Finalised implementation of compressed beacons transmitted

Add settings strings for compressed locations in strings resource
Updated javAPRSlib to support setting "csT" field on compressed pakcets
Implement compressed altitudes & course data. If speed/course is enabled then append altitude as per uncompressed packets

res/values/strings.xml

@@ -231,6 +231,10 @@
 <string name="p_locsource_summary">Manual, Periodic or SmartBeaconing™</string>
 <string name="p__location">Location Settings</string>
 <string name="p__location_summary">Configuration for sending position reports</string>
+<string name="p__location_compressed_beacons">Compressed Beacons</string>
+<string name="p__location_compressed_beacons_on">Send Compressed Beacons</string>
+<string name="p__location_compressed_beacons_off">Send Uncompressed Beacons</string>
+
 
 <string name="p_smartbeaconing">SmartBeaconing™</string>
 <string name="p_sb_help">SmartBeaconing™ help</string>

res/xml/location.xml

@@ -1,6 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
 <PreferenceScreen
         xmlns:android="http://schemas.android.com/apk/res/android">
+	<CheckBoxPreference
+		android:defaultValue="false"
+		android:key="compressed_location"
+		android:summaryOff="@string/p__location_compressed_beacons_off"
+		android:summaryOn="@string/p__location_compressed_beacons_on"
+		android:title="@string/p__location_compressed_beacons" />
         <de.duenndns.ListPreferenceWithValue
                 android:key="loc_source"
 		android:title="@string/p_locsource"

src/AprsPacket.scala

@@ -43,6 +43,20 @@ object AprsPacket {
 			""
 	}
 
+	def formatAltitudeCompressed(location : Location) : String = {
+		if (location.hasAltitude) {
+			var altitude = m2ft(location.getAltitude)
+			var compressedAltitude = ((math.log(altitude) / math.log(1.002)) + 0.5).asInstanceOf[Int]
+			var c = (compressedAltitude / 91).asInstanceOf[Byte] + 33
+			var s = (compressedAltitude % 91).asInstanceOf[Byte] + 33
+			// Negative altitudes cannot be expressed in base-91 and results in corrupt packets
+			if(c < 33) c = 33
+			if(s < 33) s = 33
+			"%c%c".format(c.asInstanceOf[Char], s.asInstanceOf[Char])
+		} else
+			""
+	}
+
 	def formatCourseSpeed(location : Location) : String = {
 		// only report speeds above 2m/s (7.2km/h)
 		if (location.hasSpeed && location.hasBearing)
@@ -53,6 +67,23 @@ object AprsPacket {
 			""
 	}
 
+	def formatCourseSpeedCompressed(location : Location) : String = {
+		// only report speeds above 2m/s (7.2km/h)
+		if (location.hasSpeed && location.hasBearing) {
+			// && location.getSpeed > 2)
+			var compressedBearing = (location.getBearing.asInstanceOf[Int] / 4).asInstanceOf[Int]
+			var compressedSpeed = ((math.log(mps2kt(location.getSpeed)) / math.log(1.08)) - 1).asInstanceOf[Int]
+			var c = compressedBearing.asInstanceOf[Byte] + 33;
+			var s = compressedSpeed.asInstanceOf[Byte] + 33;
+			// Negative speeds a courses cannot be expressed in base-91 and results in corrupt packets
+			if(c < 33) c = 33
+			if(s < 33) s = 33
+			"%c%c".format(c.asInstanceOf[Char], s.asInstanceOf[Char])
+		} else {
+			""
+		}
+	}
+
 	def formatFreq(csespd : String, freq : Float) : String = {
 		if (freq == 0) "" else {
 			val prefix = if (csespd.length() > 0) "/" else ""

src/AprsService.scala

@@ -213,13 +213,54 @@ class AprsService extends Service {
 		val pos = new Position(location.getLatitude, location.getLongitude, 0,
 				     symbol(0), symbol(1))
 		pos.setPositionAmbiguity(prefs.getStringInt("priv_ambiguity", 0))
-		val status_spd = if (prefs.getBoolean("priv_spdbear", true))
-			AprsPacket.formatCourseSpeed(location) else ""
+		val status_spd = if (prefs.getBoolean("priv_spdbear", true)) {
+			if(prefs.getBoolean("compressed_location", false)) {
+				// Compressed format
+				AprsPacket.formatCourseSpeedCompressed(location)
+			} else {
+				AprsPacket.formatCourseSpeed(location)
+			}
+		} else ""
 		val status_freq = AprsPacket.formatFreq(status_spd, prefs.getStringFloat("frequency", 0.0f))
-		val status_alt = if (prefs.getBoolean("priv_altitude", true))
-			AprsPacket.formatAltitude(location) else ""
-		newPacket(new PositionPacket(
-			pos, status_spd + status_freq + status_alt + " " + status, /* messaging = */ true))
+		val status_alt = if (prefs.getBoolean("priv_altitude", true)) {
+			// if speed is empty then use compressed altitude, otherwise use full length altitude
+			if(prefs.getBoolean("compressed_location", false) && status_spd == "") {
+				// Compressed format
+				AprsPacket.formatAltitudeCompressed(location)
+			} else {
+				AprsPacket.formatAltitude(location)
+			}
+		} else ""
+		if(prefs.getBoolean("compressed_location", false)) {
+			if(status_spd == "") {
+				// Speed is empty, so we can use a compressed altitude
+				if(status_alt == "") {
+					// Altitude is empty, so don't send any altitude data
+					pos.setCsTField(" sT")
+				} else {
+					// 3 signifies current GPS fix, GGA altitude, software compressed.
+					pos.setCsTField(status_alt + "3")
+				}
+				val packet = new PositionPacket(
+					pos, status_freq + " " + status, /* messaging = */ true)
+				packet.setCompressedFormat(true)
+				newPacket(packet)
+			} else {
+				// Speed is present, so we need to append the altitude to the end of the packet using the
+				// uncompressed method
+				// Apply the csT field with speed and course
+				// [ signifies current GPS fix, RMC speed, software compressed.
+				pos.setCsTField(status_spd + "[")
+				val packet = new PositionPacket(
+					pos, status_freq + status_alt + " " + status, /* messaging = */ true)
+				packet.setCompressedFormat(true)
+				newPacket(packet)
+			}
+		} else {
+			val packet = new PositionPacket(
+				pos, status_spd + status_freq + status_alt + " " + status, /* messaging = */ true)
+			newPacket(packet)
+		}
 	}
 
 	def sendPacket(packet : APRSPacket, status_postfix : String) {