Aprs working on 144.888, tested on aprs.fi with pymutlimonaprs

firmware/inc/aprs.h

@@ -25,7 +25,25 @@
 #ifndef APRS_H
 #define APRS_H
 
+/**
+ * Reference APRS Protocol Spec  http://www.aprs.org/doc/APRS101.PDF
+ */
+
+/**
+ * This should be a full licensed callsign you own. Not mine plz k thx bai
+ *
+ * Max. 6 characters
+ */
+#define APRS_CALLSIGN	"M0SBU"
+#define APRS_SSID	11
+
+/**
+ * APRS Map Symbol.  See Appendix 2: APRS Symbol Tables
+ */
+#define APRS_SYMBOL	"/O"    /* Balloon */
+
 void aprs_init(void);
+void aprs_start(float lat, float lon, float altitude);
 uint8_t aprs_tick(void);
 
 #endif /* APRS_H */

firmware/src/aprs.c

@@ -24,8 +24,10 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <math.h>
 
 #include "samd20.h"
+#include "aprs.h"
 #include "ax25.h"
 
 /**
@@ -36,16 +38,64 @@
  * http://k9dci.home.comcast.net/~k9dci/APRS%20Beginner%20Guide%20-%20K9DCI%20Ver%205-1.pdf
  */
 
-char addresses[50];
-
-void aprs_start(void)
+/**
+ * Encodes a base-91 representation of `value` in `str`
+ *
+ * `str` should have length `n` + 1 and should be big enough to hold `value`
+ */
+void base91_encode(char *str, uint8_t n, uint32_t value)
 {
-  sprintf(addresses, "%-6s%c%-6s%c%-6s%c",
-          "APRS", 0,
-          "M0SBU", 0,
-          "WIDE1", 1);
+  /* Start at the end of the string, adding the null terminator */
+  for(str += n, *str = '\0'; n; n--) {
 
-  ax25_start(addresses, 21, "testtest", 8);
+    *(--str) = (value % 91) + 33;
+    value /= 91;
+  }
+}
+
+
+/**
+ * Start the transmission of an aprs frame
+ */
+void aprs_start(float lat, float lon, float altitude)
+{
+  char addresses[50];
+  char information[50];
+  char compressed_lat[5];
+  char compressed_lon[5];
+
+  /* Encode the destination / source / path addresses */
+  uint32_t addresses_len = sprintf(addresses, "%-6s%c%-6s%c%-6s%c",
+                                   "APRS", 0,
+                                   APRS_CALLSIGN, APRS_SSID,
+                                   "WIDE2", 1);
+
+  /* Prepare the aprs position report */
+  uint32_t compressed_lat_value = (uint32_t)round(380926 * ( 90 - lat));
+  uint32_t compressed_lon_value = (uint32_t)round(190463 * (180 + lon));
+  base91_encode(compressed_lat, 4, compressed_lat_value);
+  base91_encode(compressed_lon, 4, compressed_lon_value);
+  uint32_t altitude_feet = altitude * 3.2808; /* Oh yeah feet! Everyone loves feet */
+
+  /* Encode the information field */
+  /* Compressed Lat/Long position report, no timestamp */
+  uint32_t information_len = sprintf(information,
+                                     "!%c%s%s%c%s%c/A=%06ld RTTY/434.6U8N2",
+                                     APRS_SYMBOL[0], /* Symbol Table ID */
+                                     compressed_lat,
+                                     compressed_lon,
+                                     APRS_SYMBOL[1], /* Symbol Code */
+                                     "  ",           /* Compressed Altitude */
+                                     ' ',            /* Compression Type */
+                                     altitude_feet   /* Altitude */
+    );
+
+  /* Let's actually try that out.. We can add comment etc. later */
+
+
+  /* Transmit the frame */
+  ax25_start(addresses,   addresses_len,
+             information, information_len);
 }
 
 uint8_t aprs_tick(void)

firmware/src/si_trx.c

@@ -30,7 +30,9 @@
 #include "si_trx_defs.h"
 #include "hw_config.h"
 
-#define RADIO_FREQUENCY	434600000
+//#define RADIO_FREQUENCY	434600000
+#define RADIO_FREQUENCY	144888000
+
 #define RADIO_POWER	0x3f
 #define VCXO_FREQUENCY	SI406X_TCXO_FREQUENCY
 #define RF_DEVIATION	200

firmware/src/telemetry.c

@@ -308,7 +308,7 @@ void telemetry_tick(void) {
 
       if (!radio_on) {
         /* APRS: We use pwm to control gpio1 */
-        aprs_start();
+        aprs_start(51.47, -2.58, 10);
 
         si_trx_on(SI_MODEM_MOD_TYPE_2GFSK, 400);
         radio_on = 1;