Trying to use Arduino Uno R3 - with USB Host Shield, LCD, and USB Sensor

Projects Programming
2 
Serial.write("$PSPA,PR\r\n");  //command to get pitch and roll


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{PSPA=Serial.read();
    if (PSPA == '!')
    {
     // Get Length of NMEA Sentence
      mLength = Serial.read();

Make a request. Expect a response within a few nanoseconds. I doubt that your device is THAT fast.