I would like to use a 430mhz RF antenna and a 1575mhz GPS antenna. Both antennas are chip and drive current in the ground plane to radiate. The RF antenna is only useful 1s every minute. The GPS has an LNA. I am wondering if the two antennas could share the same monolithic ground plane or should they each have a proper ground plane joined in a single point ?
The best answer would be - simulate! However, the antennas are usually designed to be effective on the particular frequency, so I believe there will be no problems using the same ground plane. Just remember to "sew" the GND plane around high frequency signal and add as much VIAs as possible to connect all the ground signals.
GPS signals are circularly polarised, GPRS signals are predominantly vertical E-Field.
Separate antennas are recommended, however there are many multi-band antennas available regrettably not for your combination of interest. Simulation and grass-roots development is probably not the path you want to travel as it requires a Finite Element modeler such as Serenade or Microwave office to form magnetic solutions. Combining the two antennas onto one plane won't be a problem provided you consider the parasitic loading effects of each one on the other and match them accordingly.