OGN APRS Protocol

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 37.187.40.234:14580

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 37.187.40.234:14580

OGN-specific beacons

Aircraft Beacons

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

idXXYYYYYY

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:
STttttaa

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.
-019fpm
is the climb rate in feet-per-minute.
+0.0rot
is the turn-rate as reported by FLARM (or OGN tracker). 1rot is the standard aircraft rotation rate of 1 half-turn per minute.
5.5dB
is the Signal-to-Noise ratio measured upon reception. 4-5dB is about the limit of meaningful reception.
SNR can change as a function of many factors: distance, aircraft orientation, obstacles, …
3e
is the number of bit errors corrected in the packet upon reception. It is recommended that you ignore packets that have a high CRC error rate (>5) as their information may be corrupt. However, starting version 0.1.2 positions are checked for time-position consistency thus false positions should be rare.
-4.3kHz
is the frequency offset measured upon reception. Starting version 0.1.2 this offset is correctly calibrated against the GSM signals thus should be same or close when receiving same aircraft with different receivers. This offset can slowly change as a function of temperature in the glider and/or emitter's unit supply voltage.
!W52!
is a APRS position precision enhancement. "5" is the third decimal digit of latitude minutes, "2" is the added digit of longitude minutes.

As from v0.2.5 there are additional fields available in the aircraft beacon:

example:

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..

Receiver beacons

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.   **

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