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The APRS protocol is used to transmit telemetry data, i.e. 5 analog values and 8 binary values (1 or 0). It's not an easy protocol to grasp for the first time. Let's see if we can get to the bottom of it. |
The analogic datas
The 5 analog values are transmitted in 8-bit words, and are therefore limited to values from 0 to 255.
Analog measurements (voltage, temperature, pressure, etc.) must therefore be mapped within this 255-value limit.
For example, the voltage of a 12V battery could be transmitted as binary value 12. The range would then be 0 to 255 Volts.
Accuracy would therefore be low, since we could only have an accuracy of 1 V.
If you want to increase the accuracy transmitted, you can shift and reduce the measuring range.
For example, for the same battery, only the normal operating range is useful, say from 10 to 14 V.
This would give us 255 values in the 4 V range, i.e. 4 / 255 = approx. 0.015 V per step, which is much better and sufficient to monitor a battery!
The “formula” for calculating the numerical integer value to be transmitted would therefore be (battery voltage -10) / 0.015.
For the 12V voltage above => (12 - 10) / 0.015= 133
The maximum measurable voltage is (255 / 0.015) + 10 = 13.825 V
Minimum measurable voltage is (0 / 0.015) + 10 = 10 V
At the other end of the link, the telemetry decoder will perform the opposite operation, receiving the digital value 133.
So (133 x 0.015) + 10 = 11.995V
The binary datas
In APRS telemetry, 8 binary states can be transmitted, i.e. 0 or 1.
These states can represent, for example, whether a relay is open or closed, whether a door is open or closed, etc...
Now that we know what to transmit, we need to decide how to exploit this data.
There are 2 choices: either we transmit the values as they are, because WE know how to exploit them but nobody else can, or we transmit additional information so that everybody can exploit them.
For this 2nd solution, the APRS protocol provides 4 messages for interpreting analog telemetry and one for binary telemetry.
The T message
This message transmits the actual telemetry data. It uses the T symbol followed by a 3-digit sequence number xxx, the 5-digit aaa series of analog values and the 8-digit bbbbbbb binary data.
The PARM message
This message transmits the name of the data sent. WARNING, names have a different limited length depending on their position!
For example, battery voltage could be named Battery in the 1st field.
An unused field will be set to 0 (zero), or the series can be interrupted at any position.
The UNIT message
This message transmits the units of the data sent.
For example, the battery unit could be expressed in Volts or v/100 as in the example below.
The field lengths are the same as for the PARM message.
The EQNS message
This message transmits the calculation formula for recovering the initial analog value.
For each value, 3 numbers are transmitted, a, b and c.
The header is EQNS. (dot) followed by the 15 values a,b,c in sequence.
The formula for finding the numerical value is ax² + bx + c ( sound familiar? :) )
Generally, a is set to 0, so the final value is bx + c.
x being the numerical data received, we find the values in our battery example. b is the value of each step (0.015) and c the offset (10).
The receiver therefore calculates ax² + bx + c = (0 x 133) + (0.015 x 133) + 10 = 11.995
Sans message EQNS, les coefficients par défaut sont {a,b,c} = {0,1,0}
The BITS message
This “Bit sense” and project name message indicates the sense of the corresponding bit + the project name.
For example, if bit B1 = 0 previously transmitted in message T, and the bit sense for B1 = 0, then B1 is considered TRUE.
The project name can contain up to 24 characters.
Notes
In PARM, UNIT, ENQS and BITS messages, the destination callsign must be 9 characters long. This means left-justify and space-fill up to 9.
This is not indicated in the APRS protocol reference document and cost me a few hours
Résults
Here are the data received by a digipeater and graphs obtained by reading telemetry data from an APRS station on http://aprs.fi
I hope this help ?