A Text Overview
The actual APRS protocol is text based, though using a compact representation. For example over the APRS connection you could receive messages like the following:
Cambridge>APRS,TCPIP*,qAC,GLIDERN2:/074555h5212.73NI00007.80E&/A=000066 CPU:4.0 RAM:242.7/458.8MB NTP:0.8ms/-28.6ppm +56.2C RF:+38+2.4ppm/+1.7dB FLRDDE626>APRS,qAS,EGHL:/074548h5111.32N/00102.04W'086/007/A=000607 id0ADDE626 -019fpm +0.0rot 5.5dB 3e -4.3kHz FLRDDE626>APRS,qAS,EGHL:/074557h5111.32N/00102.01W'086/006/A=000607 id0ADDE626 +020fpm +0.3rot 5.8dB 4e -4.3kHz FLRDDE626>APRS,qAS,EGHL:/074559h5111.32N/00102.00W'090/006/A=000607 id0ADDE626 +020fpm -0.7rot 8.8dB 0e -4.3kHz FLRDDE626>APRS,qAS,EGHL:/074605h5111.32N/00101.98W'090/006/A=000607 id0ADDE626 +020fpm +0.0rot 5.5dB 1e -4.2kHz # aprsc 2.0.14-g28c5a6a 29 Jun 2014 07:46:15 GMT GLIDERN1 188.8.131.52:14580
The first line is a beacon from a receiver (receiver beacon), it contains information as to where the receiver is located and in the free form fields some information about the receiver itself:
Cambridge>APRS,TCPIP*,qAC,GLIDERN2:/074555h5212.73NI00007.80E&/A=000066 v1.0.4 CPU:4.0 RAM:242.7/458.8MB NTP:0.8ms/-28.6ppm +56.2C RF:+38+2.4ppm/+1.7dB
The next lines are position reports from an aircraft (aircraft beacons)
FLRDDE626>APRS,qAS,EGHL:/074548h5111.32N/00102.04W'086/007/A=000607 id0ADDE626 -019fpm +0.0rot 5.5dB 3e -4.3kHz
And the last line is a keep alive from the APRS server. The leading hash symbol indicates a comment
# aprsc 2.0.14-g28c5a6a 29 Jun 2014 07:46:15 GMT GLIDERN1 184.108.40.206:14580
After the standard APRS information is some information that is OGN specific:
id0ADDE626 -019fpm +0.0rot 5.5dB 3e -4.3kHz
The first field combines the FLARM/ICAO/OGN address, aircraft type and privacy flags.
It is meant to be unique in the network. It is, except for rare cases. Its form is
where X and Y stand for hexadecimal digits. YYYYYY is the address.
XX encodes stealth mode S, no-tracking flag T, aircraft type tttt and address type aa as follows:
S, T, tttt, aa stand for 8 bits from most to least significant. Note that no messages with
the no-tracking set flag should ever appear on the public APRS network.
SNR can change as a function of many factors: distance, aircraft orientation, obstacles, …
As from v0.2.5 there are additional fields available in the aircraft beacon:
ICA4B4E68>APRS,qAS,Letzi:/152339h4726.50N/00814.20E'260/059/A=002253 !W65! id054B4E68 -395fpm -1.5rot 16.5dB 0e -14.3kHz gps1x2 s6.05 h4C rDF0CD1 +4.5dBm
description will follow shortly..
Each receiver transmits a beacon every 5 minutes. This beacon contains location information for the receiver and some metrics on how it is performing.
CPU:0.8 - **load average** RAM:211.1/458.7MB - **free and total ram** NTP:0.2ms/-29.3ppm - **offset from NTP reference and rate at which it is being adjusted** +45.5C - **only on some boxes (notably PI) temperature** RF:+62-0.8ppm/+4.1dB - **RTLSDR information, +62 => manually configured adjustment, -0.8ppm adjustment based on GSM / +4.1db is how much louder is noise vs internal reference. **
- Original APRS specification
- APRS Spec Addendum 1.2 Proposals (includes high precision !DAO! datum extension)
- OGN flavoured APRS-protocol documentation by svoop