SMS reading with Arduino + SM5100B-D

Hi again. Now i have function which will get altitude and longitude. How can i do a check up when i ask by SMS coordinates and my code will perform a certain function after that. Heres my new code:

#include <TinyGPS.h>
#include <SoftwareSerial.h> //Sarjaliikennekäskyille
#include <string.h>
#define RXPIN 2
#define TXPIN 3

char incoming_char = 0; //Tuleva viesti Serial Portilta
char mobilenumber[]="0451352086";
char textmessage[160];
int b=0;
float latitude, longitude;

TinyGPS gps;


SoftwareSerial cell(2,3); // 2 = RX(Receive) ja 3 = TX(Transmit)
SoftwareSerial uart_gps(RXPIN, TXPIN);

void startSMS();
void endSMS();
void SendGPSLocation();
void getgps(TinyGPS &gps);

void setup()
{
   Serial.begin(4800);
   cell.begin(4800);
   Serial.println("Starting SM5100B Communication...");
   Serial.println("Waiting for GPS signal...");
  
}
void loop()
{
  
 // getgps(gps);//-----------------------------------------------GPS-----------------------------------
/*while(uart_gps.available())     // While there is data on the RX pin...
    {
        int c = uart_gps.read();    // load the data into a variable...
        if(gps.encode(c))      // if there is a new valid sentence...
        {
          getgps(gps);         // then grab the data.
        }
    }
 //--------------------------------GPS LOPPUU---------------------------------------------
*/  
  
  //--------------------------------------------GSM----------------------------------
  if(cell.available() >0)
    {
     
       incoming_char=cell.read();    //Get the character from the cellular serial port.
       Serial.print(incoming_char);  //Print the incoming character to the terminal.
    }
  if(Serial.available() >0)
    {
      incoming_char=Serial.read();  //Get the character coming from the terminal
      cell.print(incoming_char);    //Send the character to the cellular module.
    }
 //----------------------------------GSM LOPPUU_---------------------------------------
     
}
void startSMS()
{
digitalWrite(13, HIGH);
cell.println("AT+CMGF=1"); // set SMS mode to text
cell.print("AT+CMGS=");
cell.print((char)13); // ASCII equivalent of "
cell.print(mobilenumber);
cell.println((char)13);  // ASCII equivalent of "
delay(500); // give the module some thinking time
}

void endSMS()
{
cell.println((char)26);  // ASCII equivalent of Ctrl-Z
delay(15000); // the SMS module needs time to return to OK status
digitalWrite(13, LOW);
}

 void SendGPSLocation()
{
  startSMS();
  getgps(gps); 
  endSMS();
}
void getgps(TinyGPS &gps)
{
  
  
  gps.f_get_position(&latitude, &longitude);
  
 cell.print("N  ");
 cell.print(latitude,5);
 cell.print(" W ");
 cell.println(longitude,5); 
  
 //Serial.print("Altitude (meters): "); Serial.println(gps.f_altitude()); cell.print(gps.f_altitude());
 //Serial.print("Course (degrees): "); Serial.println(gps.f_course()); cell.print(gps.f_course());
 
 
 unsigned long chars;
 unsigned short sentences, failed_checksum;
 gps.stats(&chars, &sentences, &failed_checksum);
  
  
}

I have done already how to send the coordinates by SMS. All i need now is to read my SMS and after that perform "void SendGPSLocation()"...

Pls i need and exact example or a straight answer which i can copy because i am so new to AT Commands....