PHI - Position and Homing Indicator

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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.


On the lower part of the plane there were 4 antennas with radial beams facing the ground. During the flight the return signals of the two front beams, compared to the two rear beams had a different frequency, this difference was proportional to the speed of the aircraft on the ground. With this speed difference technique, it was possible to establish the speed with high accuracy. The accuracy, however, decreased in the case of mountainous terrain or in turns. Therefore it was necessary to have two groups of beams to average the values and thus obtain a less error-prone result.

It is important to remember that the technique was possible with the constant improvement of the quality of radars and their simultaneous miniaturization due to the rapid development of this device during and after the Second World War. The PHI model used in the FIAT G91 and in the F104s from the A-B to G series and in various civil aircraft including the DC8, was mechanical (ball integrator) and with tube electronics, no transistors were still used.

This fact made the instrument rather bulky for a small and compact aircraft like the G91 and therefore its use considerably reduced the space in the forward avionics compartment where the processing unit was located.

The reliability was good, certainly superior to that of the radars of the time, but it certainly could represent a problem for long flights in sparsely inhabited areas without radio aids to navigation.

The apparatus required a lot of power, about 150W, partly with 28V voltage and partly with 400 Hz alternating at 115V needed to drive the various synchronous motors and related actuators.

Parts constituting the apparatus

Within various spaces within the aircraft:

  • 4-beam doppler transmitter and relative antenna positioned on the underside of the aircraft.
  • Computer unit.
  • Signal amplifier and Junction-Box.
  • Pitot tube to obtain the static and dynamic pressure without interfering with that used by the pneumatic instruments on board.
  • Gyro Compass with fluxgate magnetic sensor.

Inside the pilot's cabin:

  • Indicator Unit normally placed in front of the pilot at the top of the cockpit.
  • Wind control unit, a device necessary to enter the wind speed and direction in order to automatically calculate the deviation due to the wind.
  • Selector unit, keypad necessary to activate the device, together with a switch / programmer of the (1 + 4) possible routes.
  • Doppler radar management unit.

This long list of devices highlights the complexity of such an instrument which at the time cost about as much as a general aviation aircraft.