APRS Data in the AX.25 Destination and Source Address Fields¶

The AX.25 Destination Address Field¶

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APRS Software Version Number¶

The name “destination” causes a lot of confusion to many people. The term “destination” comes from AX.25 where it is the destination for the packet. APRS is sent to everyone, rather than being directed to a particular station.

APRS uses this field for other multiple purposes:

• An APRS software version (or device) number.
• Mic-E encoded data.
• A generic APRS address.
• A generic APRS address with a symbol.
• A Maidenhead Grid Locator (obsolete).
• An Alternate Net (ALTNET) address.

In all of these cases, the Destination Address SSID may specify a generic APRS digipeater path.

The AX.25 Destination Address field will most often contain the version number of the APRS software or device that is running at the station. Knowledge of the version number can be useful when troubleshooting.

Here are some early examples. The variable part (xxx) often contains a software version number.

APCxxx APRS/CE, Windows CE

APDxxx Linux aprsd server

APExxx PIC-Encoder

APIxxx Icom radios

APKxxx Kenwood radios

APMxxx MacAPRS

APPxxx pocketAPRS

APRxxx APRSdos

APSxxx APRS+SA

APWxxx WinAPRS

APXxxx X-APRS

APYxxx Yaesu radios

APZxxx Experimental

For example, a station using version 3.2.6 of MacAPRS could use the version APM326.

Developers of new products should obtain a unique software/device identifier to identify the product which is creating the packet. The traditional list at http://aprs.org/aprs11/tocalls.txt is no longer being maintained. The current location is https://github.com/aprsorg/aprs-deviceid

The APZxxx Experimental destination is designated for temporary use only while a product is being developed, before a special APRS Software Version address is assigned to it.

Mic-E Encoded Data¶

Another alternative use of the AX.25 Destination Address field is to contain Mic-E encoded data. This data includes:

• • The latitude of the station.
• A West/East Indicator and a Longitude Offset Indicator (used in longitude computations).
• A Position Comment.
• The APRS digipeater path.

This data is used with associated data in the AX.25 Information field to provide a complete Position Report and other information about the station (see Chapter 10: Mic-E Data Format).

The rest of these are already declared obsolete or probably should be.

Generic APRS A ddress with Symbol¶

Generic APRS Digipeater Path¶

(Obsolete)¶

APRS can also use this field for the display symbol. This was for the very early trackers, with extremely limited resources. They would transmit raw GPS NMEA sentences rather than formatting the data into position reports.

These special addresses are GPSxyz, GPSCnn, GPSEnn, SPCxyz and SYMxyz, and are intended for use where it is not possible to include the symbol in the AX.25 Information field.

The GPS addresses above are for general use.

The SPC addresses are intended for special events. The SYM addresses are reserved for future use.

The characters xy and nn refer to entries in the APRS Symbol Tables. The character z specifies a symbol overlay. See Chapter 20: APRS Symbols and Appendix 2 for more information.

The SSID in the Destination Address field of all packets is can be used to specify the APRS digipeater path.

If the Destination Address SSID is –0, the packet follows the standard AX.25 digipeater (“VIA”) path contained in the Digipeater Addresses field of the AX.25 frame.

If the Destination Address SSID is non-zero, the packet follows one of 15 generic APRS digipeater paths.

The SSID field in the Destination Address (i.e. in the 7th address byte) is encoded as follows:

APRS Digipeater Paths in Destination Address SSID

This was hack for a very early APRS tracker with extremely limited compute resources, and 4 DIP switches to set the destination SSID. This was many years before the WIDEn-N paradigm was created.

We are not aware of any contemporary implementations. The community consensus is that this should be considered obsolete.

Maidenhead Grid Locator in Destination Address (Obsolete)¶

The AX.25 Destination Address field may contain a 6-character Maidenhead Grid Locator. For example: IO91SX. This format is typically used by meteor scatter and satellite operators who need to keep packets as short as possible.

This format is now obsolete.

Generic APRS Destination Addresses¶

APRS uses the following generic beacon-style destination addresses:

The asterisk is a wildcard, allowing the address to be extended (up to a total of 6 alphanumeric characters). Thus, for example, WX1, WX12 and WX12CD are all valid APRS destination addresses.

All of these addresses have an SSID of –0. Non-zero SSIDs are reserved for generic APRS digipeating.

These addresses are copied by everyone. All APRS software must accept packets with these destination addresses.

The addresses DGPS and RTCM are used by differential GPS correction stations. Most software will not make use of packets using this address, other than to pass them on to an attached GPS unit.

The address SKY is used for Skywarn stations.

Packets addressed to SPCL are intended for special events. APRS software can display such packets to the exclusion of all others, to minimize clutter on from other stations not involved in the special event. The addresses TEL and TLM is used for telemetry stations.

The use of “APRS,” in the destination field, is obsolete. Use an APxxxx software version number as explained earlier.

Alternate Nets¶

Any other destination address not included in the specific generic list or the other categories mentioned above may be used in Alternate Nets (ALTNETs) by groups of individuals for special purposes. Thus they can use the APRS infrastructure for a variety of experiments without cluttering up the maps and lists of other APRS stations. Only stations using the same ALTNET address should see their data. HUH? Can we elaborate or declutter this?

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The AX.25 Source Address SSID to specify Symbols¶

The AX.25 Source Address field contains the callsign and SSID of the originating station. If the SSID is –0, APRS does not treat it in any special way.

If, however, the Source Address SSID is non-zero, APRS interprets it as a display icon. This is intended for use only with stand-alone trackers where there is no other method of specifying a display symbol or a destination address (e.g. MIM trackers or NMEA trackers).

For more information, see Chapter 20: APRS Symbols.