The Structure of this Specification¶
This specification describes the overall requirements for developing software that complies with APRS Protocol Version 1.0. The information flow starts with the standard AX.25 UI-frame, and progresses downwards into more and more detail as the use of each field in the frame is explored.
A key feature of the specification is the inclusion of dozens of detailed examples of typical APRS packets and related math computations.
Here is an outline of the chapters:
Introduction to APRS — A brief background to APRS and a summary of its main features.
The APRS Design Philosophy — The fundamentals of APRS, highlighting its use as a real-time tactical communications tool, the timing of APRS transmissions and the use of generic digipeating.
APRS and AX.25 — A brief refresher on the structure of the AX.25 UI-frame, with particular reference to the special ways in which APRS uses the Destination and Source Address fields and the Information field.
APRS Data in the AX.25 Destination and Source Address Fields — Details of generic APRS callsigns and callsigns that specify display symbols and APRS software version numbers. Also a summary of how Mic-E encoded data is stored in the Destination Address field, and how the Source Address SSID can specify a display icon.
APRS Data in the AX.25 Information Field — Details of the principal constituents of APRS data that are stored in the Information field. Contains the APRS Data Type Identifiers table, and a summary of all the different types of data that the Information field can hold.
Time and Position Formats — Information on formats for timestamps, latitude, longitude, position ambiguity, Maidenhead locators, NMEA data and altitude.
APRS Data Extensions — Details of optional data extensions for station course/speed, wind speed/direction, power/height/gain, pre-calculated radio range, DF signal strength and Area Object descriptor.
Position and DF Report Data Formats — Full details of these report formats.
Compressed Position Report Data Formats — Full details of how station position and APRS data extensions are compressed into very short packets.
Mic-E Data Format — Mic-E encoding of station lat/long position, altitude, course, speed, Mic-E position comment, telemetry data and APRS digipeater path into the AX.25 Destination Address and Information fields.
Object and Item Reports — Full information on how to set up APRS Objects and Items, and details of the encoding of Area Objects (circles, lines, ellipses etc).
Weather Reports — Full format details for weather reports from standalone (positionless) weather stations and for reports containing position information. Also details of storm data format.
Telemetry Data — A description of the MIM/KPC-3+ telemetry data format, with supporting information on how to tailor the interpretation of the raw data to individual circumstances.
Telemetry Metadata — A description of how to interpret the raw numeric telemetry data. This includes properties such as name, units, and scaling.
Messages, Bulletins and Announcements — Full format information.
Station Capabilities, Queries and Responses — Details of the ten different types of query and expected responses.
Status Reports — The format of general status messages, plus the special cases of using a status report to contain meteor scatter beam heading/power and Maidenhead locator.
Network Tunneling — The use of the Source Path Header to allow tunneling of APRS packets through third-party networks that do not understand AX.25 addresses, and the use of the third-party Data Type Identifier.
User-Defined Data Format — APRS allows users to define their own data formats for special purposes. This chapter describes how to do this.
Other Packets — A general statement on how APRS is to handle any other packet types that are not covered by this specification.
APRS Symbols — How to specify APRS symbols and symbol overlays, in position reports and in generic GPS destination callsigns.
APRS Data Formats — An appendix containing all the APRS data formats collected together for easy reference.
The APRS Symbol Tables — A complete listing of all the symbols in the Primary and Alternate Symbol Tables.
ASCII Code Table — The full ASCII code, including decimal and hex codes for each character (the decimal code is needed for compressed lat/long and altitude computations), together with the hex codes for bit-shifted ASCII characters in AX.25 addresses (useful for Mic-E decoding and general on-air packet monitoring).
Glossary — A handy one-stop reference for the many APRS-specific terms used in this specification.
References — Pointers to other documents that are relevant to this specification.