non-PC computers and TNC's in an APRS network.
All APRS communication is done using UI frames. UI frames are
transmitted with all the error detection capability of standard packets, but
they are not automatically acknowledged. The only disadvantage is that if a
collision occurs or there is interference on the channel, the UI frame is not
automatically retransmitted. In APRS this is not a problem because all
information is repeatedly and redundantly transmitted.
UI frames can be transmitted easily from a TNC in two ways. First, by
loading the TNC Beacon Text with a desired message, the beacon is repeatedly
transmitted as a UI frame. The period of retransmission is set using the B
parameter. Most standard TAPR-2 TNC's use 10 second increments so that a
B_E_6 actually results in a beacon once every minute. A value of 60 is once
every ten minutes. The BText is the primary method used by APRS to broadcast
one line messages to all stations.
The second way to send UI frames is to enter CONVERSE mode directly from
the cmd: mode without first establishing a connection. This is done simply
by using either the CONV or K command. In this mode you specify your
digipeater path by using the UNPROTO command. Once in UNPROTO CONVERSE,
everything you type is still transmitted as normal AX.25 packets, except that
lines are only transmitted once and acknowledgments do not exist. As long as
you do not experience a collision, these UNPROTO packets are just as good as
the connected protocol and are very effective at broadcasting information to
many other stations at once.
TO ADDRESS: One caution; since APRS operates in a broadcast fashion and
does not use a connected protocol, the TO ADDRESS is not required. To help
ignore non-APRS traffic which might also be on the channel, APRS stations use
the TO ADDRESS field as a type of filter. APRS will only recognize UI frames
transmitted to the broadcast addresses of CQ, QST, BEACON, MAIL, SKYWRN and
APRS. All other packets are ignored. For this reason, you must place one of
these addresses in the TO ADDRESS field in your UNPROTO command.
APRS FORMATS: Using either the BText or UNPROTO CONVERSE methods noted
above, APRS reports can still be transmitted manually. To transmit a report
repetatively, enter it as a Beacon Text. To report it once, simply type it
in the UNPROTO CONVERSE mode and hit carriage return. The following formats
must be used exactly to have the desired effect at an APRS station.
Abbreviations are D for degrees (or DAY), M for minutes (Both lat/long and
time), H for hundredths (or hours), N for North and W for West.
POSITION REPORT: To report the position LAT/LONG of your station you may
either use the abbreviated format for a fixed station or the
longer format for a mobile that includes the time of the position
and an optional course, speed, and comments. If a course is
given with a speed of 000 then APRS assumes the report is a line
of bearing for direction finding.
FIXED: !DDMM.HHN/DDDMM.HHW/comments
MOBILE: @DDHHMM/DDMM.HHN/DDDMM.HHW/CSE/SPD/comments
DF: !DDMM.HHN/DDDMM.HHW/CSE/000/CSE indicates a line of bearing
WEATHER REPORT: APRS recognizes the three characters WX: at the beginning
of the comment field to indicate that the position report
contains wind direction and speed information. This plots the
station in dark blue and prevents APRS from assuming it is a
moving mobile station. The remainder of the comment line can
contain any other weather information pertinent to the situation:
@DDHHMM/DDMM.HHN/DDDMM.HHW/CSE/SPD/WX: Hvy rain,Vis 1mi,Seas 2ft
OBJECT POSITION: To report the position of something other than your own
station, the same format for a mobile object is used but it is
preceeded with a nine digit object name. For shorter names, the
9 character field must be padded with spaces:
OBJCTNAME@DDHHMM/DDMM.HHN/DDDMM.HHW/CSE/SPD/comments
BROADCAST: Simply send anything. If you are using your TNC BText to
repeatedly send the same message, it is preferred to include
the day and time that you entered the the BText in the text
so that recepients can tell how old the information is.
@DDHHMM/comments...
MESSAGE: To direct a specific message to a specific station and have
his bell ring and the message instantly flashed on his screen,
use the following format, again padding the addressee call
with spaces to a total of nine characters followed by a colon:
W3XYZ____:one line message text......
His station will acknowledge receipt with a responding message
to you with only the letters ack in the message.
MSG ACK: If an APRS station sends you a message line, it will be repeated
over and over until you send a responding ack. To send an ack
to station W3XYZ, simply enter his call padded to nine spaces
and a colon followed by the letters ACK:
W3XYZ____:ack
USING AND DISPLAYING POSITION INFORMATION. If all manual stations in an APRS
system carry a pre-printed map of the exercise or event area, not only can
they easily determine their own location for reporting, but they can also
plot the location of other stations and objects. Usually, you do not need
the high technology of a GPS or LORAN receiver to know where you are. Most
people can find themselves on a map and read the coordinates. Then they
simply type them in to their BText and in a few minutes, everyone on the net
can see their exact location. Similarly, by monitoring all of the APRS
beacons going back and forth, the dumb terminal user can still be equally
well informed of whats going on.