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--- guides:aprs:telemetry [2014/12/11 20:19] – lz1dev
+++ guides:aprs:telemetry [2015/01/09 19:02] (current) – added C code for base91 + clean ups lz1dev
@@ Line 1: / Line 1: @@
 ====== APRS Telemetry ======
-WIP
+There are a few ways of sending telemetry via APRS.
-===== References =====
+===== Telemetry report format =====
-  - APRS Protocol v1.0 — {{:guides:aprs:aprs101.pdf|}}, http://www.aprs.org/doc/APRS101.PDF
-  - Base91 telemetry comment extension — http://he.fi/doc/aprs-base91-comment-telemetry.txt
+The packet format is described on ''page 78'' in the ''APRS Protocol reference''
+((APRS Protocol reference v1.0.1 — {{:guides:aprs:aprs101.pdf|}}, http://www.aprs.org/doc/APRS101.PDF))
+.
+It allows for 5 analogue channels and 8 digital bits.
+Here are examples of typical telemetry messages:
+<code>
+                  |seq|ch1|ch2|ch3|ch4|ch5|-8bits--|
+LZ1DEV-11>APRS,…:T#005,199,100,255,073,123,01101001
+LZ1DEV-11>APRS,…:T#MIC199,100,255,073,123,01101001
+</code>
+The downside of this method is the lack of position or timestamp.
+Plotting the data overtime could be impossible as time of arrival can be severely
+affected by delays in the APRS network.
+Furthermore, the resolution of each channel is limited to 8bits (e.g. 0-255) and
+there are only 5 analogue channels.
+==== Set up ====
+You may have noticed that the above is limited in displaying negative numbers,
+decimals and unit of measurement.
+However, there are messages which can be send, that would indicate the unit of measurement,
+constants to calculate a meaningful value and friendly name for the specific channel.
+The details are described in the ''APRS Protocol reference <sup>[1]</sup>''.
+<code>
+              |ADDRESSEE|TYPE|  ch1  | ch2 | ch3 |ch4 |ch5| 1b  | 2b | 3b|4b |5b |…
+LZ1DEV>APRS,…::LZ1DEV-11:PARM.Battery,Btemp,ATemp,Pres,Alt,Camra,Chut,Sun,10m,ATV
+              |ADDRESSEE|TYPE|  ch1  |  ch2    |  ch3    |   ch4  | ch5 |…
+LZ1DEV>APRS,…::LZ1DEV-11:EQNS.0,5.2,0,0,.53,-32,3,4.39,49,-32,3,18,1,2,3
+              |ADDRESSEE|TYPE| ch1 | ch2 | ch3 | ch4|ch5|  b1 | b2 |b3|b4|b5|
+LZ1DEV>APRS,…::LZ1DEV-11:UNIT.v/100,deg.F,deg.F,Mbar,Kft,Click,OPEN,on,on,hi
+</code>
+There are 3 types of messages that can be send that would allow to "configure" telemetry.
+**PARM** and **UNIT** are straightforward and configure the channel name and unit.
+**EQNS** is to configure the constants in the following quadratic equation ''a*x<sup>2</sup> + b*x + c''
+, which is used to calculate the final value.
+Example of calculating the final value for channel #2:
+<code>
+x = 100      a = 0, b = 0.53, c = -32
+*100^2 + 100*0.53 + (-32)
++ 53 + (-32)
+----
+Btemp: 21 def.F
+</code>
+==== Support ====
+Both [[http://aprs.fi]] and [[http://tracker.habhub.org/]] recognize the format and will graph the telemetry accordingly.
+===== Base91 comment telemetry =====
+''Base91 comment telemetry''
+((Base91 comment telemetry — {{:guides:aprs:aprs-base91-comment-telemetry.pdf|}}, http://he.fi/doc/aprs-base91-comment-telemetry.txt))
+is an alternative to the telemetry report format.
+The key advantages are:
+  * Short (4 to 16 characters)
+  * Can be added to the comment field of any format (position reports for example)
+  * Each channel has resolution of 0-8280
+  * Set up the same way
+==== Example C program ====
+<code c>
+#include <stdio.h>
+void base91_encode(char *buf, unsigned short value) {
+    // valid values are 0 - 8280
+    value = value % 8281;
+    // encode the value
+    buf[0] = 33 + (value / 91);
+    buf[1] = 33 + (value % 91);
+}
+int main(void) {
+    printf("APRS base91 telemetry extension example\n");
+    printf("---------------------------------------\n");
+    // One channel of telemetry
+    char telem1[] = "|ss11|";
+    base91_encode(&telem1[1], 1); // sequence number
+    base91_encode(&telem1[3], 0); // ch1
+    printf("Telemetry #1: %s\n", telem1);
+    // Five channels of telemetry
+    char telem2[] = "|ss1122334455|";
+    base91_encode(&telem2[1], 2); // sequence number
+    base91_encode(&telem2[3], 1111); // ch1
+    base91_encode(&telem2[5], 2222); // ch2
+    base91_encode(&telem2[7], 3333); // ch3
+    base91_encode(&telem2[9], 4444); // ch4
+    base91_encode(&telem2[11], 5555); // ch5
+    printf("Telemetry #2: %s\n", telem2);
+}
+</code>
+===== Human readable format in the comment field =====
+This method is straightforward, simply format and append telemetry to the comment field. Example:
+<code>
+LZ1DEV-11>APRS,…:=0001.00N/00001.00Wv266/024/A=000299 13.55Cint -30.51Cext 3.45V 7Sats
+                                                    |----------- comment -------------|
+</code>
+Things too keep in mind:
+  * the packet will be longer than necessary
+  * the amount of characters in the comment section is limited, depending on the packet type (e.g. 0-28, 0-43)
+  * existing system won't recognize the telemetry and provide friendly plotting over time
+====== References ======