What is APRS ?
Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time tactical digital Communications of information of immediate value in the local area. In addition, all such data are ingested into the APRS Internet System (APRS-IS) and distributed globally for ubiquitous and immediate access. Along with messages, alerts, announcements, and bulletins, the most visible aspect of APRS is its map display. Anyone may place any object or information on his or her map, and it is distributed to all maps of all users in the local RF network or monitoring the area via the Internet. Any station, radio, or object that has an attached GPS is automatically tracked. Other prominent map features are weather stations(WX), alerts and objects and other map-related amateur radio volunteer activities including Search and Rescue and signal direction finding.
APRS has been developed since the late 1980s by Bob Bruninga, callsign WB4APR, currently a senior research engineer at the United States Naval Academy. He still maintains the main APRS website. The acronym "APRS" was derived from his callsign.
In Austrila all APRS services apart form space systems use 145.175Mhz to oprate on Below are Pictures of how the APRS System works
to find out more see http://www.aprs.net.au/
Click hear to download a .kml file for Google Earth that shows real time APRS data on Google Earth
"Digipeater" is short for "Digital Repeater"; a repeater for packet data rather than voice. Unlike the standard voice repeater that receives on one frequency and retransmits what it hears simultaneously on another frequency, the usual digipeater is a single frequency device. It receives a packet of data, stores it in internal memory and then a moment later retransmits it on the SAME frequency.
Digpeating is much more critical to APRS than to conventional packet because APRS heavily involves packet data transmission to and from moving vehicles. Traditional packet was overwhelmingly used between fixed locations, typically with better and higher antennas.
Signal levels that may be considered adequate on voice probably WON'T be adequate on packet, because data transmission is an all-or-nothing proposition. ALL of a packet has to be received PERFECTLY to recover ANY data from it. The kind of noisy, scratchy, not-completely-noise-free, operation so many people inflict on voice repeaters, especially with underpowered handhelds, JUST WONT WORK on data transmissions. A pop, a momentary burst of white noise, flutter, or multipath-induced phase distortion that you don't even notice on voice WILL be fatal to a packet transmission.With APRS, the problem is more prevalent than with conventional (connected) packet because it operates in a non-connected mode. With traditional packet, a station receiving a defective packet will automatically send a request for retransmission to the sending station, or the sending station will automatically retry if the receiving station doesn't acknowledge in a reasonable time. With APRS there is no ACK/NAK (Negative Acknowledgement) handshaking process. The sending station broadcasts packets at intervals and "hopes" the receiving station(s) get them error-free. The receiving station ignores the packet if it is defective in anyway.
Signals to/from mobile units can and do fluctuate in strength by 15-20 dB as the mobile moves over even a short distance. For reliable data transmission, you must have good signal strength over the intended path. The signal should be enough that even with a 20dB drop, the signal will remain noiseless and hard quieted.
Remember that APRS will performance will vary with local conditions an weather. For instance in Melbourne you can use a 10 watt transmitter and 1/2 wave ground plain antenna and have an 80% success rate with your beacons. In Sydney the situation is much different due to the hilly terrain. You would need more power and a different antenna, probability a 1/4 wave to increase your success rate in the valleys.
An IGate's (Internet Gateway) job to pass all packets heard on Radio Freqecney (RF) to Internet. The first part of this is sometimes misunderstood. It should be to pass "all valid packets" from the Radio wave into the internet. With this passing of all message packets also includes passing the sending station's position along with the message. When APRS was small, we did this using historical position packets. This has become problematic as it introduces historical data on to RF. The IGate should note the station(s) it has gated messages to RF for and pass the next position packet seen for that station(s) to RF.
An IGate should is set for the minimum number of hops necessary to cover the intended area (both in the transmit path and what is considered a local station). It should only gate to RF for local stations heard within 1 hour (or less) and within the intended coverage area (should be based on digipeater hops, not distance,
WX or Weather stations on APRS send weather reports from their location about every ¼ of a hour as a text string that can be read off by others in some parts of the world emergency services use this as a real time weather map and some national weather services also do the same.