PHI - Position and Homing Indicator
When you look at the cockpit of the G91 (but also of the F104) you can see in the center at the top a very particular instrument, particularly large and with 4 knobs on the sides. That instrument was one of the first navigation systems that used Doppler radar to determine ground speed.
The accuracy of the instrument was, for those times, rather high, as it allowed to travel a long route up to 999 miles with a deviation in the position of the aircraft of 2-3%, a contained error that could be exploited both for flights both military and civilian in the transatlantic route, and in tactical flight in enemy territory or in areas with little radio assistance coverage. However, the real disadvantage in military use was that the instrument allowed to achieve this accuracy only with the use of the doppler radar, in the event that the pilot had disengaged the radar device, in order to be less visible to the enemy, the accuracy decreased a lot and was linked to the real knowledge of the local weather situation.