Glossary

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APRS Glossary

A

  • 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).

B

  • 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.

D

  • 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.

E

  • ECHO - A generic APRS digipeater callsign alias, for an HF digipeater.

F

  • findu - a web site at http://www.findu.com 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
  • FCS - Frame Check Sequence. A sequence of 16 bits that follows the AX.25 Information field, used to verify the integrity of the packet.

G

  • GATE - A gateway between HF and VHF APRS networks. Used primarily to relay longdistance HF APRS traffic onto local VHF networks. Short for IGate.
  • GGA Sentence - A standard NMEA sentence, containing the receiving station’s lat/long position and antenna height relative to mean sea level, and other data.
  • GLL Sentence - A standard NMEA sentence, containing the receiving station’s lat/long position and other data.
  • GPS - Global Positioning System. A global network of 24 satellites that provide lat/long and antenna height of a receiving station. 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
  • GPSxyz - An APRS destination callsign that specifies a display symbol from either the Primary Symbol Table or the Alternate Symbol Table. Some symbols from the Alternate Symbol Table can be overlaid with a digit or a letter. Used by trackers that cannot specify the symbol in the AX.25 Information field.

H

  • 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

I

  • IGate - Internet Gateway, a radio and computer setup for receiving packet data via radio and sending those packets to the Internet (via APRS-IS)
  • Information field - The AX.25 Information field containing APRS information.

K

  • Knots - International nautical miles per hour.

L

  • LORAN - Long Range Navigation System (a terrestrial precursor to GPS).

M

  • Maidenhead Locator - A 4- or 6-character grid locator specifying a station’s position.
  • Message - A one-line text message addressed to a particular station.
  • Message Group - A user-defined group to receive messages.
  • Mic-E - Originally Microphone Encoder, a unit that encodes location, course and speed information into a very short packet, for transmission when releasing the microphone PTT button. The Mic-E encoding algorithm is now used in other devices (e.g. in the PIC-E and the Kenwood TH-D7/TM-D700 radios).

N

  • Net Cycle Time - The time within which it should be possible to gain the complete picture of APRS activity (typically 10, 20 or 30 minutes, depending on the number of digipeaters traversed and local conditions). Stations should not transmit status or position information more frequently unless mobile, or in response to a Query.
  • NMEA (Received) Sentence - The ASCII data stream received from navigation equipment (such as GPS receivers) conforming to the NMEA 0182 Version 2.0 specification. APRS supports five NMEA Sentences: GGA, GLL, RMC, VTG and WPT.
  • NRQ - Number/Rate/Quality. A measure of confidence in DF Bearing reports.
  • NWS - National Weather Service (United States).

O

  • Object - A display object that is (usually) not a station. For example, a weather front or a marathon runner.
  • Object Report - A report containing the position of an object, with optional timestamp and APRS Data Extension.

P

  • 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.

Q

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

R

  • 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.

S

  • 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.

T

  • 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).

U

  • 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.

V

  • 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.

W

  • 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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