PHI - Position and Homing Indicator

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PHI - Position and Homing Indicator

When you look at the cockpit of the [FIAT 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.

PHI mod MK2 - Principle of operation from confidential doc AD091920[1] (1955)

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.

The main documentation found on the net that technically describes the operation of the device is that of the PDF file: AD091920 aut. RL JEFFERIES year 1955 [1]. This device performance evaluation document gives a clear idea of how it was designed and the parts that make it up. Its reading is really useful and has allowed us to create a sufficiently real model for an instrument that has not been operational for at least 20 years.

Programming the routes

PHI Station Selector - NCU in the FIAT G91R1B airplane

Programming The instrument allows you to sequentially follow up to 5 routes stored in the Station Selector (unit with the code NCU located to the right of the pilot). A route consists of two values: N-S and E-W coordinates measured in air miles of the next point [1]. The NCU allowed the insertion of a box containing 4 multiturn resistors (10 turns) to enter the distance N-S and other 4 multiturn resistors to enter the distance E-W. Once the route was planned, the navigator made a list of max. 5 pairs of coordinates, the first pair was not inserted into the device, but only communicated to the pilot, who set it on the display, the other 4 pairs were set by a technician using a special programming device and a screwdriver that turned the potentiometers placed on the programming box. The cassette could be inserted into the compartment with a light pressure and extracted using the release button [2] dell NCU unit.
Programming The instrument allows sequentially follow up to 5 routes stored in the Station Selector (unit with the code NCU located to the right of the pilot). A route consists of two values: N-S and E-W coordinates measured in air miles of the next point [1].
In the simulation, as will see later, the two coordinates have been replaced by a distance and an angle (heading), with respect to the geographic North. In this way it is possible to quickly integrate the polar coordinate programming method of the routes obtained from the route manager of the FGFS program with the N-S and E-W system used by the PHI.

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.

  1. 1.0 1.1 1.2 1.3 The canadian position and homing indicator mk.2 doc: AD091920
    CLASSIFICATION CHANGES. TO: unclassified. FROM: confidential. Approved for public release, distribution unlimited. Aut. RL JEFFERIES · ‎1955
  2. The release button in the G91R1B works, and then ejects/retracts the program cassette.