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APRS packet anatomy

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An APRS packet is very often presented in a readable text form:

SOURCE>DEST,PATH:DATA

Example:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

At first glance, this looks like a single format.

In reality, this notation combines information from several AX.25 frame elements and the actual APRS data in a single line.

In simplified form:

SQ9MDD-7 > APRS , WIDE1-1 : !5012.34N/01956.78E>
^^^^^^^^   ^^^^   ^^^^^^^   ^^^^^^^^^^^^^^^^^^^^^
 SOURCE    DEST     PATH           DATA
  AX.25    AX.25    AX.25          APRS

Understanding this division is the foundation for further protocol analysis.

General packet form

The most common text representation of an APRS packet has the form:

SOURCE>DEST,PATH:INFORMATION

Not all elements must always be present.

A packet without a path can look like this:

SOURCE>DEST:INFORMATION

Example:

SQ9MDD-7>APRS:>QRV 145.550 MHz

A packet with a path can look like this:

SOURCE>DEST,WIDE1-1,WIDE2-1:INFORMATION

Example:

SQ9MDD-7>APRS,WIDE1-1,WIDE2-1:>QRV 145.550 MHz

Logically, it can therefore be divided into:

SOURCE
DEST
PATH
INFORMATION

The first three elements are related to AX.25 addressing.

The INFORMATION field contains the actual APRS information.

Step-by-step example

Consider the packet:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

We can break it down into the following parts:

SQ9MDD-7

packet source,

>

separator used in the text representation,

APRS

destination address,

,

separator before the path,

WIDE1-1

digipeater path,

:

separator between the header and the information field,

!

Data Type Identifier,

5012.34N/01956.78E>

actual APRS data.

A single line therefore shows both AX.25 layer information and APRS protocol data.

SOURCE

The first element is the source address:

SOURCE

Example:

SQ9MDD-7

In typical APRS use, this is the station callsign together with an optional SSID.

It can therefore appear, for example, as:

SQ9MDD
SQ9MDD-4
SQ9MDD-7
SQ9MDD-9

SOURCE identifies the station that created the AX.25 frame.

It is an AX.25 addressing element, not part of the APRS information field.

Callsign and SSID

An AX.25 address can contain a callsign and an SSID.

Example:

SQ9MDD-7

can be read logically as:

callsign: SQ9MDD
SSID:     7

SSID allows the same callsign to be used by several logical stations.

For example, an operator may use separate SSIDs for:

  • a base station,
  • a mobile station,
  • a handheld radio,
  • a digipeater,
  • an IGate,
  • a tracker.

There are historical and practical conventions regarding the meaning of individual SSIDs, but they should not be treated as an absolute protocol rule defining device function.

Details of callsigns and SSIDs are described separately.

The > separator

After the source address, the following character appears in the text representation:

>

For example:

SQ9MDD-7>APRS

It separates SOURCE from DEST.

It is an element of the packet’s text representation.

The > character should not be treated as a separate byte located at that point in the raw AX.25 frame transmitted over radio.

DEST

After the > character comes the destination address.

Example:

APRS

Together:

SQ9MDD-7>APRS

In AX.25, this is the destination address.

In classic packet radio, it can identify the destination station.

In APRS, its meaning is often different.

Destination in APRS

In many APRS packets, the destination field does not identify the actual recipient of the information.

It can be used, among other things, to identify:

  • the device type,
  • the manufacturer,
  • the software,
  • the application family,
  • the method used to generate the packet.

For this purpose, APRS uses, among other mechanisms, the TOCALL system.

Example values can look like:

APRS
APDWxx
APRSxx

Their exact meaning depends on the TOCALL assignment.

Therefore, the packet:

SQ9MDD-7>APRS:...

does not mean:

SQ9MDD-7 sends data to station APRS

Here, the destination field is primarily part of the AX.25 header and is also used by APRS for additional identification.

Destination is not the APRS message recipient

This is one of the most important things to remember.

Consider:

SQ9MDD-7>APRS,WIDE1-1::SP9XYZ   :Hello

The AX.25 destination is:

APRS

but the message recipient is:

SP9XYZ

The recipient is located inside the APRS data.

We therefore have two different concepts:

AX.25 destination

and:

APRS message addressee

They should not be confused with each other.

PATH

A path may follow the destination:

SOURCE>DEST,PATH:DATA

Example:

SQ9MDD-7>APRS,WIDE1-1,WIDE2-1:...

The path can contain one or more elements:

WIDE1-1
WIDE2-1

In the text representation, they are separated by commas.

PATH is an AX.25 element

The path is not inside the APRS payload.

It represents the list of AX.25 digipeater addresses.

Logically:

SOURCE>DEST,WIDE1-1,WIDE2-1:DATA

can be understood as:

AX.25:
    SOURCE
    DEST
    DIGI 1
    DIGI 2


APRS:
    DATA

This matters when analysing packets.

WIDE1-1 is not a command stored inside the APRS data.

It is part of AX.25 frame addressing.

Multiple path elements

The path can contain several addresses.

Example:

SQ9MDD-7>APRS,WIDE1-1,WIDE2-1:...

or after passing through infrastructure:

SQ9MDD-7>APRS,SP9ABC-2*,WIDE2-1:...

The character:

*

visible in the monitor representation can indicate that the given path element has already been used.

Details of the H-bit mechanism, WIDE aliases, the New-N Paradigm, and path processing are described in a separate article.

The : separator

The most important boundary in the text representation of a packet is:

:

Example:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

Everything before the first : separator forms the text representation of header-related information.

What follows it is the information field.

We can therefore write:

SOURCE>DEST,PATH : INFORMATION
^^^^^^^^^^^^^^^^   ^^^^^^^^^^^
     header           data

For APRS analysis, the actual APRS content begins on the right-hand side of this separator.

INFORMATION

The field after the : separator contains the data carried in the AX.25 Information field.

Example:

!5012.34N/01956.78E>

This is where the actual APRS data protocol is located.

The contents can represent, among other things:

  • position,
  • message,
  • status,
  • object,
  • item,
  • weather,
  • telemetry,
  • query,
  • station capabilities,
  • special data.

Interpretation of the field depends primarily on how it begins.

Data Type Identifier

The first character of the APRS Information field is very often the Data Type Identifier, or DTI.

Example:

!5012.34N/01956.78E>
^
|
DTI

The DTI defines how the remaining data should be interpreted.

Example DTIs include:

DTI General meaning
! position without timestamp
= position without timestamp, with messaging support
/ position with timestamp
@ position with timestamp, with messaging support
: message
; object
) item
> status
? query
_ weather data without position
T telemetry
} third-party traffic

This is not a complete table of all possible values.

A complete DTI overview is provided in the article devoted to APRS frame types.

DTI is not always enough for full interpretation

Recognising the first character is only the beginning of packet parsing.

Example:

!5012.34N/01956.78E>

DTI:

!

indicates a position without a timestamp.

Only the following characters define:

  • latitude,
  • symbol table,
  • longitude,
  • symbol,
  • optional extensions,
  • comment.

An APRS parser must therefore first recognise the data type and then apply the rules appropriate to that type.

Position example

The packet:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

can be broken down as follows:

SQ9MDD-7

SOURCE,

APRS

DEST,

WIDE1-1

PATH,

!

DTI for a position without timestamp,

5012.34N

latitude,

/

symbol table identifier,

01956.78E

longitude,

>

symbol code.

In this example, the whole packet can therefore be viewed as:

SQ9MDD-7 > APRS , WIDE1-1 : ! 5012.34N / 01956.78E >
SOURCE      DEST    PATH      DTI LATITUDE   LONGITUDE  SYMBOL

The symbol is part of the position format

In an APRS position packet, the symbol is not transmitted as a name:

car
house
repeater

Instead, it is defined by characters located at specific positions in the position format.

In a classic uncompressed position, the important elements are:

  • the symbol table character,
  • the symbol code.

Therefore:

/

inside a position packet is not a text separator like > or :.

It is part of the APRS data and is significant when interpreting the symbol.

Message example

Consider:

SQ9MDD-7>APRS,WIDE1-1::SP9XYZ   :Test

The breakdown is as follows:

SQ9MDD-7

SOURCE,

APRS

DEST,

WIDE1-1

PATH,

and the information field:

:SP9XYZ   :Test

begins with:

:

which is the message DTI.

Next comes the addressee field:

SP9XYZ

and the content:

Test

It is worth noting the two different colons:

...WIDE1-1::SP9XYZ   :Test
           ^^

The first:

:

separates the header from Information.

The second:

:

is the APRS message DTI.

This repetition is not accidental.

Status example

The packet:

SQ9MDD-7>APRS:>QRV 145.550 MHz

contains:

>

as the first character of the information field.

Here, > is the status DTI.

This shows why the same character can have different meanings depending on where it appears.

In the header:

SOURCE>DEST

> is a separator in the text representation.

In Information:

>QRV 145.550 MHz

> is an APRS DTI.

A parser must always take context into account.

Object example

An object packet begins in Information with:

;

Schematically:

SOURCE>DEST,PATH:;OBJECTNAM*...
                 ^
                 |
                DTI

After the DTI come object-specific fields, including its name, state, time, and position.

The detailed object syntax is described in a separate part of the documentation.

Comment

Many APRS formats allow an additional comment after the basic data.

For example, a position packet can contain:

!5012.34N/01956.78E>QRV 145.550 MHz

The first part defines the position and symbol.

The following part:

QRV 145.550 MHz

is the comment.

The comment can also contain specific APRS data extensions.

Therefore, not everything following the position should automatically be treated as arbitrary plain text.

Data extensions

Additional information can appear after the basic APRS position.

Depending on the format and context, these can describe, among other things:

  • course and speed,
  • altitude,
  • PHG,
  • range,
  • frequency,
  • tone,
  • offset,
  • DAO,
  • other extensions.

An example can therefore be much more elaborate than a simple position:

SOURCE>DEST,PATH:POSITION + EXTENSION + COMMENT

The exact rules depend on the specific data type and are described in the relevant documentation sections.

Information field length matters

APRS operates in a bandwidth-limited environment, especially on the classic 1200 baud VHF channel.

Every additional character increases transmission length.

A well-designed packet should therefore convey the required information without unnecessary content expansion.

This applies especially to:

  • comments,
  • position beacons,
  • objects,
  • statuses,
  • messages.

A long packet occupies the channel longer than a short packet.

The text shown in a monitor is TNC2 format

The form:

SOURCE>DEST,PATH:INFORMATION

is commonly referred to as TNC2 monitor format or TNC2 representation.

It is used, among other things, by:

  • terminals,
  • APRS software,
  • logs,
  • APRS-IS servers,
  • diagnostic tools.

It allows an AX.25 frame to be presented in a form that is easy for a human to read.

This does not mean that exactly this character string was transmitted over radio.

What is actually inside an AX.25 frame?

On RF, addresses are not transmitted as literal text:

SQ9MDD-7>APRS,WIDE1-1

An AX.25 frame has its own binary encoding for addresses and control fields.

It includes, among other things:

Destination
Source
Digipeater addresses
Control
PID
Information
FCS

For typical APRS:

Control = UI
PID     = no layer 3

The Information field, however, contains the APRS data.

The TNC2 representation hides many AX.25 layer details to make the packet easier to analyse.

FCS is not visible in TNC2

A real radio frame includes an error-checking mechanism, FCS.

However, it is not visible in a typical line:

SOURCE>DEST,PATH:DATA

The receiver, TNC, or modem checks frame validity earlier.

If the frame is accepted and passed to the application, the monitor representation usually does not contain the FCS.

This is another example of the difference between the real AX.25 frame and its text representation.

Packet received directly

Example of a packet seen directly on RF:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

We can interpret it as:

SOURCE: SQ9MDD-7
DEST:   APRS
PATH:   WIDE1-1
DATA:   !5012.34N/01956.78E>

This is one of the simplest analysis cases.

Packet after passing through a digipeater

After retransmission, the representation may contain information about the path element that was used.

For example:

SQ9MDD-7>APRS,SP9ABC-2*,WIDE2-1:!5012.34N/01956.78E>

The monitor can then show through which station the packet was repeated.

This does not mean that the APRS data:

!5012.34N/01956.78E>

changed.

The information related to AX.25 transport changed.

Packet in APRS-IS

After being forwarded by an IGate, the packet may look like this:

SQ9MDD-7>APRS,WIDE1-1,qAR,SQ9MDD-4:!5012.34N/01956.78E>

Here we can see the elements:

qAR
SQ9MDD-4

which relate to APRS-IS.

The entire string between destination and : should therefore not be analysed as only a radio AX.25 path.

In APRS-IS, the packet representation can also contain information added by the Internet infrastructure.

The boundary between RF and APRS-IS

Compare:

RF packet

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

Packet visible in APRS-IS

SQ9MDD-7>APRS,WIDE1-1,qAR,SQ9MDD-4:!5012.34N/01956.78E>

The actual APRS information:

!5012.34N/01956.78E>

remains the same.

What changes is the information describing how the packet reached the given environment.

Third-party packet

APRS also has a format that allows an entire packet to be placed inside another packet.

DTI:

}

means third-party traffic.

Schematically:

SOURCE>DEST,PATH:}ORIGINAL>DEST,PATH:DATA
                 ^
                 |
          third-party DTI

After the } character comes the representation of the original packet.

We therefore have a situation where another packet representation is contained inside the Information field of one frame.

A parser must recognise this case and handle the nested information appropriately.

A packet can contain nested information

Third-party traffic demonstrates an important APRS property.

A packet cannot always be analysed using the simple assumption:

find the first character
read the rest as data

Some data types create an additional internal structure.

A correct parser must therefore recognise the format implied by the DTI and only then interpret the remaining content.

The meaning of character position

When analysing APRS, a character is significant not only because of its value but also because of where it appears.

For example, the character:

:

can be:

  • a separator between the header and Information in TNC2 representation,
  • a message DTI,
  • a separator inside the message format.

The character:

>

can be:

  • a separator between SOURCE and DEST,
  • a status DTI,
  • a symbol code in position data.

APRS should therefore not be analysed by searching for individual characters without considering structure.

Parsing order

A practical packet parser can logically operate in the following order.

First, separate:

HEADER:INFORMATION

Then recognise from the header:

SOURCE
DEST
PATH

And then from the Information field:

DTI

and select the appropriate data parser based on the DTI.

Diagram:

packet
  |
  +-> header
  |     |
  |     +-> source
  |     +-> destination
  |     +-> path
  |
  +-> information
        |
        +-> DTI
              |
              +-> position parser
              +-> message parser
              +-> object parser
              +-> weather parser
              +-> ...

This is much safer than trying to interpret the entire packet as one string with a fixed structure.

Not every position packet looks the same

Example:

!5012.34N/01956.78E>

is a classic uncompressed position.

APRS also supports other position encodings, including:

  • position with timestamp,
  • compressed position,
  • Mic-E,
  • precision extensions.

A parser should therefore not assume that a position always contains readable coordinates in the form:

DDMM.mmN
DDDMM.mmE

Detailed position formats are described in a separate section.

Not every Information field begins with a simple DTI

Most common formats can be recognised easily from the first character.

However, there are historical formats, special formats, and constructions requiring further analysis.

A complete APRS implementation therefore requires support not only for the most common:

!
=
:
;
>

but for the full set of formats defined by the specification and later extensions.

What does the user see and what does the modem see?

The user can see:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>

The APRS application sees logical fields:

source
destination
path
information

The APRS parser sees inside Information:

DTI
payload

The AX.25 layer operates on the actual frame fields.

The modem operates on the bitstream and signal.

The radio ultimately carries the RF signal.

Each layer therefore looks at the same packet from a different perspective.

Common interpretation errors

Everything before : is APRS

No.

A large part of that information describes the AX.25 header and path.

WIDE1-1 is part of the APRS payload

No.

It is an AX.25 path element.

Destination is the message recipient

Not always.

The APRS message addressee is located inside Information.

Every : character has the same meaning

No.

Its meaning depends on where it appears.

The text form of the packet is the exact RF contents

No.

It is a monitor representation of the real AX.25 frame.

Everything between destination and : comes from RF

Not always.

APRS-IS can contain elements added by the infrastructure, for example q-constructs.

Every position has the form DDMM.mmN/DDDMM.mmE

No.

APRS has several position formats.

How to read an APRS packet

For the packet:

SQ9MDD-7>APRS,WIDE1-1:!5012.34N/01956.78E>QRV 145.550

it is useful to read it in stages.

1. Who transmitted it?

SQ9MDD-7

2. What is the destination?

APRS

3. What is the path?

WIDE1-1

4. Where does Information begin?

After:

:

5. What is the DTI?

!

meaning a position without a timestamp.

6. What are the actual data?

5012.34N/01956.78E>

position and symbol.

7. Is there additional information?

QRV 145.550

comment.

This approach allows even much more complex packets to be analysed step by step.

The entire packet as layers

The same example can be shown as follows:

SQ9MDD-7 > APRS , WIDE1-1 : ! 5012.34N / 01956.78E > QRV 145.550
|          |      |          | |          |           | |
|          |      |          | |          |           | +-- comment
|          |      |          | |          |           +---- symbol
|          |      |          | |          +---------------- longitude
|          |      |          | +--------------------------- latitude
|          |      |          +----------------------------- DTI
|          |      +---------------------------------------- path
|          +----------------------------------------------- destination
+---------------------------------------------------------- source

At the layer level:

AX.25
+---------------------------------------------------+
| SOURCE | DEST | PATH | INFORMATION               |
+---------------------------------------------------+
                       |
                       v
APRS
+---------------------------------------------------+
| DTI | packet-type data | extensions / comment    |
+---------------------------------------------------+

This is the most important model to remember.

Key points to remember

A text APRS packet combines AX.25 and APRS information.

The form:

SOURCE>DEST,PATH:DATA

is not one flat format.

SOURCE, DEST, and PATH belong to the AX.25 addressing layer.

They are not the actual APRS payload.

The Information field begins after the : separator in TNC2 representation.

This is where the APRS data is located.

The first character of Information is often the DTI.

It defines how the rest of the packet should be interpreted.

Destination does not have to be the actual recipient.

In APRS, it can identify a device or software.

The APRS message addressee is located in Information.

It is different from the AX.25 destination.

The digipeater path is not part of the APRS data.

It belongs to AX.25.

TNC2 representation is not the raw radio frame.

It hides some AX.25 fields and binary details.

A packet visible in APRS-IS can contain information that was not present on RF.

q-constructs are an example.

The meaning of a character depends on its position in the structure.

The packet should therefore be analysed by layer and according to the format implied by the DTI.

Next

After understanding the anatomy of a single packet, its individual elements can be discussed in more detail.

The next topics include:

  • source callsign,
  • SSID,
  • destination address,
  • TOCALL,
  • AX.25 paths,
  • the WIDE mechanism,
  • Data Type Identifier,
  • Information field,
  • timestamp,
  • symbols,
  • data extensions,
  • position formats,
  • Mic-E,
  • messages,
  • objects,
  • telemetry,
  • weather,
  • third-party traffic,
  • q-constructs.

From this point onward, individual APRS types can be analysed without mixing information that belongs to AX.25, APRS, and APRS-IS.