Track HAE altitude separately to baro altitude where possible.

dump1090.h

@@ -163,6 +163,8 @@ typedef struct rtlsdr_dev rtlsdr_dev_t;
 #define MODES_ACFLAGS_REL_CPR_USED   (1<<16) // Lat/lon derived from relative CPR
 #define MODES_ACFLAGS_CATEGORY_VALID (1<<17) // Aircraft category is known
 #define MODES_ACFLAGS_FROM_MLAT      (1<<18) // Data was derived from multilateration
+#define MODES_ACFLAGS_ALTITUDE_HAE_VALID (1<<19) // altitude_hae is valid
+#define MODES_ACFLAGS_HAE_DELTA_VALID    (1<<20) // hae_delta is valid
 
 #define MODES_ACFLAGS_LLEITHER_VALID (MODES_ACFLAGS_LLEVEN_VALID | MODES_ACFLAGS_LLODD_VALID)
 #define MODES_ACFLAGS_LLBOTH_VALID   (MODES_ACFLAGS_LLEVEN_VALID | MODES_ACFLAGS_LLODD_VALID)
@@ -389,6 +391,8 @@ struct modesMessage {
     int    vert_rate;           // Vertical rate.
     int    velocity;            // Reported by aircraft, or computed from from EW and NS velocity
     unsigned category;          // A0 - D7 encoded as a single hex byte
+    int    altitude_hae;        // altitude reported as GNSS HAE
+    int    hae_delta;           // difference between HAE and baro alt
 
     // DF 18
     int    cf;                  // Control Field

mode_s.c

@@ -846,6 +846,11 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
             }
         }
 
+        if (msg[10] != 0) {
+            mm->bFlags |= MODES_ACFLAGS_HAE_DELTA_VALID;
+            mm->hae_delta = ((msg[10] & 0x80) ? -25 : 25) * ((msg[10] & 0x7f) - 1);
+        }
+
         break;
     }
         
@@ -910,10 +915,19 @@ static void decodeExtendedSquitter(struct modesMessage *mm)
         }
 
         if (AC12Field) {// Only attempt to decode if a valid (non zero) altitude is present
+            if (metype == 20 || metype == 21 || metype == 22) {
+                // Position reported as HAE
+                mm->altitude_hae = decodeAC12Field(AC12Field, &mm->unit);
+                if (mm->altitude_hae != INVALID_ALTITUDE) {
+                    mm->bFlags  |= MODES_ACFLAGS_ALTITUDE_HAE_VALID;
+                }
+            } else {
                 mm->altitude = decodeAC12Field(AC12Field, &mm->unit);
-            if (mm->altitude != INVALID_ALTITUDE)
+                if (mm->altitude != INVALID_ALTITUDE) {
                     mm->bFlags  |= MODES_ACFLAGS_ALTITUDE_VALID;
                 }
+            }
+        }
 
         if (metype == 0 || metype == 18 || metype == 22)
             mm->nuc_p = 0;
@@ -1011,15 +1025,22 @@ static void displayExtendedSquitter(struct modesMessage *mm) {
             printf("    Vertical status   : %s\n", (mm->bFlags & MODES_ACFLAGS_VERTRATE_VALID) ? "Valid" : "Unavailable");
             printf("    Vertical rate src : %d\n", ((mm->msg[8] >> 4) & 1));
             printf("    Vertical rate     : %d\n", mm->vert_rate);
-            
         } else {
             printf("    Unrecognized ME subtype: %d subtype: %d\n", mm->metype, mm->mesub);
         }
+
+        if (mm->bFlags & MODES_ACFLAGS_HAE_DELTA_VALID) {
+            printf("    HAE - Barometric  : %d ft\n", mm->hae_delta);
+        } else {
+            printf("    HAE - Barometric  : not valid\n");
+        }
     } else if (mm->metype >= 5 && mm->metype <= 22) { // Airborne position Baro
         printf("    F flag   : %s\n", (mm->msg[6] & 0x04) ? "odd" : "even");
         printf("    T flag   : %s\n", (mm->msg[6] & 0x08) ? "UTC" : "non-UTC");
         if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_VALID)
-            printf("    Altitude : %d feet\n", mm->altitude);
+            printf("    Altitude : %d feet barometric\n", mm->altitude);
+        else if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_HAE_VALID)
+            printf("    Altitude : %d feet HAE\n", mm->altitude_hae);
         else
             printf("    Altitude : not valid\n");
         if (mm->bFlags & MODES_ACFLAGS_LATLON_VALID) {

track.c

@@ -522,9 +522,34 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
             a->altitude = mm->altitude;
             a->modeC    = (mm->altitude + 49) / 100;
             a->seenAltitude = now;
+
+            // reporting of HAE and baro altitudes is mutually exclusive
+            // so if we see a baro altitude, assume the HAE altitude is invalid
+            // we will recalculate it from baro + HAE delta below, where possible
+            a->bFlags &= ~MODES_ACFLAGS_ALTITUDE_HAE_VALID;
         }
     }
 
+    // If a (new) HAE altitude has been received, copy it to the aircraft structure
+    if (mm->bFlags & MODES_ACFLAGS_ALTITUDE_HAE_VALID) {
+        a->altitude_hae = mm->altitude_hae;
+
+        // reporting of HAE and baro altitudes is mutually exclusive
+        // if you have both, you're meant to report baro and a HAE delta,
+        // so if we see explicit HAE then assume the delta is invalid too
+        a->bFlags &= ~(MODES_ACFLAGS_ALTITUDE_VALID | MODES_ACFLAGS_HAE_DELTA_VALID);
+    }
+
+    // If a (new) HAE/barometric difference has been received, copy it to the aircraft structure
+    if (mm->bFlags & MODES_ACFLAGS_HAE_DELTA_VALID) {
+        a->hae_delta = mm->hae_delta;
+
+        // reporting of HAE and baro altitudes is mutually exclusive
+        // if you have both, you're meant to report baro and a HAE delta,
+        // so if we see a HAE delta then assume the HAE altitude is invalid
+        a->bFlags &= ~MODES_ACFLAGS_ALTITUDE_HAE_VALID;
+    }
+
     // If a (new) SQUAWK has been received, copy it to the aircraft structure
     if (mm->bFlags & MODES_ACFLAGS_SQUAWK_VALID) {
         if (a->modeA != mm->modeA) {
@@ -559,6 +584,12 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm)
     // Update the aircrafts a->bFlags to reflect the newly received mm->bFlags;
     a->bFlags |= mm->bFlags;
 
+    // If we have a baro altitude and a HAE delta from baro, calculate the HAE altitude
+    if ((a->bFlags & MODES_ACFLAGS_ALTITUDE_VALID) && (a->bFlags & MODES_ACFLAGS_HAE_DELTA_VALID)) {
+        a->altitude_hae = a->altitude + a->hae_delta;
+        a->bFlags |= MODES_ACFLAGS_ALTITUDE_HAE_VALID;
+    }
+
     // Update mlat flags. The mlat flags indicate which bits in bFlags
     // were last set based on a mlat-derived message.
     if (mm->bFlags & MODES_ACFLAGS_FROM_MLAT)

track.h

@@ -64,7 +64,9 @@ struct aircraft {
     uint32_t      addr;           // ICAO address
     char          flight[16];     // Flight number
     double        signalLevel[8]; // Last 8 Signal Amplitudes
-    int           altitude;       // Altitude
+    int           altitude;       // Altitude (Baro)
+    int           altitude_hae;   // Altitude (HAE)
+    int           hae_delta;      // Difference between HAE and Baro altitudes
     int           speed;          // Velocity
     int           track;          // Angle of flight
     int           vert_rate;      // Vertical rate.