1. A method of generating an image of a volume ahead of an aerial vehicle, the method comprising: determining a position of the aerial vehicle using global positioning system (GPS) position data when GPS position data is available, and when GPS position data is not available, determining the position of the aerial vehicle by: determining an altitude of the aerial vehicle using a radar altimeter; and calculating the position of the aerial vehicle by correlating the altitude with a digital terrain elevation map and correlating attitude data from an inertial measurement unit (IMU) pertaining to an attitude of the aerial vehicle with the elevation of the aerial vehicle; generating a terrain image corresponding to ground features correlated to the position of the aerial vehicle; determining obstacle data pertaining to a set of obstacles ahead of the aerial vehicle with a forward looking sensor; generating an obstacle overlay image; and overlaying the obstacle overlay image onto the terrain image to generate a composite image, wherein the IMU is operable to smooth the transition between when GPS position data is available and when GPS position data is not available by: determining a position difference between the GPS position data and the position data calculated by correlating the altitude with a digital terrain elevation map; and establishing an offset for the GPS position data and an offset for the position data calculated by correlating the altitude with a digital terrain elevation map based upon the position difference.