From The Manual.
Even though the rapidFIRE is calibrated in the factory, it is possible that over time it will need recalibrating. This calibration removes any slight differences in RSSI reading between the two receiver modules.
After a firmware upgrade it is also advisable to perform a calibration.
1. Tune to RaceBand Channel 5 (or IRC channel 4, approx 5800MHz)
2. Enter the calibration menu from the Tools menu
3. Remove antennas, and ensure that you are not anywhere near a transmitting video transmitter.
4. Press the joystick center button when ready.
5. Wait for the progress bar to complete
6. Now turn on a transmitter, on the same frequency selected above, and place it about 2 meters (6 feet) from the receiver
7. Press the joystick center button when ready
8. While the progress bar is moving, move the goggles around a bit
9. Once calibration is complete, a summary screen will show the calibration status (with success/fail).
If the calibration procedure fails, then it is likely due to one of two causes. The first is that the selected channel was not ‘empty’ when the first part of the calibration was attempted. The second is that the selected channel did not have a transmitting VTX when the second part of the calibration was attempted.
Note also that calibration will fail if the module is in low power mode, since both receiver modules need to be powered up to calibrate.