Could anyone give me an information about the VCO requirement in the GPS system? I means what is the phase noise or required output power do they need usually?
Because GPS is a spread spectrum system, phase noise is not a critical parameter, and even a value around 80–85 dBc/Hz at 100 kHz offset is still acceptable.
Normal values for LO phase noise of the GPS receivers on the market are better than –95dBc/Hz @ 100kHz.
Output level of the LO is dictated by the mixer type, and in this case could be somewhere between –10dBm and 0dBm.
GPS is spread spectrum modulation (there's no bands, just one common band with all the signals on top of each other) so you just design for a fixed frequency which ensures downconversion of the GPS signal to the IF you have chosen.