Isolation btw GSM and GPS antenna

It is the band selection filters and selectivity of the GPS receiver that will make if the GPS receiver has significant influence of the GSM transmission signals.

figure out;

1- the GPS signal input @ sensitivity, in terms of a dBm/Hz. This is the minimum input power for the GPS signal.
2-Figure out how much interfering signal you want to allow for. I'd start out with a interferer power level 10 dB below the minimum GPS signal power. This will degrade the sensitivity by ~ 0.25 dB, not too much.
3-Figure out how much power the GSM transmitter puts out at the GPS signal frequency. You need to keep in mind both high band and low band for this.

The difference between the number in 2 and the number in 3 is the isolation that you need.

Pretty straightforward.

Dave
www.keystoneradio.com