AP_Airspeed: add MATLAB based NMEA sensor example

libraries/AP_Airspeed/models/NMEA_Sensor.m

@@ -0,0 +1,55 @@
+clc
+clear
+
+% This is a example of using MATLAB to test a sensor drive using TCP
+% we send a NMEA messages at 1hz, note that this assumes that SITL is
+% running at real time.
+
+% NMEA is a basic one way protocol but more complex protocols can be
+% implemented in the same way
+
+% Use the same TCP/UDP libbary that is used for MALTAB SITL
+addpath(genpath('../../SITL/examples/JSON/MATLAB/tcp_udp_ip_2.0.6'))
+
+% if this dosn't work try the MALTAB SITL example first
+pnet('closeall')
+
+% Init the TCP port, 5763 is serial 2
+u = pnet('tcpconnect','127.0.0.1',5763);
+
+flipflop = true;
+while(true)
+    
+    if flipflop
+        % send MTW temp message
+        water_temp = 10 + randn();
+        NMEA_string = sprintf('$YXMTW,%0.1f,C',water_temp);
+    else 
+        % send VHW speed message
+        water_speed_knots = 5 + randn()*2;
+        water_speed_kph = water_speed_knots * 1.852;
+        NMEA_string = sprintf('$VWVHW,,T,,M,%0.1f,N,%0.1f,F',water_speed_knots,water_speed_kph);
+    end
+    flipflop = ~flipflop;
+
+    % Calculate the correct checksum
+    NMEA_string = add_checksum(NMEA_string);
+
+    % send to ap
+    pnet(u,'printf',sprintf('%s\r\n',NMEA_string));
+    pnet(u,'writepacket');
+
+    % also print to MATLAB console
+    fprintf("%s\n",NMEA_string);
+
+    % 1hz (ish)
+    pause(1);
+end
+
+function NMEA_string_out = add_checksum(NMEA_string_in)  
+    checksum = uint8(0);
+    for i = 2:numel(NMEA_string_in)
+        checksum = bitxor(checksum,uint8(NMEA_string_in(i)),'uint8');
+    end    
+    NMEA_string_out = sprintf('%s*%s',NMEA_string_in,dec2hex(checksum));
+end