1. HRDF (High Resolution Direction Finder) Purpose, Principles & Aerial System

2. Doppler
A change in frequency of an RF signal caused by the relative motion between the source and the observer
Doppler is a natural phenomenon

3. Doppler
Stretch
Squeeze
Develop the idea of what you would hear if a police car with it’s siren sounding was moving towards you and then moving away from you
Q. Is there a difference in the sound of the siren when the car is coming towards you, then away from you
Yes
Q. Why do you think this is?
When moving towards you the frequency of the siren is squeezed and when moving away from you it is stretched
The result is Frequency Modulation
Relative movement of a sound source towards and away from a fixed point causes frequency modulation or a Doppler effect
This is a natural phenomenon

4. Doppler C D A B
Q. If blindfolded, would you be able to accurately determine from what direction the car was coming from?
No. You could tell whether the car was coming towards or away from you, but not accurately

5. Doppler Squeeze Stretch
A radio transmission from an aircraft would be affected in exactly the same way.
When the aircraft is moving towards a fixed reception point
- the frequency of the transmission will be squeezed
When the aircraft is moving away from a fixed reception point
the frequency of the transmission will be stretched
in exactly the same way no matter which direction the aircraft came from

6. Doppler
(1, 2) As with the police car example, using a single uni-polar antenna we would be able to detect the squeeze and stretch of the aircraft radio transmission, but would not be able to determine the direction of the aircraft
We would get squeeze and stretch on a radio transmission irrespective of wherever it was coming from
(3) In addition to this, stationary aircraft such as helicopters also need to be displayed by the HRDF system
The HRDF system therefore needs to produce its own ‘man-made’ dopper effect

7. Doppler Q. Using a single aerial, how could you make it directional?
A. Add a reflector to your system
(1) … Reflector
(2) … Then as long as we know the position of the reflector, we could determine the direction of the aircraft
(3) … Rotate reflector
Q. What are the disadvantages of this solution?
A. Miss a transmission if not ‘looking’ in that direction
A. Moving mechanical parts
A. Cost
Q. How can we get the rotation we require without moving reflector
A. Circular aerial array

8. 6 7 9 8 10 11 5 4 3 12 13 16 17 18 1 15 2 14
+fm
-fm fc 6 10
The stretch and squeeze on the signal is Frequency Modulation and can be represented graphically as a maximum +Ve, No and maximum –Ve
Modulation frequency.
If we consider a stationary helicopter transmitting from north of our aerial array that has aerial 1 at west.
Remembering the aerials are being switched on in a clockwise direction:
Q. Remembering the direction of rotation, is Ae1 moving away or towards the helicopter?
Towards
Q. What will be the effect on the transmission at that time?
The transmission would be squeezed – (maximum squeeze ie Max +Ve Frequency Modulation)
Q. What will be the effect on the transmission at aerial 6?
Neither squeeze or stretch – (hence no modulation)
Q. What will be the effect on the transmission at aerial 10?
The transmission would be stretched – (maximum stretch ie Max –Ve Frequency Modulation)
Q. What will be the effect on the transmission at aerial 15?
Neither squeeze or stretch
The graph shows the relative amount of squeeze and stretch or frequency deviation for any aerial in one revolution of rotation 1 15

9. 6 7 9 8 10 11 5 4 3 12 13 16 17 18 1 15 2 14
+fm
-fm fc 6 10 1 15
Lets say the helicopter now moves around to south of the aerial array and then transmits from there
Q. What will be the effect on the transmission at aerial 1?
The transmission would be stretched – (maximum stretch)
Q. What will be the effect on the transmission at aerial 6?
Neither squeeze or stretch
Q. What will be the effect on the transmission at aerial 10?
The transmission would be squeezed – (maximum squeeze)
Q. What will be the effect on the transmission at aerial 15?
The graph shows the relative amount of squeeze and stretch or frequency deviation for any aerial in one revolution of rotation

10. 6 7 9 8 10 11 5 4 3 12 13 16 17 18 1 15 2 14
+fm
-fm fc 6 10 1 15
Consider the helicopter now moves around to west and transmits
Q. What will be the effect on the transmission at aerial 1?
Neither squeeze or stretch
Q. What will be the effect on the transmission at aerial 6?
The transmission would be stretched – (maximum stretch)
Q. What will be the effect on the transmission at aerial 10?
Q. What will be the effect on the transmission at aerial 15?
The transmission would be squeezed – (maximum squeeze)
The graph shows the relative amount of squeeze and stretch or frequency deviation for any aerial in one revolution of rotation

11. 6 7 9 8 10 11 5 4 3 12 13 16 17 18 1 15 2 14
+fm
-fm fc 6 10 1 15
Finally, the helicopter now moves around to east and transmits
Q. What will be the effect on the transmission at aerial 1?
Neither squeeze or stretch
Q. What will be the effect on the transmission at aerial 6?
The transmission would be squeezed – (maximum squeeze)
Q. What will be the effect on the transmission at aerial 10?
Q. What will be the effect on the transmission at aerial 15?
The transmission would be stretched – (maximum stretch)
The graph shows the relative amount of squeeze and stretch or frequency deviation for any aerial in one revolution of rotation

12. Pseudo Doppler
The man made frequency modulation induced by the electronic rotation of the HRDF aerial array is what we will now refer to as Pseudo Doppler
It is the phase of this Pseudo Doppler that will ultimately determine the bearing of the aircraft

13. UHF Aerial Array 18 unipole aerials, 1.67m in diameter
7.62m diameter counterpoise 3m above the ground
fm is ± MHz
fm is ± 400 MHz
fm is ± 60Hz at 400MHz
fm is ± 33Hz at 225MHZ

14. Auxiliary Aerial
It is the phase of the Pseudo Doppler induced by the electronic rotation of the HRDF aerial array that will ultimately determine the bearing of the aircraft
We need to ensure that Aircraft Doppler (frequency modulation produced by the aircraft movement) is not considered in bearing calculation process
(1) … It is the phases of ...
Earlier we did the pseudo doppler phase calculations using a stationary helicopter
Q. What else would influence these calculations for we swapped the helicopter with a conventional aircraft?
Aircraft movement would introduce some frequency modulation (Doppler) on the transmission
(2) … We need to ensure

15. Auxiliary Aerial Pseudo Doppler + Aircraft Doppler only10 11 13 12 14 15 9 8 7 16 17 2 3 4 5 1 6 18
Pseudo Doppler +
Aircraft Doppler
Aux
Aircraft Doppler only
Q. What doppler will be produced from the commutating array?
A. Pseudo Doppler due to the electrical rotation of the array and Aircraft Doppler due to the movement of the aircraft
Q. What doppler will be produced from the auxiliary aerial?
A. It is a single aerial, so only Aircraft Doppler
We will see later:
auxiliary aerial output is essentially subtracted from the commutating array output to remove the Aircraft Doppler, leaving only the Pseudo Doppler with which to calculate the aircraft bearing

16. Advantages Low Operating Cost Requires little aircraft equipment
Interface with Console

17. Any Questions?