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APRS Glossary


  • Announcement - An APRS message that is repeated a few times an hour, perhaps for several days.
  • APRS - Automated Packet Reporting System
  • APRS Data - The data that follows the APRS Data Type Identifier in the AX.25 Information field and precedes the APRS Data Extension.
  • APRS Data Extension - A 7-byte extension to APRS Data. The Data Extension includes one of Course/Speed, Wind Direction/Wind Speed, Station Power/Antenna Effective Height/Gain/Directivity, Pre-Calculated Radio Range, DF Signal Strength/Effective Antenna Height/Gain, Area Object Descriptor.
  • APRS Digipeater Path - A digipeater path via repeaters with RELAY, WIDE and related aliases. Used in Mic-E compressed location format.
  • APRS Data Type Identifier - The single-byte identifier that specifies what kind of APRS information is contained in the AX.25 Information field.
  • APRS-IS - APRS-Internet System
  • Area Object - A user-defined graphic object (circle, ellipse, triangle, box and line).


  • Base 91 - Number base used to ensure that numeric values are transmitted as printable ASCII characters. To obtain the character string corresponding to a numeric value, divide the value progressively by decreasing powers of 91, and add 33 decimal to the result at each step. Printable characters are in the range !..{. Used in compressed lat/long and altitude computation.
  • Bulletin - An APRS message that is repeated several times an hour, for a small number of hours. A General Bulletin is addressed to no-one in particular. A Group Bulletin is addressed to a named group (e.g. WX).
  • Bulletin Identifier - A single digit 0-9 that identifies a particular bulletin.


  • Destination Address field - The AX.25 Destination Address field, which can contain an APRS destination callsign or Mic-E encoded data.
  • DF Report - A report containing DF bearing and range.
  • DGPS - Differential GPS. Used to overcome the errors arising from Selective Availability.
  • digi - short for "digipeater"
  • digipeater - A station that relays AX.25 packets. A chain of up to 8 digipeaters may be specified.
  • Digipeater Addresses field - The AX.25 field containing 0–8 digipeater callsigns (or aliases).
  • DX Cluster - A network host that collects and disseminates user reports of DX activity.


  • findu - a web site at which collects APRS data from APRS-IS and can answer queries on the database. Also, generically, the Internet aspect of APRS, though there are many other useful APRS sites


  • gate - short for IGate
  • GPS - Global Positioning System. A //GPS receiver// uses signals from satellites to determine its position. For APRS use, the GPS receiver will then send the location information to a TNC for transmission by radio


  • ham - a person who uses amateur radio. The origin of the nickname is obscure
  • hop - when a digipeater repeats a packet, it's considered a "hop." If a second digi receives the packet from the first digi and retransmits it, that counts as a second hop


  • IGate - Internet Gateway, a radio and computer setup for receiving packet data via radio and sending those packets to the Internet (via APRS-IS)


  • packet - a bundle of data (like an APRS user's call sign and location, or a weather station's report on conditions) plus a path and other information, represented as a set of bytes in a computer or transmitted via radio as a series of rapidly-changing tones
  • path - a specification of the path a packet should take through the digipeater system. Can be explicit (using the callsigns of the intended repeaters and the ultimate destination) or symbolic, like "WIDE2-2" which digipeaters will interpret to mean "repeat twice."
  • PHG - APRS Data Extension specifying Power, Effective Antenna Height/Gain/Directivity.
  • Position Ambiguity - A reduction in the accuracy of APRS position information (implemented by replacing low-order lat/long digits with spaces). Used when the exact position is not known.
  • Position Report - A report containing lat/long position, optionally with timestamp and Data Extension.


  • Query - A request for information. Queries may be addressed to stations in general or to specific stations.


  • Range Circle - Usable radio range (in miles), computed from PHG data.
  • RELAY - A generic APRS digipeater callsign alias, for a VHF/UHF digipeater with limited local coverage.
  • RMC Sentence - A standard NMEA sentence, containing the receiving station’s lat/long position, course and speed, and other data.


  • Sentence - See NMEA (Received) Sentence.
  • Source Address Field - The AX.25 Source Address field, containing the callsign of the originating station. A non-zero SSID specifies a display symbol.
  • SSID - Secondary Station Identifier. A number in the range 0-15, as an adjunct to an AX.25 address. If the SSID in a source address is non-zero, it specifies a display symbol. (This is used when the station is unable to specify the symbol in the AX.25 Destination Address field or Information field).
  • Status Report - A report containing station status information (and optionally a Maidenhead locator).
  • Symbol - A display icon. Consists of a Symbol Table Identifier/Symbol Code pair. Generically, /$ represents a symbol from the Primary Symbol Table, and \$ represents a symbol from the Alternate Symbol Table.
  • Symbol Table Identifier - An ASCII code specifying the Primary Symbol Table (/) or Alternate Symbol Table (\). The Symbol Table Identifier is also implicit in GPSCnn and GPSEnn destination callsigns.


  • TH-D7 - A combined VHF/UHF handheld radio and APRS-compatible TNC from Kenwood.
  • TM-D700 - A combined VHF/UHF mobile radio and APRS-compatible TNC from Kenwood.
  • TM-D710 - A combined VHF/UHF mobile radio and APRS-compatible TNC from Kenwood.
  • Third-Party Header - A Path Header with the Third-Party Network Identifier and the callsign of the receiving gateway inserted.
  • TNC - Terminal Node Controller. A device which can control a radio and transmit computer data (like a user's call sign and GPS position information) using a series of rapidly-changing tones
  • Trace - An APRS query that asks for the route taken by a packet to a specified station.
  • TRACE - A generic digipeater callsign alias, for digipeaters that performs callsign substitution. These digipeaters self-identify packets they digipeat, by inserting their own callsign in place of RELAY,WIDE or TRACE.
  • Tracker - A unit comprising a GPS receiver (to obtain the current geographical position) and a radio transmitter (to transmit the position to other stations).


  • UI-Frame - AX.25 Unnumbered Information frame. APRS uses only UI-frames — that is, it operates entirely in connectionless (UNPROTO) mode.
  • UNPROTO Path - The digipeater path to the destination callsign.


  • Voice Alert - is a way of setting up an APRS station to notify you if other active stations come into simplex voice range
  • VTG Received Sentence - A standard NMEA sentence, containing the receiving station’s course and speed.


  • WIDE - A generic APRS digipeater callsign alias, for a digipeater with wide area coverage.
  • WIDEn-N - A generic APRS digipeater callsign alias, for a digipeater with wide area coverage (N=0-7). As a packet passes through a digipeater, the value of N is decremented by 1 until it reaches zero. The digipeater keeps a record of each packet (or its FCS) as it passes through, and will not digipeat the packet again if it has digipeated it already within the last 28 seconds.
  • WPT Sentence - A standard NMEA sentence, containing waypoints.
  • WX - Weather.
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