A portable head up display system for an aircraft comprising: a display, for disposition in a cockpit of the aircraft in a forward line of sight of a pilot when seated in a pilot's seat in the cockpit; a camera, for disposition on the aircraft to provide a field of view that at least partially intersects the line of sight of the pilot when seated in the pilot's seat in the cockpit; and a processor; wherein: the display is configurable to display an image captured by the camera of the external environment of the aircraft that the pilot would observe in the direction of the display in the absence of the display and any aircraft objects; the processor is adapted to receive avionic information from instrumentation, either of the aircraft or of the system; the processor is adapted to calculate, on the basis of the instrumentation, a first flight path marker (FPM I); characterised in that the processor is adapted to calculate, on the basis of flow of pixels in successive images captured by the camera, a second flight path marker (FPM O), to correlate with the FPM I; and the processor is adapted to overlay, on the image displayed by the display, one of said first and second flight path markers, or a combination thereof and provide an alarm if said correlation indicates a lack of correspondence between FPM I and FPM O greater than a predetermined threshold. a display, for disposition in a cockpit of the aircraft in a forward line of sight of a pilot when seated in a pilot's seat in the cockpit; a camera, for disposition on the aircraft to provide a field of view that at least partially intersects the line of sight of the pilot when seated in the pilot's seat in the cockpit; and a processor; wherein: the display is configurable to display an image captured by the camera of the external environment of the aircraft that the pilot would observe in the direction of the display in the absence of the display and any aircraft objects; the processor is adapted to receive avionic information from instrumentation, either of the aircraft or of the system; the processor is adapted to calculate, on the basis of the instrumentation, a first flight path marker (FPM I); characterised in that the processor is adapted to calculate, on the basis of flow of pixels in successive images captured by the camera, a second flight path marker (FPM O), to correlate with the FPM I; and the processor is adapted to overlay, on the image displayed by the display, one of said first and second flight path markers, or a combination thereof and provide an alarm if said correlation indicates a lack of correspondence between FPM I and FPM O greater than a predetermined threshold. the display is configurable to display an image captured by the camera of the external environment of the aircraft that the pilot would observe in the direction of the display in the absence of the display and any aircraft objects; the processor is adapted to receive avionic information from instrumentation, either of the aircraft or of the system; the processor is adapted to calculate, on the basis of the instrumentation, a first flight path marker (FPM I); characterised in that the processor is adapted to calculate, on the basis of flow of pixels in successive images captured by the camera, a second flight path marker (FPM O), to correlate with the FPM I; and the processor is adapted to overlay, on the image displayed by the display, one of said first and second flight path markers, or a combination thereof and provide an alarm if said correlation indicates a lack of correspondence between FPM I and FPM O greater than a predetermined threshold.