1. Introduction to APRS Digipeaters
Rich Casey, N5CSU
Many slides are from Intro to APRS, by John Beedles, N5OOM
Hamcom 2004

2. What is APRS?
APRS, or Amateur Position Reporting System, developed by Bob Bruninga, WB4APR is a system that allows users to transmit location and other data in single data packets. Usually stations being tracked use GPS receivers to provide real time tracking data.
APRS uses existing packet TNCs (terminal node controllers) and small, low cost microcontroller driven units to transmit standard AX.25 packets on a ground frequency (in the U.S.) of MHz at 1200 baud. APRS can also be used over HF and satellite links.
Hamcom 2004

3. APRS Applications Skywarn Nets Spotter assignment Storm tracking
Post Disaster Management
Damage assessment
Liaison tracking
Logistics management
Staging site talk in
Search & Rescue
Public Service Events
Bike rallies
Parades
Crime prevention patrols
Other
Weather Stations
Repeater advertising
Event talk-in
Hamcom 2004

4. How APRS Works
An APRS station broadcasts a packet of information, typically a GPS coordinate and other information.
The packet is received and decoded by stations in range.
Digipeater stations hear the packet and rebroadcast it based on
rules in the digipeater software and
routing that a user puts in the packet.
Hamcom 2004

5. Limitations with APRS
APRS on VHF is a single 1200 baud shared channel.
APRS packets take a finite amount of time to transmit; therefore, only a limited number of users may operate in a given area.
The number of digipeaters that retransmit a packet is dependent on how each user configures his TNC.
Misconfiguring a TNC can flood the channel. Smart digipeating can help.
Hamcom 2004

6. Example (1) – Mobile Station Beacons
Step 1. An APRS station beacons and is heard by every other APRS station in direct range
Hamcom 2004

7. Example (2) – Local Digipeater relays
Step 2. The packet is rebroadcast by the local digipeater; this packet is heard by everyone in direct range of that digipeater. It is also received by other area digipeaters.
Hamcom 2004

8. Example (3) – Area digipeaters relay
Step 3. The packet is then rebroadcast by the other area digipeaters. The packet is heard by every APRS station in direct range of this second set of digipeaters, including the original digipeater.
Hamcom 2004

9. Good, Bad & Ugly The Good News: Everyone’s a digipeater! The Bad News:
Hamcom 2004

10. Digipeaters: two flavors
Relay
A default alias that all stations share. We’re all relays, but this is unnecessary in urban areas with good wides!
Wide
Exceptional stations… coordinated with area hams to provide specific wide area coverage
Hamcom 2004

11. More about WIDES!
Wide area digipeaters need to be coordinated with the user community
As your network grows, a wide may need to become a relay if it will improve the network.
A good relay is hard to find! Moving from wide to relay shows the network is getting better!
Example: The Dallas County experience
Hamcom 2004

12. Avoiding the ping-pong
With a via address of relay,wide, your signal will be repeated twice
First relay (or wide) that hears you
Next wide that hears you repeats you
But what if someone uses wide-wide-wide?
In the old days, you could ping, pong back and forth between digis
Hamcom 2004

13. Digipeaters: Getting Smarter
Callsign substitution on retransmission
Digipeater replaces the relay or wide with its own callsign
Available in KPC3 w/8.2 or higher or special ROM
Eliminates dupes
Widen-n the next step
Shorter user packets and smarter digipeaters
Hamcom 2004

14. How does Widen-n work?
A widen-n digi simply repeats any packet with the via address of widen-n; but ONLY ONCE.
It keeps a copy (or checksum) of the last 30 seconds of packets, and compares each new packet that it hears with these last ones to avoid dupes.
This eliminates the multiple looping of packets caused by multiple generic paths such as WIDE,WIDE,WIDE when call sign substituting digis are not used (as many as 21 copies!) In a widen-n network, however, there would only be three packets.
From digis.txt file in the APRS DOS SW by WB4APR
Hamcom 2004

15. Decoding Widen-n Widen-n Example: First N= total number to digipeat
Second N = the number left to go
Example:
N5CSU-7>APRS,relay,wide2-2 retransmit via one relay and 2 wides
N5CSU-7>APRS,K5QBM-2*,wide2-2 in transit… Relay has been changed to discrete call.  Wide2-2 left to go.
N5csu-7>APRS,K5QBM-2,wide2-1* in transit… one wide down and looking for another!
Hamcom 2004

16. Wide Area Digipeater Settings
Set aliases to digipeat on relay, wide, trace, ss
Use TNC that provides callsign substitution and provides widen-n function
Set your POSIT power-height-gain correctly
Watch your network for possible changes
KPC users: turn ID to off (not needed and disrupts first digi identifier) ref:
Hamcom 2004

17. Options for Wide Area Digis
KPC-3 or KPC-3+
PacComm ROM in any TAPR-2 clone tnc
DIGI_NED software on any clone PC
UIDIGI ROM in a TAPR-2 tnc
Hamcom 2004

18. TNC Settings for KPC-3
MYCall W3XYZ-x
UIDIGI ON RELAY, WIDE, TRACE, SS <= these do callsign substitution
UIFLOOD WIDE,28,NOID <= turn off to kill DX long hops
UITRACE TRACE
LT 1 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments
LT 2 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
LT 3 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
LT 4 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
LTP 1 APRS
LTP 2 APRS VIA WIDE
LTP 3 APRS VIA WIDE2-2
LTP 4 APRS VIA TRACE3-3
BLT 1 E 00:20:00 At 00:00:00 Sends local posit locally every 20 mins
BLT 2 E 00:40:00 At 00:10:00 Sends posit out 1 hops every 40 minutes
BLT 3 E 01:20:00 At 00:30:00 Sends a 2 hop path once every 80 mins
BLT 4 E 02:40:00 At 01:10:00 Sends a 3 hop path once every 160 Mins
From digis.txt file in the APRS DOS SW by WB4APR
Hamcom 2004

19. Recommendations: Home QTH
For your own station, set via with real callsigns, not relay,wide
In an urban area with a busy channel, consider turning your own digipeat function off. If stations can see a wide, save your transmitter and the channel’s throughput!
If your station is enabled as a relay digipeater, use a tnc that provides callsign substitution
Hamcom 2004

20. Learning More digis.txt file in the APRS DOS software by WB4APR
Ongoing discussions on the TAPR SIGs
Local resources
North TX APRS group on Yahoo!
Have fun and ask questions!
Hamcom 2004