XRPi Documentation - Miscellaneous

APRS

Name

APRS -- Automatic Packet Reporting System.

Description

APRS is (currently) an acronym for "Automatic Packet Reporting System", although the name tends to change from time to time! (it was originally called "Automatic *Position* Reporting System, and there has been talk of re-branding it to Automatic PRESENCE Reporting System).

It is a protocol that uses AX25 UI frames and digipeaters to report a wide variety of parameters such as position, weather, speed, bearing, status, objects, frequencies and so on.

XRPi includes the following sub-systems that support or make use of APRS:

• APRS Generic Digipeating
• APRS Igate
• APRS Server
• APRS Messaging Shell
• APRS Weather reports
• APRS DX recording
• APRS Queries
• Positions, distances & directions in MH lists

Generic digipeating is a complex type of digipeating which responds to special digipeater addresses, and modifies a packet's digipeater addresses in transit. It also prevents duplicates and looping.

The Igate is a client daemon that allows APRS data to flow to and from between the Internet APRS system and RF ports, messaging shell etc.

The APRS server allows APRS applications such as UI-view to use XRPi to access all the APRS data handled by XRPi.

The APRS messaging shell allows users to send and receive APRS messages and bulletins.

Weather reports are received via RF and/or Igate, and are made available for users to read using the WX command.

The DX feature stores a list of the furthest stations heard via RF.

APRS Queries allow RF users to query which nodes are on channel, what DX they've heard, what messages are waiting etc.

MHeard lists are able to display the positions, bearings and distances of stations that broadcast APRS data.

Specifying XRPi's Position

In order for most of these systems to work, XRPi needs to know its position on the globe. There are currently 2 ways to achieve this...

The easiest method is to use the "LOCATOR=" directive in XROUTER.CFG, which enables you to specify an approximate location at the centre of a 1Km "Maidenhead" locator square, e.g. "LOCATOR=IO82VJ".

The second method, which allows more precise (or less precise if you prefer) positioning, is to include an APRS-style position in IDTEXT, starting within the first 40 characters.

The format is "!ddmm.mmN/dddmm.mmE#" where dd represents degrees of latitude/longitude and mm.mm represents minutes to two decimal places. "N" and "E" may be replaced by "S" and "W" as appropriate. For example:

	IDTEXT
	!5224.00N/00215.00W# Kidderminster Router (KDRMIN)
	***

You are urged to use at least one of these methods to define XRPi's position. It really does make Packet more interesting!

APRS Generic Digipeating

XRPi supports APRS generic digipeating for RELAY, WIDE, TRACE, TRACEn-N and WIDEn-N aliases.

Generic digipeating is configured on a port by port basis, using the flags marked "*" in "DIGIFLAG" as follows:

	 1    Digipeat UI frames (note 1)
	 2    Digipeat non-UI frames (note 1)
	*4    Enable RELAY generic digipeating.
	*8    Enable TRACE generic digipeating.
	*16   Enable WIDE (Well sited stations only!)
	*32   Allow APRS 3rd party digi via L4.
	*64   Allow digipeating to Internet (IGate).
	*128  Allow digipeating from Internet (IGate).
	*256  Enable UITRACE digipeating
	*512  Enable UIFLOOD digipeating

Add the appropriate numbers together to enable the desired combination of services.

Note 1: Irrespective of the generic digipeater settings, you may choose to allow regular digipeating (i.e. using XRPi's normal callsign or alias) on APRS ports, allow regular UI-only digipeating, or disable regular digipeating altogether, by manipulating bits 0 and 1 of DIGIFLAG.

SSID Substitution Digipeating

UITRACE and UIFLOOD represent two special addresses that are suffixed with pseudo-SSID's, e.g. "TRACE4-4" and "WIDE2-2".

These addresses can digipeat several times. The first digit specifies the maximum number of hops, and the second is the hop counter, which is decremented each time the frame is digipeated.

These two addresses behave slightly differently however. When a frame is digipeated on the alias specified by UITRACE, each digipeater inserts its own callsign in the digipeater list, and decrements the "SSID". Frames digipeated on the UIFLOOD address have their SSIDs decremented, but the digi doesn't insert its own callsign.

For the sake of consistency with UI-View, UITRACE defaults to "TRACE", giving TRACEn-n digipeating, and UIFLOOD defaults to WIDE, giving WIDEn-n digipeating.

New-N Paradigm

According to the APRS "New-N Paradigm", RELAY, TRACE and WIDE are deprecated, UITRACE should be set to "WIDE", and UIFLOOD should be set to a "state" code (e.g. "GBR" for the UK).

One of the main reasons for the New-N Paradigm was the fact that some of the older digipeaters would digipeat the same packets over and over. This does not happen with XRPi however.

Not everyone agrees with the "New-N Paradigm, so the choice of which features to enable is left you you.

Mixing Modes

In quiet areas, you may wish to mix APRS and normal connected-mode operations on the same port, and that is the default if you enable any of the above flags in DIGIFLAG.

However, in most areas, APRS tends to be on a separate frequency reserved for "unconnected nets", and you may wish to prevent people from connecting to the node or downlinking from it on your APRS-only ports.

The CFLAGS keyword can be used in the PORT section of the XROUTER.CFG file to control uplinking and downlinking as follows:

	0     Prevent uplinking and downlinking.
	1     allow uplinking only.
	2     allow downlinking only.
	3     allow both up and downlinking.

ID Beacons

Whilst all sysops are urged to include an APRS position in their normal IDTEXT, a dedicated APRS port may require a more detailed and cryptic ID beacon, therefore you may define a different IDTEXT for each port if necessary.

A "regular" port would include a position followed by some human-readable text, whereas the APRS-only ports would include additional data such as power / height / gain / direction, wind speed, bowel movements etc., in encoded format.

The IDTEXT may be sent via digipeaters by including the IDPATH keyword in the relevant port configuration section of XROUTER.CFG.

Dupe Suppression

XRPi checks for its own callsign or alias in previously-used digipeaters to prevent digi looping. It will not digipeat frames it originated, and will not digipeat the same frame within 9 seconds.

DX Facility

The "DX [port]" command shows the best received APRS DX. It only works if XRPi's position has been defined as described earlier.

The DXFLAGS keyword in the .CFG file controls whether or not the DX list contains callsigns heard via digipeaters.

The DX list may be queried by RF stations, by means of APRS queries.

APRS / UI-View queries

XRPi responds to the following general queries:

• ?APRS? All stations query.

• ~\xFD~ UI-View general query.

• ?IGATE? Igate query.

The response to the first two is the ID beacon for the port, which should contain the APRS position and station type. The response to the ?IGATE? query shows the message and local user counts.

The following "directed" queries (directed at portcall) are supported:

?APRSD - Directly heard stations list.

Responds with a list of stations heard directly on the receiving port (i.e. not via digipeaters or via 3rd party networks)

?APRSM - Un-delivered messages query.

If there are any un-delivered or expired messages addressed to the sender of the query, they are re-activated and transmitted on the port which received the query.

?APRSP - Station Position.

If the sysop has defined XRPi's position, it is sent in response to this query.

?APRSS - Station status.

The response consists of the software type and version plus a list of the enabled generic digipeater calls.

?PING? ?APRST - Trace Route.

Both of these return the route by which the query was received.

~\xFE~n - UI-View "ping".

The response to this query is a UI-View ack for the ping id.

~\FC~n - UI-View "DX" query.

Responds with a UI-View ack, followed by details of the best DX heard directly by XRPi (digipeated packets are NOT included!)

See www.aprs.org for more info about APRS.

See also

AMSG(1) -- APRS Messaging Mode.
APRS-SRV(6) -- APRS Server.
DX(1) -- Display Distant APRS stations.
IGATE(9) -- APRS Igate.
MHEARD(1) -- Display Recently Heard Stations.
MHEARD(9) -- About the MHeard Facility.
WX(1) -- Display APRS Weather Information.
XROUTER.CFG -- Main Configuration File.