Background

A joint project between South West Hampshire Raynet and the South Hampshire Repeater Group, MB7UUV was commissioned on 3rd April 2023 with the aim of providing street-level APRS coverage to the City of Southampton and surrounding areas. Southampton's digipeater coverage has been patchy since the loss of Raynet's MB7UUW whose antenna on a site in Winchester failed some years ago.

Co-sited with the South Hampshire Repeater Group's repeaters ~100m ASL the digipeater provides excellent area coverage within the City, along the major trunk roads and into the New Forest. MB7UUV works in conjunction with MB7RSH, a South West Hampshire Raynet iGate located at Dock Gate 20, South East Hampshire Raynet's MB7UP located atop Portsdown Hill, and North East Hampshire Raynet's MB7UG located in Alton.

Equipment

Hardware consists of a Motorola GM380 coupled to a Rasberry Pi 3B+ running Dire Wolf v1.6 and associated control logic. Antenna is 0dBd omni vertical, feeder & filter a ¼-wave bandpass resonator with ~1dB total insertion loss; reliable decodes appear to be around -110dBm at the feedpoint.

The digipeater is configured as a "medium fill-in" WIDE1-1,WIDE2-2 digipeater / iGate, beaconing WIDE3-3. Being an "end-of-the-line" digipeater the configuration is conservative; as further licensed digis appear westwards we will likely revise the configuration to a wide area digipeater. Furthermore, Raynet aliases are currently in place for event, exercise and emergency work.

For more information regarding the implementation please contact Jason M0MZF. Technical input and equipment from Geoff G7UHE, Alan G8IPG and Andy G4MYS was crucial to the success of the project!

What is APRS?

APRS (automatic packet reporting system) uses AX.25 and a network of digipeaters to allow position reports, messages and other "tactical" information to be broadcast to listening stations providing a real-time view of locations, points of interest and a way to pass text messages. Stations transmit these packets, digipeaters hear and re-transmit these packets, and iGates hear the packets and upload them to the APRS-IS (APRS internet service). Uploaded information can be viewed through aggregation websites, most notably aprs.fi.

Like regular AX.25 the packet contains a source, destination, path and information. The difference with APRS is that the destination instead identifies what TNC the station is using, and the paths define how digipeaters will treat the packet.

What's in these packets?

A typical packet looks like this:

M0MZF-7>APDW16,WIDE1-1,WIDE2-2:!/4fF/N/GA#{8GOn the move!

Let's break down a typical position report:

M0MZF-7
Callsign of the originating station. The -7 is an SSID and is used to separate multiple stations from a given callsign. The number also identifies the type of station by convention, e.g. -7 is hand portable, -9 is mobile etc.

APDW16,
"Destination", though this name is misleading. Usually this identifies the type of hardware or TNC in operation, for example here APDW16 means "Dire wolf". Yaesu and Kenwood radios will often use this to send position information too as part of the Mic-E encoding scheme.

WIDE1-1,
The first path, i.e. how the packet is treated by the digipeater

WIDE2-2:
The second path (there can be further paths!)
! Type of packet, e.g. ! indicates a position report.

/4fF/N/GA#{8G
Compressed position data and symbol / icon. Modern TNCs will decode this as a lat/long, it's shorter than sending the lat/long in plain text.

On the move! Information contained within the packet.

When a digipeater hears this packet, it makes a change and re-transmits it. For example:

M0MZF-7>APDW16,MB7UUV*,WIDE2-2:!/4fF/N/GA#{8GOn the move!

Notice that WIDE1-1 has been replaced by MB7UUV*.
The digipeater has been configured to forward WIDE1 and WIDE2 packets, and as the WIDE1-1 path has "run out" of WIDE1 hops, the digipeater replaces it with it's own callsign. The * indicates the packet came from that particular digipeater.

The next digipeater makes another change and retransmits it:

M0MZF-7>APDW16,MB7UUV,MB7UP*,WIDE2-1:!/4fF/N/GA#{8GOn the move!

The next digipeater sees the WIDE2-2 path, adds its own callsign and decrements the WIDE2-2 to WIDE2-1. Again the * indicated the packet has been digipeated. (The previous * has not been removed, it's just generally not displayed by software.)
The next digipeater does the same:

M0MZF-7>APDW16,MB7UUV,MB7UP,MB7UG*:!/4fF/N/GA#{8GOn he move!

Again, the digipeater sees the WIDE2-1 path, removes it as it's run out of hops, then adds it's callsign and a *.

Now there are no more WIDE paths left in the packet any subsequent digipeater will not retransmit the packet - this packet has traversed three digipeaters and has finished its journey!

The digipeater will almost always hear a packet it just transmitted arrive back, but as it can see its own callsign it will not transmit the packet again; this deduplication prevents unnecessary clogging of the frequency.

Tell me more about this WIDEN-n path thing!

With AX.25 an operator would intentionally configure the path to choose which digipeaters would forward the packet towards the intended destination. With APRS there is no intended destination per se, instead paths are used to configure how far the packet will travel across the network. These paths are called "aliases" as the digipeater will be configured to respond to them despite them not actually being the digipeater's callsign.

The WIDEN-n notation says "I want my packet to be repeated N times, and there are n hops left". Thus, a WIDE1-1,WIDE2-2 path will travel three hops. A WIDE1-1,WIDE2-2,WIDE3-3 path will travel six hops.

The APRS network would be very noisy and the shared frequency clogged if everyone used, say, WIDE7-7 paths. The system is designed to share "local" or "tactical" information, and mostly people want their packets to hit the nearest iGate so they appear on the internet. In the UK there are a many, many iGates, and each digipeater is often in view of an iGate - furthermore some digipeaters are themselves iGates, so in reality a sensible and friendly path in the UK is WIDE1-1,WIDE2-1. Two hops will get you to your nearest digipeater (and therefore probably an iGate) and to the next digipeater, allowing local stations monitoring the frequency to hear your activity within a two-digipeater radius.

For stations in range of MB7UUV two hops will have you digipeated by MB7UUV in Southampton, MB7UP in Portsmouth and MB7UG in Alton. WIDE1-1,WIDE2-2 will get you as far as MB7UJ in Bracknell and MB7VW in Warminster, as well as MB7UBZ near Brize Norton and MB7VG in Crawley. As you can see, paths go far pretty quickly!

What's Dire Wolf?

Dire wolf is a highly capable, modern software modem/TNC which uses a sound card to receive packet data from the radio and software to decode it and perform digipeater logic. It has lots of advanced alias, filtering, telemetry, deduplication and decoding features and is spectacularly well-documented, making it THE modern replacement for a hardware TNC. Multi-port operation can be achieved with multiple sound cards, and it can be used for far more than APRS - it really is the swiss-army knife of AX.25. The author WB2OSZ has even implemented a backwards-compatible version of AX.25 called FX.25 complete with forward error correction!

Any other juicy technical details about MB7UUV?

MB7UUV runs on a Raspberry Pi 3B+, and the full Linux stack coupled with the Pi's GPIO hardware makes for a powerful repeater controller. In a nutshell, the Pi's filesystem is configured to be read-only, such that power outages etc. do not corrupt it's storage and a series of scripts control start-up and running of the digipeater. Logic-level GPIO signals are used to control relays which can interrupt the transceiver's power and PTT lines, and further logic ensures that the digipeater does not hang up the PTT if it gets into a "funny state". After a power failure the digipeater reverts to iGate only operation, until it can be verified that it's operating correctly and the transmitter can then be manually re-enabled.
Eventually the digipeater will be running from an extremely capable battery-backed power supply, and should provide many years of 24/7 operation - but that's the next project!

Is that it?

No, but this'll get you up and running :) Enjoy, and happy APRSing! 73 de Jason M0MZF