Measure signal power / noise power (at least in 2.4MHz mode).

Switch signalLevel back to a power measurement, don't put SNR in there.
But make it a 0.0 - 1.0 double so we're not scaling everywhere.

Adjust for the amplitude offset when calculating power.

Adapt everything else to the new scheme.

demod_2000.c

@@ -540,7 +540,7 @@ void demodulate2000(uint16_t *m, uint32_t mlen) {
           && (errors      <= MODES_MSG_ENCODER_ERRS) ) {
             // Set initial mm structure details
             mm.timestampMsg = Modes.timestampBlk + (j*6);
-            mm.signalLevel = (snr > 255 ? 255 : (uint8_t)snr);
+            mm.signalLevel = (365.0*60 + sigLevel + noiseLevel) * (365.0*60 + sigLevel + noiseLevel) / MAX_POWER / 60 / 60;
             mm.phase_corrected = use_correction;
 
             // Decode the received message

demod_2400.c

@@ -153,10 +153,15 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
     struct modesMessage mm;
     unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
     uint32_t j;
+    uint32_t last_message_end = 0;
 
     unsigned char *bestmsg;
     int bestscore, bestphase;
 
+    // noise floor:
+    uint32_t noise_power_count = 0;
+    uint64_t noise_power_sum = 0;
+
     memset(&mm, 0, sizeof(mm));
     msg = msg1;
 
@@ -167,6 +172,17 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
         int initial_phase, first_phase, last_phase, try_phase;
         int msglen;
 
+        // update noise for all samples that aren't part of a message
+        // (we don't know if m[j] is or not, yet, so work one sample
+        // in arrears)
+        if (j > last_message_end+1) {
+            // There seems to be a weird compiler bug I'm hitting here..
+            // if you compute the square directly, it occasionally gets mangled.
+            uint64_t s = TRUE_AMPLITUDE(m[j-1]);
+            noise_power_sum += s * s;
+            noise_power_count++;
+        }
+
         // Look for a message starting at around sample 0 with phase offset 3..7
 
         // Ideal sample values for preambles with different phase
@@ -403,6 +419,28 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
         mm.score = bestscore;
         mm.bFlags = mm.correctedbits   = 0;
 
+        // measure signal power
+        {
+            uint64_t signal_power_sum = 0;
+            double signal_power;
+            int signal_len = msglen*12/5 + 1;
+            int k;
+
+            for (k = 0; k < signal_len; ++k) {
+                uint64_t s = TRUE_AMPLITUDE(m[j+19+k]);
+                signal_power_sum += s * s;
+            }
+
+            mm.signalLevel = signal_power = signal_power_sum / MAX_POWER / signal_len;
+            Modes.stats_current.signal_power_sum += signal_power;
+            Modes.stats_current.signal_power_count ++;
+
+            if (signal_power > Modes.stats_current.peak_signal_power)
+                Modes.stats_current.peak_signal_power = signal_power;
+            if (signal_power > 0.50119)
+                Modes.stats_current.strong_signal_count++; // signal power above -3dBFS
+        }
+
         // Decode the received message
         if (decodeModesMessage(&mm, bestmsg) < 0)
             continue;
@@ -426,10 +464,14 @@ void demodulate2400(uint16_t *m, uint32_t mlen) {
         //  where the preamble of the second message clobbered the last
         //  few bits of the first message, but the message bits didn't
         //  overlap)
+        last_message_end = j + (8 + msglen)*12/5;
         j += (8 + msglen - 8)*12/5 - 1;
             
         // Pass data to the next layer
         useModesMessage(&mm);
     }
+
+    Modes.stats_current.noise_power_sum += (noise_power_sum / MAX_POWER / noise_power_count);
+    Modes.stats_current.noise_power_count ++;
 }
 

dump1090.h

@@ -209,6 +209,11 @@
 
 #define MODES_NOTUSED(V) ((void) V)
 
+// adjust for zero offset of amplitude values
+#define TRUE_AMPLITUDE(x) ((x) + 365)
+#define MAX_AMPLITUDE TRUE_AMPLITUDE(65535)
+#define MAX_POWER (1.0 * MAX_AMPLITUDE * MAX_AMPLITUDE)
+
 // Include subheaders after all the #defines are in place
 
 #include "anet.h"
@@ -235,7 +240,7 @@ struct client {
 struct aircraft {
     uint32_t      addr;           // ICAO address
     char          flight[16];     // Flight number
-    unsigned char signalLevel[8]; // Last 8 Signal Amplitudes
+    double        signalLevel[8]; // Last 8 Signal Amplitudes
     int           altitude;       // Altitude
     int           speed;          // Velocity
     int           track;          // Angle of flight
@@ -402,7 +407,7 @@ struct modesMessage {
     int           phase_corrected;                // True if phase correction was applied
     uint64_t      timestampMsg;                   // Timestamp of the message
     int           remote;                         // If set this message is from a remote station
-    unsigned char signalLevel;                    // Signal Amplitude
+    double        signalLevel;                    // RSSI, in the range [0..1], as a fraction of full-scale power
     int           score;
 
     // DF 11, DF 17

interactive.c

@@ -68,16 +68,18 @@ static uint64_t mstime(void) {
 // of aircraft
 //
 struct aircraft *interactiveCreateAircraft(struct modesMessage *mm) {
+    static struct aircraft zeroAircraft;
     struct aircraft *a = (struct aircraft *) malloc(sizeof(*a));
+    int i;
 
     // Default everything to zero/NULL
-    memset(a, 0, sizeof(*a));
+    *a = zeroAircraft;
 
     // Now initialise things that should not be 0/NULL to their defaults
     a->addr = mm->addr;
-    a->lat  = a->lon = 0.0;
+    for (i = 0; i < 8; ++i)
-    memset(a->signalLevel, mm->signalLevel, 8); // First time, initialise everything
+        a->signalLevel[i] = mm->signalLevel;  // First time, initialise everything
-                                                // to the first signal strength
+                                              // to the first signal strength
 
     // mm->msgtype 32 is used to represent Mode A/C. These values can never change, so 
     // set them once here during initialisation, and don't bother to set them every 
@@ -521,7 +523,7 @@ void interactiveShowData(void) {
 
     if (Modes.interactive_rtl1090 == 0) {
         printf (
-"Hex     Mode  Sqwk  Flight   Alt    Spd  Hdg    Lat      Long   Sig  Msgs   Ti%c\n", progress);
+"Hex     Mode  Sqwk  Flight   Alt    Spd  Hdg    Lat      Long   RSSI  Msgs  Ti%c\n", progress);
     } else {
         printf (
 "Hex    Flight   Alt      V/S GS  TT  SSR  G*456^ Msgs    Seen %c\n", progress);
@@ -576,9 +578,9 @@ void interactiveShowData(void) {
                     char strMode[5]               = "    ";
                     char strLat[8]                = " ";
                     char strLon[9]                = " ";
-                    unsigned char * pSig       = a->signalLevel;
+                    double * pSig                 = a->signalLevel;
-                    unsigned int signalAverage = (pSig[0] + pSig[1] + pSig[2] + pSig[3] + 
+                    double signalAverage = (pSig[0] + pSig[1] + pSig[2] + pSig[3] + 
-                                                  pSig[4] + pSig[5] + pSig[6] + pSig[7] + 3) >> 3; 
+                                            pSig[4] + pSig[5] + pSig[6] + pSig[7]) / 8.0; 
 
                     if ((flags & MODEAC_MSG_FLAG) == 0) {
                         strMode[0] = 'S';
@@ -599,9 +601,9 @@ void interactiveShowData(void) {
                         snprintf(strFl, 6, "%5d", altitude);
                     }
 
-                    printf("%06X  %-4s  %-4s  %-8s %5s  %3s  %3s  %7s %8s %2d.%1d %5d  %2d\n",
+                    printf("%06X  %-4s  %-4s  %-8s %5s  %3s  %3s  %7s %8s %5.1f %5d %2d\n",
-                    a->addr, strMode, strSquawk, a->flight, strFl, strGs, strTt,
+                           a->addr, strMode, strSquawk, a->flight, strFl, strGs, strTt,
-                    strLat, strLon, signalAverage/5, 2*(signalAverage%5), msgs, (int)(now - a->seen));
+                           strLat, strLon, 10 * log10(signalAverage), msgs, (int)(now - a->seen));
                 }
                 count++;
             }

mode_ac.c

@@ -144,7 +144,6 @@ int detectModeA(uint16_t *m, struct modesMessage *mm)
   int F1_sig, F1_noise;
   int F2_sig, F2_noise;
   int fSig, fNoise, fLevel, fLoLo;
-  int snr;
 
   // m[0] contains the energy from    0 ->  499 nS
   // m[1] contains the energy from  500 ->  999 nS
@@ -307,14 +306,7 @@ int detectModeA(uint16_t *m, struct modesMessage *mm)
   if ((ModeABits < 3) || (ModeABits & 0xFFFF8808) || (ModeAErrs) )
     {return (ModeABits = 0);}
 
-  // snr = 5 * 20log10(fSig / (fSig+fNoise))         (in units of 0.2dB)
+  mm->signalLevel = 1.0 * TRUE_AMPLITUDE(fSig + fNoise) * TRUE_AMPLITUDE(fSig + fNoise) / MAX_POWER;
-  //     = 100log10(fSig) - 100log10(fSig+fNoise)
-  while (fSig > 65535 || (fSig + fNoise) > 65535) {
-      fSig >>= 1;
-      fNoise >>= 1;
-  }
-  snr = Modes.log10lut[fSig] - Modes.log10lut[fSig + fNoise];
-  mm->signalLevel = ((snr < 255) ? (uint8_t)snr : 255);
 
   return ModeABits;
   }

mode_s.c

@@ -1005,7 +1005,8 @@ void displayModesMessage(struct modesMessage *mm) {
     if (mm->correctedbits != 0)
         printf("No. of bit errors fixed: %d\n", mm->correctedbits);
 
-    printf("SNR: %d.%d dB\n", mm->signalLevel/5, 2*(mm->signalLevel%5));
+    if (mm->signalLevel > 0)
+        printf("RSSI: %.1f dBFS\n", 10 * log10(mm->signalLevel));
 
     if (mm->score)
         printf("Score: %d\n", mm->score);

net_io.c

@@ -293,7 +293,7 @@ void modesSendBeastOutput(struct modesMessage *mm) {
         if (0x1A == ch) {*p++ = ch; }
     }
 
-    *p++ = (ch = mm->signalLevel);
+    *p++ = ch = (char) round(mm->signalLevel * 255);
     if (0x1A == ch) {*p++ = ch; }
 
     for (j = 0; j < msgLen; j++) {
@@ -568,7 +568,8 @@ int decodeBinMessage(struct client *c, char *p) {
             if (0x1A == ch) {p++;}
         }
 
-        mm.signalLevel = ch = *p++;  // Grab the signal level
+        ch = *p++;  // Grab the signal level
+        mm.signalLevel = ((unsigned)ch / 255.0);
         if (0x1A == ch) {p++;}
 
         for (j = 0; j < msgLen; j++) { // and the data
@@ -627,7 +628,7 @@ int decodeHexMessage(struct client *c, char *hex) {
     // Mark messages received over the internet as remote so that we don't try to
     // pass them off as being received by this instance when forwarding them
     mm.remote      =    1;
-    mm.signalLevel = 0xFF;
+    mm.signalLevel =    0.0;
 
     // Remove spaces on the left and on the right
     while(l && isspace(hex[l-1])) {
@@ -644,7 +645,7 @@ int decodeHexMessage(struct client *c, char *hex) {
 
     switch(hex[0]) {
         case '<': {
-            mm.signalLevel = (hexDigitVal(hex[13])<<4) | hexDigitVal(hex[14]);
+            mm.signalLevel = ((hexDigitVal(hex[13])<<4) | hexDigitVal(hex[14])) / 255.0;
             hex += 15; l -= 16; // Skip <, timestamp and siglevel, and ;
             break;}
 

stats.c

@@ -110,6 +110,22 @@ void display_stats(struct stats *st) {
             printf("%d phase enhancement attempts\n",                 st->out_of_phase);
             display_demod_stats("phase enhanced ", &st->demod_phasecorrected);
         }
+
+        if (st->noise_power_count) {
+            printf("%.1f dBFS noise floor\n",
+                   10 * log10(st->noise_power_sum / st->noise_power_count));
+        }
+
+        if (st->signal_power_count) {
+            printf("%.1f dBFS mean signal power\n",
+                   10 * log10(st->signal_power_sum / st->signal_power_count));
+        }
+
+        printf("%.1f dBFS peak signal power\n",
+               10 * log10(st->peak_signal_power));
+
+        printf("%u messages with signal power above -3dBFS\n",
+               st->strong_signal_count);
     }
 
     printf("%d remote messages accepted\n"
@@ -206,6 +222,23 @@ void add_stats(const struct stats *st1, const struct stats *st2, struct stats *t
     // total messages:
     target->messages_total = st1->messages_total + st2->messages_total;
 
+    // noise floor:
+    target->noise_power_sum = st1->noise_power_sum + st2->noise_power_sum;
+    target->noise_power_count = st1->noise_power_count + st2->noise_power_count;
+
+    // mean signal power:
+    target->signal_power_sum = st1->signal_power_sum + st2->signal_power_sum;
+    target->signal_power_count = st1->signal_power_count + st2->signal_power_count;
+
+    // peak signal power seen
+    if (st1->peak_signal_power > st2->peak_signal_power)
+        target->peak_signal_power = st1->peak_signal_power;
+    else
+        target->peak_signal_power = st2->peak_signal_power;
+
+    // strong signals
+    target->strong_signal_count = st1->strong_signal_count + st2->strong_signal_count;
+
     // CPR decoding:
     target->cpr_global_ok = st1->cpr_global_ok + st2->cpr_global_ok;
     target->cpr_global_bad = st1->cpr_global_bad + st2->cpr_global_bad;

stats.h

@@ -91,6 +91,20 @@ struct stats {
 
     struct timespec cputime;
 
+    // noise floor:
+    double noise_power_sum;
+    uint32_t noise_power_count;
+
+    // mean signal power:
+    double signal_power_sum;
+    uint32_t signal_power_count;
+
+    // peak signal power seen
+    double peak_signal_power;
+
+    // number of signals with power > -3dBFS
+    uint32_t strong_signal_count;
+
     // remote messages:
     unsigned int remote_accepted;
     unsigned int remote_rejected;