Upgrade your APM2.5 to 2.6 by adding this new external GPS/Compass module.

This new design incorporates the HMC5883L digital compass, providing a convenient method of mounting the compass away from sources of interference that may be present in the confines of the vehicle. This is our best GPS module, significantly outperforming the Mediatek GPS due to the larger antenna and next-gen chipset. An excellent solution for multicopters and rovers in particular, where GPS accuracy is paramount. It features active circuitry for the ceramic patch antenna, rechargeable backup battery for warm starts, and I2C EEPROM for configuration storage. The units ship pre-configured for use with 3DR APM 2.6 autopilot; the configuration file is available in the repository.

See here how to upgrade your APM2.5 to 2.6 -