How to improve the multipath effect of GPS?

I designed a LNA for GPSMET, i.e. GPS meteorology to detect the water vapor in the atmosphere.
But how to improve the multipath effect?
My LNA configuration is as following:
L1 Filter
LNA1 + Power Divider+ + Power combiner + LNA2 + Bandpass (L1+L2) + LNA3.
L2 Filter

Any suggestions is welcome.
Thanks.

I pasted the system design as a picture.