Im doing a similar project but sending my gps tracking unit up via air balloon... so im no expert but it seems that you are starting the gsm unit at one baud rate then changing it to a lower baud rate.
Sim900Serial.begin(115200); // the GPRS baud rate
delay(500);
Sim900Serial.println("AT+IPR=19200");
delay(500);
Sim900Serial.begin(19200); // the GPRS baud rate
try just using the 19200 the sim900 calls for.