OhMyCod:
At the moment I'm testing for data at the port using the available() function. Is there any interupt that I can use to tell me when some data has arrived? If there was, I might be able to have a routine that as soon as more than say 30 bytes arrived I could copy the data to another bigger buffer and process it later when nothing else was happening.cheers
what I have done (probably not the best way, but it works well) :
I have a "readGPS" function with a while(Serial.available()) {.......}
wich reads each character, until it is a "13" , or until the number of char read is 159 (just to be sure I don't stay in the while loop and I don't write after the last byte of the string ) ,
and put them in a string (which is 160 byte long ) .
I call that function in the loop() , until I get the string I need (which begin with GPRMC or GPGTV)
It exists a interruption on serial event, but it didn't match my needs 
edit : here http://arduino.cc/en/Tutorial/SerialEvent