Why a SAW filter is used at the LNA input or output?

I bought a GPS Patch antenna with a LNA, the specifications say that some LNA has a SAW filter in the input or at the output, some one know why the are using SAW filters at the LNA input or at the output, wich is the difference and If I put the SAW filter I will creases the noise figure?.

SAW filter before LNA increase the NF by IL of SAW filter, SAW after LNA effects it's linearity -> P1DB is decreased by IL of SAW filter