The Inertial Navigation System (INAS)
The FGR2 Phantom was fitted with an inertial navigation attack system known as INAS which used analogue technology. The equipment was designed for TSR2 by Ferranti but when that project was cancelled the Phantom inherited the capability. Designed for the ground attack role, the INAS gave a comprehensive bombing capability but most of those functions were not needed in the air defence role. The equipment provided the primary source of attitude information to the flight instruments in the FGR2 version.
Located on the right hand console in the back cockpit the two main components were the inertial navigation unit or INU and the inertial navigation control unit. The INCU allowed the navigator to align the system to true north before flight. Three inertial gyros then sensed any movement of the aircraft in flight. There were a number of alignment modes but the principle options were a full align which took 12 minutes or a set heading align which, after a pre alignment, could be completed in just 90 seconds. The quick align was less accurate but was used mainly on Quick Reaction Alert. After a full alignment, typical accuracy was 2 miles drift error per hour. The system could be updated using the radar, TACAN or by visually fixing the position.
The main panel allowed the navigator to program waypoints using a small joystick. The navigator entered the system position before alignment. Six navigation points could be entered into “destination pots”, small buttons on the lower left of the main unit. A further target of opportunity or TO could be selected using a mode switch.
Range and bearing to a selected target could be read in either cockpit. The pilot read navigation information from the horizontal situation indicator or HSI located on the main instrument panel in the lower central position. The navigator had a smaller range and bearing indicator located on his flight instrument panel.
The FG1 Phantom only had a simple and very inaccurate air position indicator which gave an estimated position based on a wind velocity which the navigator entered into the equipment.