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--- guides:aprs:telemetry [2015/01/09 18:36] – [Support] changed tracker url to the new one lz1dev
+++ guides:aprs:telemetry [2015/01/09 19:02] (current) – added C code for base91 + clean ups lz1dev
@@ Line 1: / Line 1: @@
-''WORK IN PROGRESS''
------
 ====== APRS Telemetry ======
@@ Line 8: / Line 5: @@
 ===== Telemetry report format =====
-The packet format is described on ''page 78'' in the ''APRS Protocol reference <sup>[1]</sup>''.
+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:
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 there are only 5 analogue channels.
-==== Setup ====
+==== Set up ====
 You may have noticed that the above is limited in displaying negative numbers,
@@ Line 62: / Line 61: @@
 Both [[http://aprs.fi]] and [[http://tracker.habhub.org/]] recognize the format and will graph the telemetry accordingly.
-===== Base91 encoded as comment extension =====
+===== Base91 comment telemetry =====
-FIXME
+''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 =====
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   * 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 ======
-  - APRS Protocol reference v1.0.1 — {{:guides:aprs:aprs101.pdf|}}, http://www.aprs.org/doc/APRS101.PDF
-  - Base91 telemetry comment extension — {{:guides:aprs:aprs-base91-comment-telemetry.pdf|}}, http://he.fi/doc/aprs-base91-comment-telemetry.txt