fixed TTN decoder for gps altitude

src/TTN/packed_decoder.js

@@ -151,7 +151,13 @@ var altitude = function (bytes) {
     if (bytes.length !== altitude.BYTES) {
         throw new Error('Altitude must have exactly 2 bytes');
     }
-    return +(bytesToInt(bytes) / 4 - 1000).toFixed(1);
+
+    var alt = bytesToInt(bytes);
+    if (alt > 32767) {
+        alt -= 65536;
+    }
+
+    return +(alt / 4 - 1000).toFixed(1);
 };
 altitude.BYTES = 2;
 

src/TTN/plain_decoder.js

@@ -41,7 +41,11 @@ function Decoder(bytes, port) {
     decoded.longitude = ((bytes[i++] << 24) | (bytes[i++] << 16) | (bytes[i++] << 8) | bytes[i++]);
     decoded.sats = bytes[i++];
     decoded.hdop = (bytes[i++] << 8) | (bytes[i++]);
-    decoded.altitude = ((bytes[i++] << 8) | (bytes[i++])) / 4 - 1000;
+    var alt = ((bytes[i++] << 8) | (bytes[i++]));
+    if (alt > 32767) {
+      alt -= 65536;
+    }
+    decoded.altitude = alt / 4 - 1000;
   }
 
   if (port === 5) {