CPR position decoding. Track calculation from velocity components.

dump1090.c

@@ -85,8 +85,17 @@ struct aircraft {
     char flight[9];     /* Flight number */
     int altitude;       /* Altitude */
     int speed;          /* Velocity computed from EW and NS components. */
+    int track;          /* Angle of flight. */
     time_t seen;        /* Time at which the last packet was received. */
     long messages;      /* Number of Mode S messages received. */
+    /* Encoded latitude and longitude as extracted by odd and even
+     * CPR encoded messages. */
+    int odd_cprlat;
+    int odd_cprlon;
+    int even_cprlat;
+    int even_cprlon;
+    double lat, lon;    /* Coordinated obtained from CPR encoded data. */
+    time_t odd_cprtime, even_cprtime;
     struct aircraft *next; /* Next aircraft in our linked list. */
 };
 
@@ -157,14 +166,14 @@ struct modesMessage {
     int heading_is_valid;
     int heading;
     int aircraft_type;
-    int fflag;                  /* Odd or Even CPR message? */
+    int fflag;                  /* 1 = Odd, 0 = Even CPR message. */
     int tflag;                  /* UTC synchronized? */
     int raw_latitude;           /* Non decoded latitude */
     int raw_longitude;          /* Non decoded longitude */
     char flight[9];             /* 8 chars flight number. */
-    int ew_dir;                 /* E/W direction. */
+    int ew_dir;                 /* 0 = East, 1 = West. */
     int ew_velocity;            /* E/W velocity. */
-    int ns_dir;                 /* N/S direction. */
+    int ns_dir;                 /* 0 = North, 1 = South. */
     int ns_velocity;            /* N/S velocity. */
     int vert_rate_source;       /* Vertical rate source. */
     int vert_rate_sign;         /* Vertical rate sign. */
@@ -867,8 +876,24 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
                 mm->vert_rate_source = (msg[8]&0x10) >> 4;
                 mm->vert_rate_sign = (msg[8]&0x8) >> 5;
                 mm->vert_rate = ((msg[8]&7) << 6) | ((msg[9]&0xfc) >> 2);
+                /* Compute velocity and angle from the two speed
+                 * components. */
                 mm->velocity = sqrt(mm->ns_velocity*mm->ns_velocity+
                                     mm->ew_velocity*mm->ew_velocity);
+                if (mm->velocity) {
+                    int ewv = mm->ew_velocity;
+                    int nsv = mm->ns_velocity;
+                    double heading;
+
+                    if (mm->ew_dir) ewv *= -1;
+                    if (mm->ns_dir) nsv *= -1;
+                    heading = atan2(ewv,nsv);
+
+                    /* Convert to degrees. */
+                    mm->heading = heading * 360 / (M_PI*2);
+                    /* We don't want negative values but a 0-360 scale. */
+                    if (mm->heading < 0) mm->heading += 360;
+                }
             } else if (mm->mesub == 3 || mm->mesub == 4) {
                 mm->heading_is_valid = msg[5] & (1<<2);
                 mm->heading = (360.0/128) * (((msg[5] & 3) << 5) |
@@ -1207,6 +1232,15 @@ struct aircraft *interactiveCreateAircraft(uint32_t addr) {
     a->flight[0] = '\0';
     a->altitude = 0;
     a->speed = 0;
+    a->track = 0;
+    a->odd_cprlat = 0;
+    a->odd_cprlon = 0;
+    a->odd_cprtime = 0;
+    a->even_cprlat = 0;
+    a->even_cprlon = 0;
+    a->even_cprtime = 0;
+    a->lat = 0;
+    a->lon = 0;
     a->seen = time(NULL);
     a->messages = 0;
     a->next = NULL;
@@ -1225,6 +1259,130 @@ struct aircraft *interactiveFindAircraft(uint32_t addr) {
     return NULL;
 }
 
+/* Always positive MOD operation, used for CPR decoding. */
+int cprModFunction(int a, int b) {
+    int res = a % b;
+    if (res < 0) res += b;
+    return res;
+}
+
+/* The NL function uses the precomputed table from 1090-WP-9-14 */
+int cprNLFunction(double lat) {
+    if (lat < 10.47047130) return 59;
+    if (lat < 14.82817437) return 58;
+    if (lat < 18.18626357) return 57;
+    if (lat < 21.02939493) return 56;
+    if (lat < 23.54504487) return 55;
+    if (lat < 25.82924707) return 54;
+    if (lat < 27.93898710) return 53;
+    if (lat < 29.91135686) return 52;
+    if (lat < 31.77209708) return 51;
+    if (lat < 33.53993436) return 50;
+    if (lat < 35.22899598) return 49;
+    if (lat < 36.85025108) return 48;
+    if (lat < 38.41241892) return 47;
+    if (lat < 39.92256684) return 46;
+    if (lat < 41.38651832) return 45;
+    if (lat < 42.80914012) return 44;
+    if (lat < 44.19454951) return 43;
+    if (lat < 45.54626723) return 42;
+    if (lat < 46.86733252) return 41;
+    if (lat < 48.16039128) return 40;
+    if (lat < 49.42776439) return 39;
+    if (lat < 50.67150166) return 38;
+    if (lat < 51.89342469) return 37;
+    if (lat < 53.09516153) return 36;
+    if (lat < 54.27817472) return 35;
+    if (lat < 55.44378444) return 34;
+    if (lat < 56.59318756) return 33;
+    if (lat < 57.72747354) return 32;
+    if (lat < 58.84763776) return 31;
+    if (lat < 59.95459277) return 30;
+    if (lat < 61.04917774) return 29;
+    if (lat < 62.13216659) return 28;
+    if (lat < 63.20427479) return 27;
+    if (lat < 64.26616523) return 26;
+    if (lat < 65.31845310) return 25;
+    if (lat < 66.36171008) return 24;
+    if (lat < 67.39646774) return 23;
+    if (lat < 68.42322022) return 22;
+    if (lat < 69.44242631) return 21;
+    if (lat < 70.45451075) return 20;
+    if (lat < 71.45986473) return 19;
+    if (lat < 72.45884545) return 18;
+    if (lat < 73.45177442) return 17;
+    if (lat < 74.43893416) return 16;
+    if (lat < 75.42056257) return 15;
+    if (lat < 76.39684391) return 14;
+    if (lat < 77.36789461) return 13;
+    if (lat < 78.33374083) return 12;
+    if (lat < 79.29428225) return 11;
+    if (lat < 80.24923213) return 10;
+    if (lat < 81.19801349) return 9;
+    if (lat < 82.13956981) return 8;
+    if (lat < 83.07199445) return 7;
+    if (lat < 83.99173563) return 6;
+    if (lat < 84.89166191) return 5;
+    if (lat < 85.75541621) return 4;
+    if (lat < 86.53536998) return 3;
+    if (lat < 87.00000000) return 2;
+    else return 1;
+}
+
+int cprNFunction(int i, int isodd) {
+    int nl = cprNLFunction(i) - isodd;
+    if (nl < 1) nl = 1;
+    return nl;
+}
+
+double cprDlonFunction(int i, int isodd) {
+    return 360.0 / cprNFunction(i, isodd);
+}
+
+/* This algorithm comes from:
+ * http://www.lll.lu/~edward/edward/adsb/DecodingADSBposition.html.
+ *
+ *
+ * A few remarks:
+ * 1) 131072 is 2^17 since CPR latitude and longitude are encoded in 17 bits.
+ * 2) We assume that we always received the odd packet as last packet for
+ *    simplicity. This may provide a position that is less fresh of a few
+ *    seconds.
+ */
+void decodeCPR(struct aircraft *a) {
+    const double AirDlat0 = 6;
+    const double AirDlat1 = 360.0 / 59;
+    double lat0 = a->even_cprlat;
+    double lat1 = a->odd_cprlat;
+    double lon0 = a->even_cprlon;
+    double lon1 = a->odd_cprlon;
+
+    /* Compute the Latitude Index "j" */
+    int j = ((59*lat0 - 60*lat1) / 131072) + 0.5;
+    double rlat0 = AirDlat0 * (cprModFunction(j,60) + lat0 / 131072);
+    double rlat1 = AirDlat1 * (cprModFunction(j,59) + lat1 / 131072);
+
+    /* Check that both are in the same latitude zone, or abort. */
+    if (cprNLFunction(rlat0) != cprNLFunction(rlat1)) return;
+
+    /* Compute ni and the longitude index m */
+    if (a->even_cprtime > a->odd_cprtime) {
+        /* Use even packet. */
+        int ni = cprNFunction(rlat0,0);
+        int m = (((lon0 * (cprNLFunction(rlat0)-1)) -
+                  (lon1 * cprNLFunction(rlat0))) / 131072) + 0.5;
+        a->lon = cprDlonFunction(rlat0,0) * (cprModFunction(m,ni)+lon0/131072);
+        a->lat = rlat0;
+    } else {
+        /* Use odd packet. */
+        int ni = cprNFunction(rlat1,1);
+        int m = (((lon0 * (cprNLFunction(rlat1)-1)) -
+                  (lon1 * cprNLFunction(rlat1))) / 131072) + 0.5;
+        a->lon = cprDlonFunction(rlat1,1) * (cprModFunction(m,ni)+lon1/131072);
+        a->lat = rlat1;
+    }
+}
+
 /* Receive new messages and populate the interactive mode with more info. */
 void interactiveReceiveData(struct modesMessage *mm) {
     uint32_t addr;
@@ -1268,9 +1426,23 @@ void interactiveReceiveData(struct modesMessage *mm) {
             memcpy(a->flight, mm->flight, sizeof(a->flight));
         } else if (mm->metype >= 9 && mm->metype <= 18) {
             a->altitude = mm->altitude;
+            if (mm->fflag) {
+                a->odd_cprlat = mm->raw_latitude;
+                a->odd_cprlon = mm->raw_longitude;
+                a->odd_cprtime = a->seen; /* Current time. */
+            } else {
+                a->even_cprlat = mm->raw_latitude;
+                a->even_cprlon = mm->raw_longitude;
+                a->even_cprtime = a->seen; /* Current time. */
+            }
+            /* If the two data is less than 10 seconds apart, compute
+             * the position. */
+            if (abs(a->even_cprtime - a->odd_cprtime) <= 10)
+                decodeCPR(a);
         } else if (mm->metype == 19) {
             if (mm->mesub == 1 || mm->mesub == 2) {
                 a->speed = mm->velocity;
+                a->track = mm->heading;
             }
         }
     }
@@ -1288,9 +1460,10 @@ void interactiveShowData(void) {
     progress[3] = '\0';
 
     printf("\x1b[H\x1b[2J");    /* Clear the screen */
-    printf("Hex    Flight   Altitude  Speed     Messages  Seen  %s\n",
+    printf(
+"Hex    Flight   Altitude  Speed   Lat       Lon       Track  Messages Seen %s\n"
+"--------------------------------------------------------------------------------\n",
         progress);
-    printf("-------------------------------------------------------\n");
 
     while(a && count < Modes.interactive_rows) {
         int altitude = a->altitude, speed = a->speed;
@@ -1301,8 +1474,9 @@ void interactiveShowData(void) {
             speed *= 1.852;
         }
 
-        printf("%-6s %-8s %-9d %-9d %-9ld %d sec ago\n",
+        printf("%-6s %-8s %-9d %-7d %-7.03f   %-7.03f   %-3d   %-9ld %d sec\n",
-            a->hexaddr, a->flight, altitude, speed, a->messages,
+            a->hexaddr, a->flight, altitude, speed,
+            a->lat, a->lon, a->track, a->messages,
             (int)(now - a->seen));
         a = a->next;
         count++;