Introduction to APRS Digipeaters Rich Casey, N5CSU n5csu@arrl.net www.richcasey.net Many slides are from Intro to APRS, by John Beedles, N5OOM Hamcom 2004

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 144.39 MHz at 1200 baud. APRS can also be used over HF and satellite links. Hamcom 2004

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

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

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

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

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

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

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

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

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

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

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

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

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

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: http://www.ew.usna.edu/~bruninga/aprs/id-noid.txt Hamcom 2004

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

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

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

Learning More digis.txt file in the APRS DOS software by WB4APR Ongoing discussions on the TAPR SIGs www.tapr.org Local resources North TX APRS group on Yahoo! http://groups.yahoo.com/group/ntx_aprs_ug/ www.dfwaprs.net Have fun and ask questions! Hamcom 2004