I want to buy RTK GPS for UAV. My plane use Pixhawk 2.1. I don’t have knowledge about RTK GPS for UAV. If you help me, I will appreciated.

I think that a RTK Gps for a plane is quite unuseful, you’ll never reach centimetre precision during a plane mission basically because the plane minimum speed is much greater than the one of a copter.

Anyway first read the Drotek documentation about RTK (Learn section of this website) to get some knowledge then ask specific questions if something still unclear .

thank you for your interest. I must give knowledge about my plane. my plane is a VTOL. Max speed is 15 m/sn.

I have no experience with Pixhawk 2.1 , but read about many problems in Arducopter forum .

For the RTK the simplest solution is to buy the Drotek RTK in Tiny or XL versions depending on your setup and use Mission Planner to inject RTK corrections to the rover (your plane) via the mavlink telemetry.

The only doubt would be if Arduplanet configure automatically the M8P gps as Arducopter 3.5 does.

In the Arduplane change log several release reports support for RTK but never specifically for the M8P family.
The last beta release says " much improved RTK GPS support"

As a final note consider that Here+ use M8P as Drotek RTK Tiny or XL so if you find documentation about the Here+ and Arduplane it should be right also for Drotek Tiny or XL M8P GPS