APRS on the Internet
What is the APRS-IS?
APRS-IS (Automatic Packet Reporting System-Internet Service) is the common name given to the Internet-based network which inter-connects various APRS radio networks throughout the world (and space).
The APRS-IS is a network of multiple servers around the world cross-connected in two tiers that constantly exchange heard data with each other. In turn, thousands of I-Gates and home stations around the world are logged into these servers. The I-Gates feed packets heard off-the-air in their vicinity into the IS, and under some conditions allow Internet data to go back to RF.
In turn, Web based front ends such as FindU.com and aprs.fi connect to some of these servers, capture and archive everything the APRS-IS hears, and then perform database queries everytime someone hits them with a request for a map.
Any standard Windows or Linux APRS end-user program can connect to any of the Internet servers in addition to (or instead of) your off-air serial-port-interfaced TNC. That is, you can install an APRS program on your computer, log into one of the APRS servers via your broadband connection, and play APRS with no radio at all.
The full, unfiltered Internet feed representing heard stations all over the world is now a constant non-stop stream of data. You can not only track mobiles but also send / receive APRS messages to / from mobiles "out there".
The APRS-IS is made up of three layers:
- The Core
- The Tier2 Network
- Regional Servers & Users
Early on, the APRS-IS was soley centralized by 3 Core servers inter-linked together. Although the Core still exists, global distribution networking operates more efficiently over a mix of end-users, Igates, regional servers, and other servers. Core servers need to be reserved for lower-level servers. Today there is zero justification for end-user (non-server) connections to APRS-IS Core servers.
All I-Gates and home stations that connect to the Internet should be connected to the local Tier 2 servers. The Tier 2 servers connect upwards into the APRS-IS Core.
Although all data transmitted into one APRS-IS server is available on all other servers, it is recommended that all connections be made to a local server, due to bandwidth considerations.
Most APRS software can (and should) be configured to connect to multiple servers in a 'failover' configuration. This means that if one server is unavailable for any reason, the other server is automatically used. Alternatively, a "rotate" hostname exists that will cycle through the available servers using DNS based load balancing.
Verified vs Non Verified connection
Since the APRS network consists of both Internet and RF, we need to have a method to stop unlicensed users being transmitted onto the RF network.
When a connection to an APRS-IS server is defined, one of the fields that needs to be entered is the Validation or Verification number. This number is like a password, and is unique based on your callsign.
The validation number is a simple check that your callsign is valid. Only if the validation number matches the connected callsign will the data be allowed to be transmitted to RF. Using a validation number of -1 forces an UNVERIFIED connection, and whilst you will SEE all the data available in the APRS-IS, any data sent by your client will NEVER be transmitted onto RF.