As we know, we can calculate altitude from barometer readings, and a UAV can hold height by referring to these data. However the real air pressure is varying with many conditions. These variance will cause the UAV's height unstable. Is it possible to avoid height drifting due to pressure changes using sensor fusion without GPS?
You could not prevent long term barometer offset just by using the barometer and IMU, as both are accurate in the fairly short term but both are susceptible to drifting over time, as pressures change over the course of the day or error builds in your IMU position solution.
Over a Baro only solution a Baro + IMU will perform better, but again will not provide long term stability.
If you are looking for a long term solution I would suggest an Ultrasonic sensor or 1D LIDAR sensor to provide an absolute altitude reading.