[charoncs]

Do you know any GPS receiver that gives us the appropriate carrier phase measurements , so we can calculate relative positions in cm level? As I know Novatel and Trimble receivers have this capability. However they are so expensive.

[typer]

I'm currently using the Ublox LEA-6T evaluation kit (EVK-6T), single frequency receiver with 10Hz, costs USD 349 + shipping. Works with the application RTKLib (open source, developed in japan).

RTKLib also supports the cheaper SkyTraq S1315F, although I have not tested this. The chip has been tested on the michelebavaro blog.

You could take a look at rtklib.com for their supported chips.