Dear Srivatsa:
Regarding the drift Gyroscope and Accelerometer sensors may be used in combination.
One measure the rate of rotation and the other used for orientation
For instance, A 3axis gyroscope shows the rate of rotation on its X(side) ,Y(front) and Z(vertical) axes.
Z-axis(for heli) output may help to understand/co-relates the cyclic rotation created by the main rotor, accordingly tail rotor speed may be regulated to counterbalance rotational force.
Secondly, it also helps to counter balance the air pressure. where all the axis play together.
Using an accelerometer, we get axis tilt output - i.e. the tilt of the body.
For instance, a 3-axis accelerometer will provide tilt data on axis X,Y,Z.
Regarding altitude:from GPS you get altitute data with speed, latitude, longitude, time. The accuracy of which majorly depends on the trigonometric calculation of the number of satellites tracked/visible to the GPS.
You may datalog the path of travel using lat & long data.
if you just wish to measure only altitude, you may also use a barometric pressure sensor - we have such sensor - named altimeter
Here is the link -
http://shop.sumeetinstruments.com/index.php?route=product/category&path=35_54http://shop.sumeetinstruments.com/index.php?route=product/category&path=35_56