Added a function that retrieves RSSI values from Mode-S messages received by PiAware devices

pyModeS/extra/tcpclient.py

@@ -6,6 +6,7 @@
 import pyModeS as pms
 import traceback
 import zmq
+import math
 
 
 class TcpClient(object):
@@ -149,6 +150,103 @@ def read_beast_buffer(self):
             messages.append([msg, ts])
         return messages
 
+    def read_beast_buffer_rssi_piaware(self):
+        """Handle mode-s beast data type.
+
+        <esc> "1" : 6 byte MLAT timestamp, 1 byte signal level,
+            2 byte Mode-AC
+        <esc> "2" : 6 byte MLAT timestamp, 1 byte signal level,
+            7 byte Mode-S short frame
+        <esc> "3" : 6 byte MLAT timestamp, 1 byte signal level,
+            14 byte Mode-S long frame
+        <esc> "4" : 6 byte MLAT timestamp, status data, DIP switch
+            configuration settings (not on Mode-S Beast classic)
+        <esc><esc>: true 0x1a
+        <esc> is 0x1a, and "1", "2" and "3" are 0x31, 0x32 and 0x33
+
+        timestamp:
+        wiki.modesbeast.com/Radarcape:Firmware_Versions#The_GPS_timestamp
+        """
+        messages_mlat = []
+        msg = []
+        i = 0
+
+        # process the buffer until the last divider <esc> 0x1a
+        # then, reset the self.buffer with the remainder
+
+        while i < len(self.buffer):
+            if self.buffer[i : i + 2] == [0x1A, 0x1A]:
+                msg.append(0x1A)
+                i += 1
+            elif (i == len(self.buffer) - 1) and (self.buffer[i] == 0x1A):
+                # special case where the last bit is 0x1a
+                msg.append(0x1A)
+            elif self.buffer[i] == 0x1A:
+                if i == len(self.buffer) - 1:
+                    # special case where the last bit is 0x1a
+                    msg.append(0x1A)
+                elif len(msg) > 0:
+                    messages_mlat.append(msg)
+                    msg = []
+            else:
+                msg.append(self.buffer[i])
+            i += 1
+
+        # save the reminder for next reading cycle, if not empty
+        if len(msg) > 0:
+            reminder = []
+            for i, m in enumerate(msg):
+                if (m == 0x1A) and (i < len(msg) - 1):
+                    # rewind 0x1a, except when it is at the last bit
+                    reminder.extend([m, m])
+                else:
+                    reminder.append(m)
+            self.buffer = [0x1A] + msg
+        else:
+            self.buffer = []
+
+        # extract messages
+        messages = []
+        for mm in messages_mlat:
+            ts = time.time()
+
+            msgtype = mm[0]
+            # print(''.join('%02X' % i for i in mm))
+
+            if msgtype == 0x32:
+                # Mode-S Short Message, 7 byte, 14-len hexstr
+                msg = "".join("%02X" % i for i in mm[8:15])
+            elif msgtype == 0x33:
+                # Mode-S Long Message, 14 byte, 28-len hexstr
+                msg = "".join("%02X" % i for i in mm[8:22])
+            else:
+                # Other message tupe
+                continue
+
+            if len(msg) not in [14, 28]:
+                continue
+
+            '''
+                we get the raw 0-255 byte value (raw_rssi = mm[7])
+                we scale it to 0.0 - 1.0 (voltage = raw_rssi / 255)
+                we convert it to a dBFS power value (rolling the squaring of the voltage into the dB calculation)
+            '''
+
+            df = pms.df(msg)
+            raw_rssi = mm[7] # eighth byte of Mode-S message should contain RSSI value
+            rssi_ratio = raw_rssi / 255 
+            signalLevel = rssi_ratio ** 2 
+            dbfs_rssi = 10 * math.log10(signalLevel) 
+
+            # skip incomplete message
+            if df in [0, 4, 5, 11] and len(msg) != 14:
+                continue
+            if df in [16, 17, 18, 19, 20, 21, 24] and len(msg) != 28:
+                continue
+
+            messages.append([msg, dbfs_rssi, ts])
+        return messages
+
     def read_skysense_buffer(self):
         """Skysense stream format.