and Update from 6.02.2012

Since there are 20,000 APRS users on the air, the APRS maps and data can get very cluttered.  Also, most operators have multiple stations on the air, such as their HOME, CAR, BOAT, and HT. It is very convenient to beable to recognize these typical applications at a glance.

  Here are those common defaults:

  -0   -     Home Station, Home Station running IGate.
  -1   -     Digipeater, Home Station running a Relay Digi, Wx Digipeater
  -2   -     Digipeater [#2 or] on 70CM
  -3   -     Digipeater [#3]
  -4   -     HF to VHF Gateway
  -5   -     IGate (Not home station)
  -6   -     is for Operations via Satellite
  -7   -     Kenwood D7 HH
  -8   -     is for boats, sailboats and ships (maybe 802.11 in the future)
  -9   -     is for Mobiles
-10   -     is for operation via The internet only
-11   -     is for APRStouch-tone users  (and the occasional Balloons)
-12   -     Portable Units such as Laptops, Camp Sites etc.
-14   -     is for Truckers
-15   -     is for HF

Originally, in 1992, the first APRS GPS mobiles just transmitted raw NMEA-0183 GPS stings and so there was no way to insert a SYMBOL byte in  the pre-formatted NMEA 183 packet.  Hence we defined 14 common SYMBOLs as defaults to the 14 available SSID's that a mobile could use. These included BIKES, HELICOPTERS, AIRCRAFT, VANS and many others in addition to the ones above. This is now OBSOLETE.

In the late 1990's we came up with a better solution and defined the AX.25 TOCALL of GPSxyz for such use.  In this case, the "xyz" characters can be used to define any of the hundreds of APRS symbols including Overlays.   Just look up the xyz in the SYMBOLS.TXT file included in all copies of APRSdos (or these days, on the WEB).

Since this GPSxyz concept was so powerful, it completely replaced the original SSID assignments and so those original NMEA defaults have been removed from the Documentation.

However the convention for the 6 most useful ones remains and should be used in the absesnce of any other overriding conditions.  Except for the TH-D7 and Truckers, the other SSID conventions mark APRS "TYPES" of users that are independent of the many SYMBOLS they can use.  Thus it is nice to know HOW someone is getting into the system, whether via satellite, HF, the internet, or touchtone.  For example, using the -6 only for Space  applications is so that the global APRS WEB data bases will keep  statistics and data for when you are operating through the satellites  separate from when you are working normally.

So stick to the standards above for the obvious applications...  Of course these are not rigid.  If you have more than 4 digipeaters, use any SSID you want. 
These are only guidelines...
de WB4APR, Bob

APRS SSID Recommendations       6 Feb 2012

Updated 6 Feb 2012 to clarify the title of this document to be
SSID RECOMMENDATIONS not to imply any kind of decoding "standard".
That original NMEA decodign standard is covered in the original spec
Updated 9 June 2010 for more flexibility using 1,2,3,4 and 15
Revised 2 June 2004 to add -10, -11, 12 and -15 SSID's have seen two different uses in APRS. Initially as an ICON indicator back in the early 1990's. But that is obsolete for over a decade. Now SSID's are used as an informal way of indicating one of several different typical APRS applications.

Since many small displays for the handheld and mobile operator show nearby APRS station callsigns that flash up on the screen, it is nice to have some idea of what type of station or activity might be involved simply from the callsign SSID without having to push buttons, search lists, or look at maps to find out more about them.

SSID RECOMMENDATIONS: It is very convenient to other mobile operators or others looking at callsigns flashing by, to be able to recognize some common applications at a glance. Here are the recommendations for the 16 possible SSID's (the limit of 16 comes from the 4 bits available in the AX.25 protocol. Note, The SSID of zero is dropped by most display applications. So a callsign with no SSID has an SSID of 0.

  -0 Your primary station usually fixed and message capable
  -1 generic additional station, digi, mobile, wx, etc
  -2 generic additional station, digi, mobile, wx, etc
  -3 generic additional station, digi, mobile, wx, etc
  -4 generic additional station, digi, mobile, wx, etc
  -5 Other networks (Dstar, Iphones, Androids, Blackberry's etc)
  -6 Special activity, Satellite ops, camping or 6 meters, etc
  -7 walkie talkies, HT's or other human portable
  -8 boats, sailboats, RV's or second main mobile
  -9 Primary Mobile (usually message capable)
-10 internet, Igates, echolink, winlink, AVRS, APRN, etc
-11 balloons, aircraft, spacecraft, etc
-12 APRStt, DTMF, RFID, devices, one-way trackers*, etc
-13 Weather stations
-14 Truckers or generally full time drivers
-15 generic additional station, digi, mobile, wx, etc

* One-way trackers should best use the -12 one-way SSID indicator because the -9's usually mean a ham in full APRS communication both message and voice. The -9's can be contacted by APRS message or by Voice on his frequency included in his beacon, or on Voice Alert if he is in simplex range. The -12's are just moving Icons on the map and since they have no 2 way communication for ham radio they are not generally of routine interest to other operators.

OBJECTS or INTERNET: In addition, Objects or internet generated stations can have any SSID, not just the original 16, since Objects are not constrained by the AX.25 header and can have a 9 byte name. Here are some common OBJECT/Internet SSID's:
-63 for PSK63 HF stations
-tt for APRS TouchTone users (DTMF)
-ID for RFID
-A through -Z for Dstar

SSID BACKGROUND: Originally, in 1992, we had to use the SSID as a way of indicating the type of station that transmitted a raw NMEA-0183 GPS sting. But in the mid 1990's we began indicating any of the nearly 200 APRS symbols by the setting of the AX.25 TOCALL of "GPSxyz". The "xyz" characters define the symbol from the standard APRS symbol table www.aprs.org/symbols.html.

The GPSxyz concept worked so well, the original SSID associations are no longer a required part of the spec. But the conventions that evolved from those early SSID's are still encouraged as noted above, for easy recognition of station type or activity by when only the callsign is seen.

The -1, -2, -3, -4 and -15 are kept generic so that anyone with as many as 6 digipeaters, or 6 trackers or 6 weather stations or 6 vehicles can still have unique SSID's for each of his stations. Beyond 6, people will just have to use any SSID that suits their fancy. In some areas there might be 15 digipeaters all under one guy's call!

SSID USAGE: The SSID's also might give a hint as to how someone is getting into APRS whether via satellite, a one-way tracker, a mobile, an HT or even via DTMF or an RFID device or whether he is doing something special.

For example, if you are doing something special, change your SSID to -6 to alert others to your excitement, or to make the track-history begin and end on site, and not be tied to all your other -9 travels. Or use -6 SSID for a packet sent via the ISS or APRS satellite or for a 6 meter test so the successful packet is preserved and not overwritten by the same radio the next time you use it not via the ISS on the 144.39 national channel. By using separate SSID's the WEB data bases will keep statistics and data separate from when you are working normally on other bands with other SSID's.

So stick to the suggestions above for the obvious applications where you can. Of course these are not rigid. If you have more than 6 digipeaters, use any SSID you want. These are only guidelines to hint at a station's possible application when all you can see easily is the callsign on a screen or in a list....
de WB4APR, Bob