SurfacePosition

Initial attempt at implementing SurfacePosition. Algorithm is mostly shared with AirbornePosition, so the common elements are moved to the PositionUpdate class, which maintains a cache of even/odd frames, decides whether Global or Local calculation can be used, and generally contains the complexity of the CPR system. Some further sanity checks are required, and also a way to choose between multiple possible solutions for lat/lon for Surface data.

Author: IDBeswick

examples/gui/src/main/java/example/Demo.java

@@ -23,7 +23,7 @@
 public class Demo extends JPanel {
     private static final Logger logger = LoggerFactory.getLogger(Demo.class);
 
-    private static final String IP = "192.168.178.190";
+    private static final String IP = "127.0.0.1";
     private static final int PORT = 30002;
     private static final double LAT = 51;
     private static final double LON = 2;

src/main/java/aero/t2s/modes/CprPosition.java

@@ -5,18 +5,33 @@
 public class CprPosition {
     private double lat;
     private double lon;
-    private boolean valid;
+    private boolean surface = false;
+    private boolean valid = false;
+    private double latZone;
+    private double lonZone;
     private long time;
 
     public CprPosition() {
         this.lat = 0.0;
         this.lon = 0.0;
+        this.surface = false;
         this.valid = false;
     }
+
+    public CprPosition(double lat, double lon, boolean surface) {
+        setLatLon(lat ,lon);
+        this.surface = surface;
+    }
+
     public CprPosition(double lat, double lon) {
         setLatLon(lat ,lon);
     }
 
+    public CprPosition(int cprLat, int cprLon, boolean surface) {
+        setLatLon(cprLat / (double) (1 << 17), cprLon / (double) (1 << 17));
+        this.surface = surface;
+    }
+
     public void setLatLon(double lat, double lon) {
         this.lat = lat;
         this.lon = lon;
@@ -40,6 +55,41 @@ public double getLon() {
         return lon;
     }
 
+    public void setSurface(boolean surface) {
+        this.surface = surface;
+    }
+
+    public boolean getSurface() {
+        return this.surface;
+    }
+
+    public void validateSurface() {
+        // For SurfacePositions we get 8 possible solutions: 2x latitude, 4x longitude zones at 90 degree increments
+        // TODO Could use receiver/reference lat/lon to chose which possibile solution is closest... or build-up an average over data received to-date
+        // Just for temporary, assume northern hemisphere and west of prime meridian
+        this.lon = this.lon - 90.0;
+    }
+
+    public void setZones(double latZone, double lonZone) {
+        this.latZone = latZone;
+        this.lonZone = lonZone;
+    }
+    public void setLatZone(double zone) {
+        this.latZone = zone;
+    }
+
+    public double getLatZone() {
+        return this.latZone;
+    }
+
+    public void setLonZone(double zone) {
+        this.lonZone = zone;
+    }
+
+    public double getLonZone() {
+        return this.lonZone;
+    }
+
     public void setTime(long time) {
         this.time = time;
     }
@@ -52,6 +102,10 @@ public boolean isValid() {
         return valid;
     }
 
+    public void setValid(boolean valid) {
+        this.valid = valid;
+    }
+
     public boolean isExpired() {
         return time < Instant.now().minusSeconds(10).toEpochMilli();
     }

src/main/java/aero/t2s/modes/ModeSHandler.java

@@ -1,7 +1,7 @@
 package aero.t2s.modes;
 
 import aero.t2s.modes.decoder.df.DownlinkFormat;
-import aero.t2s.modes.decoder.df.df17.AirbornePosition;
+import aero.t2s.modes.decoder.df.df17.PositionUpdate;
 
 import java.util.function.Consumer;
 
@@ -43,10 +43,10 @@ protected short[] toData(final String input) throws EmptyMessageException, ModeA
     }
 
     public void start() {
-        AirbornePosition.start();
+        PositionUpdate.start();
     }
 
     public void stop() {
-        AirbornePosition.stop();
+        PositionUpdate.stop();
     }
 }

src/main/java/aero/t2s/modes/decoder/df/DF17.java

@@ -41,7 +41,7 @@ public DF17 decode() {
             case 6:
             case 7:
             case 8:
-                extendedSquitter = new SurfacePosition(data);
+                extendedSquitter = new SurfacePosition(data, getIcao());
                 break;
             case 19:
                 extendedSquitter = new AirborneVelocity(data);

src/main/java/aero/t2s/modes/decoder/df/DF18.java

@@ -25,7 +25,7 @@ public DF18 decode() {
             case 6:
             case 7:
             case 8:
-                extendedSquitter = new SurfacePosition(data);
+                extendedSquitter = new SurfacePosition(data, getIcao());
                 break;
             case 19:
                 extendedSquitter = new AirborneVelocity(data);

src/main/java/aero/t2s/modes/decoder/df/df17/AirbornePosition.java

@@ -3,7 +3,6 @@
 import aero.t2s.modes.Track;
 import aero.t2s.modes.CprPosition;
 import aero.t2s.modes.constants.*;
-import aero.t2s.modes.decoder.Common;
 import aero.t2s.modes.registers.Register05;
 import aero.t2s.modes.registers.Register05V0;
 import aero.t2s.modes.registers.Register05V2;
@@ -18,12 +17,9 @@ public class AirbornePosition extends ExtendedSquitter {
     private boolean altitudeSourceBaro;
     private int altitude;
 
-    private boolean positionAvailable;
-
     private double lat;
     private double lon;
-    private static Map<String, PositionUpdate> cache = new HashMap<>();
-    private static Timer cacheCleanup;
+    private boolean positionAvailable;
 
     public AirbornePosition(short[] data, String address) {
         super(data);
@@ -58,39 +54,14 @@ public AirbornePosition decode() {
         cprLon = cprLon | (data[9] << 8);
         cprLon = cprLon | data[10];
 
-
-        if (!cache.containsKey(address)) {
-            if (!isCprEven) {
-                return this;
-            }
-
-            synchronized (cache) {
-                cache.putIfAbsent(address, new PositionUpdate(
-                    new CprPosition(cprLat / (double)(1 << 17), cprLon / (double)(1 << 17))
-                ));
-            }
-        }
-
-        PositionUpdate positionUpdate;
-        synchronized (cache) {
-            positionUpdate = cache.get(address);
-        }
-        if (isCprEven) {
-            positionUpdate.setEven(new CprPosition(cprLat / (double) (1 << 17), cprLon / (double) (1 << 17)));
-        }  else {
-            positionUpdate.setOdd(new CprPosition(cprLat / (double) (1 << 17), cprLon / (double) (1 << 17)));
-        }
-
-        if (positionUpdate.isComplete()) {
-            calculateGlobal(positionUpdate.even, positionUpdate.odd);
-        } else if (positionUpdate.isPreviousPositionAvailable() && positionUpdate.isPreviousPositionAvailable()) {
-            calculateLocal(positionUpdate.odd, true, positionUpdate.previousLat, positionUpdate.previousLon);
-        }
-
-        if (positionAvailable) {
-            positionUpdate.setPreviousPosition(this.lat, this.lon);
+        CprPosition newPosition = PositionUpdate.calculate(address, isCprEven, new CprPosition(cprLat, cprLon, false));
+        if (newPosition != null) {
+            this.lat = newPosition.getLat();
+            this.lon = newPosition.getLon();
+            this.positionAvailable = true;
+        } else {
+            this.positionAvailable = false;
         }
-
         return this;
     }
 
@@ -230,88 +201,6 @@ private AltitudeSource determineAltitudeSource() {
         return AltitudeSource.GNSS_HAE;
     }
 
-    private void calculateLocal(CprPosition cpr, boolean isOdd, double previousLat, double previousLon) {
-
-        double dlat = isOdd ? 360.0 / 59.0 : 360.0 / 60.0;
-
-        double j = Math.floor(previousLat / dlat) + Math.floor((previousLat % dlat) / dlat -  cpr.getLat() + 0.5);
-
-        double newLat = dlat * (j + previousLat);
-
-        double nl = NL(newLat) - (isOdd ? 1.0 : 0.0);
-        double dlon = nl > 0 ? 360.0 / nl : 360;
-
-        double m = Math.floor(previousLon / dlon) + Math.floor((previousLon % dlon) / dlon - cpr.getLon() + 0.5);
-        double newLon = dlon * (m + lon);
-
-        //TODO Should be a sanity-check here to make sure the calculated position isn't outside receiver origin range
-        //TODO Should be a sanity-check here to see if the calculated movement since the last update is too far
-        this.lat = newLat;
-        this.lon = newLon;
-        this.positionAvailable = true;
-    }
-
-    private void calculateGlobal(CprPosition cprEven, CprPosition cprOdd) {
-        double dLat0 = 360.0 / 60.0;
-        double dLat1 = 360.0 / 59.0;
-
-        double j = Math.floor(59.0 * cprEven.getLat() - 60.0 * cprOdd.getLat() + 0.5);
-
-        double latEven = dLat0 * (j % 60.0 + cprEven.getLat());
-        double latOdd = dLat1 * (j % 59.0 + cprOdd.getLat());
-
-        if (latEven >= 270.0 && latEven <= 360.0) {
-            latEven -= 360.0;
-        }
-
-        if (latOdd >= 270.0 && latOdd <= 360.0) {
-            latOdd -= 360.0;
-        }
-
-        if (NL(latEven) != NL(latOdd)) {
-            return;
-        }
-
-        double lat;
-        double lon;
-        if (cprEven.getTime() > cprOdd.getTime()) {
-            double ni = cprN(latEven, 0);
-            double m = Math.floor(cprEven.getLon() * (NL(latEven) - 1) - cprOdd.getLon() * NL(latEven) + 0.5);
-
-            lat = latEven;
-            lon = (360d / ni) * (m % ni + cprEven.getLon());
-        } else {
-            double ni = cprN(latOdd, 1);
-            double m = Math.floor(cprEven.getLon() * (NL(latOdd) - 1) - cprOdd.getLon() * NL(latOdd) + 0.5);
-
-            lat = latOdd;
-            lon = (360d / ni) * (m % ni + cprOdd.getLon());
-        }
-
-        if (lon > 180d) {
-            lon -= 360d;
-        }
-
-        //TODO Should be a sanity-check here to make sure the calculated position isn't outside receiver origin range,
-        this.lat = lat;
-        this.lon = lon;
-        this.positionAvailable = true;
-    }
-    private double cprN(double lat, double isOdd) {
-        double nl = NL(lat) - isOdd;
-
-        return nl > 1 ? nl : 1;
-    }
-
-    private double NL(double lat) {
-        if (lat == 0) return 59;
-        else if (Math.abs(lat) == 87) return 2;
-        else if (Math.abs(lat) > 87) return 1;
-
-        double tmp = 1 - (1 - Math.cos(Math.PI / (2.0 * 15.0))) / Math.pow(Math.cos(Math.PI / 180.0 * Math.abs(lat)), 2);
-        return Math.floor(2 * Math.PI / Math.acos(tmp));
-    }
-
     private int calculateAltitude(short[] data, int typeCode) {
         // TODO this should use AltitudeEncoding class flagged with mBit false feature.
         int n = (data[5] >>> 1) << 4;
@@ -321,66 +210,4 @@ private int calculateAltitude(short[] data, int typeCode) {
 
         return (n * qBit) - 1000;
     }
-
-    public static void start() {
-        AirbornePosition.cache.clear();
-        AirbornePosition.cacheCleanup.schedule(new TimerTask() {
-            @Override
-            public void run() {
-                List<String> expired = new LinkedList<>();
-
-                synchronized (cache) {
-                    cache.entrySet().stream().filter(entry -> entry.getValue().isExpired()).forEach(entry -> expired.add(entry.getKey()));
-                    expired.forEach(cache::remove);
-                }
-            }
-        }, 0, 10_000);
-    }
-
-    public static void stop() {
-        AirbornePosition.cacheCleanup.cancel();
-        AirbornePosition.cacheCleanup = null;
-
-        AirbornePosition.cache.clear();
-    }
-
-    class PositionUpdate {
-        private CprPosition even;
-        private CprPosition odd;
-
-
-        private boolean previousPositionAvailable = false;
-        private double previousLat;
-        private double previousLon;
-
-        public PositionUpdate(CprPosition even) {
-            this.even = even;
-        }
-
-        public void setEven(CprPosition even) {
-            this.even = even;
-            this.odd = null;
-        }
-
-        public void setOdd(CprPosition odd) {
-            this.odd = odd;
-        }
-
-        public void setPreviousPosition(double lat, double lon) {
-            this.previousLat = lat;
-            this.previousLon = lon;
-        }
-
-        public boolean isPreviousPositionAvailable() {
-            return this.previousPositionAvailable;
-        }
-
-        public boolean isComplete() {
-            return even != null && odd != null;
-        }
-
-        public boolean isExpired() {
-            return even.isExpired() || odd.isExpired();
-        }
-    }
 }