Add --no-decode option.

This disables most decoding of the contents of Mode S messages, aircraft tracking, and some output modes that depend on them.
It's intended for edge receivers that just forward to a central hub rather than processing data locally.

dump1090.c

@@ -434,6 +434,7 @@ void showHelp(void) {
 "--debug <flags>          Debug mode (verbose), see README for details\n"
 "--quiet                  Disable output to stdout. Use for daemon applications\n"
 "--ppm <error>            Set receiver error in parts per million (default 0)\n"
+"--no-decode              Don't decode the message contents beyond the minimum necessary\n"
 "--help                   Show this help\n"
 "\n"
 "Debug mode flags: d = Log frames decoded with errors\n"
@@ -681,6 +682,8 @@ int main(int argc, char **argv) {
         } else if (!strcmp(argv[j],"--interactive-rtl1090")) {
             Modes.interactive = 1;
             Modes.interactive_rtl1090 = 1;
+        } else if (!strcmp(argv[j],"--no-decode")) {
+            Modes.no_decode = 1;
         } else {
             fprintf(stderr,
                 "Unknown or not enough arguments for option '%s'.\n\n",
@@ -690,6 +693,29 @@ int main(int argc, char **argv) {
         }
     }
 
+    // Handle --no-decode, which turns off various parts of decoding
+    // that are not useful for an "edge" dump1090 that purely forwards
+    // raw data to a central hub elsewhere.
+    if (Modes.no_decode) {
+        if (Modes.interactive) {
+            fprintf(stderr, "--no-decode and --interactive cannot be specified together.\n");
+            exit(1);
+        }
+
+        if (Modes.net_output_sbs_port != MODES_NET_OUTPUT_SBS_PORT) {
+            fprintf(stderr, "--no-decode and --net-sbs-port cannot be specified together.\n");
+            exit(1);
+        }
+
+        if (Modes.net_http_port != MODES_NET_HTTP_PORT) {
+            fprintf(stderr, "--no-decode and --net-http-port cannot be specified together.\n");
+            exit(1);
+        }
+
+        Modes.net_output_sbs_port = 0;
+        Modes.net_http_port = 0;
+    }
+
 #ifdef _WIN32
     // Try to comply with the Copyright license conditions for binary distribution
     if (!Modes.quiet) {showCopyright();}

dump1090.h

@@ -317,6 +317,7 @@ struct {                             // Internal state
     int   metric;                    // Use metric units
     int   mlat;                      // Use Beast ascii format for raw data output, i.e. @...; iso *...;
     int   interactive_rtl1090;       // flight table in interactive mode is formatted like RTL1090
+    int   no_decode;                 // Disable decoding and aircraft tracking
 
     // User details
     double fUserLat;                // Users receiver/antenna lat/lon needed for initial surface location

mode_s.c

@@ -911,6 +911,9 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
     // of the data contents, so save time and give up now.
     if ((Modes.check_crc) && (!mm->crcok)) { return;}
 
+    // If decoding is disabled, this is as far as we go.
+    if (Modes.no_decode) return;
+
     // Fields for DF0, DF16
     if (mm->msgtype == 0  || mm->msgtype == 16) {
         if (msg[0] & 0x04) {                       // VS Bit
@@ -1176,6 +1179,12 @@ void displayModesMessage(struct modesMessage *mm) {
     if (mm->correctedbits != 0)
         printf("No. of bit errors fixed: %d\n", mm->correctedbits);
 
+    if (Modes.no_decode) {
+        // Show DF type and address only; the rest is not decoded.
+        printf("DF %d; address: %06x\n", mm->msgtype, mm->addr);
+        return;
+    }
+
     if (mm->msgtype == 0) { // DF 0
         printf("DF 0: Short Air-Air Surveillance.\n");
         printf("  VS             : %s\n",  (mm->msg[0] & 0x04) ? "Ground" : "Airborne");
@@ -1900,8 +1909,10 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
 void useModesMessage(struct modesMessage *mm) {
     if ((Modes.check_crc == 0) || (mm->crcok) || (mm->correctedbits)) { // not checking, ok or fixed
 
-        // Always track aircraft
+        // If we are decoding, track aircraft
-        interactiveReceiveData(mm);
+        if (!Modes.no_decode) {
+            interactiveReceiveData(mm);
+        }
 
         // In non-interactive non-quiet mode, display messages on standard output
         if (!Modes.interactive && !Modes.quiet) {