Introduction
1. Early in the history of radio, it became obvious that, if a station could determine the direction from which a signal was coming, that information could be very useful for navigation. Marconi had actually patented a method of radio direction finding in 1905. Of course, the military were the first to use it for their own purposes. Presently, VHF direction finding provides a useful aid to navigation in the air and to identification for Air Traffic Control.
Principles
2. A standard half-wave dipole antenna receives signals from all directions equally, and has a horizontal polar diagram in the form of a circle, as in Fig 4-1(a). If two half-wave dipole antennas are held in a position half a wavelength apart, their reception and
re-transmission characteristics are such that their horizontal polar diagrams form a figure of 8 as in Fig 4-1(b). This means that if the pair of antennas are rotated, they will receive varying signal strengths from maximum to zero. Because there are two very sharp null points, it is easy to find the direction of an incoming signal. However, there are two possible directions, 1800 apart, so a further pair of antennas can be employed as switchable reflectors (sense antennas) to affect the strength of signal received close to one of the nulls, as in Fig 4-1(c). An operator could note the direction of the null with the sense antennas switched off, turn back until he
received the signal again, then switch in the sense antennas. The signal strength would either increase or decrease depending on the actual direction of the signal source. This set of four antennas was referred to as an ‘Adcock' aerial.
(a) (b) (c)
Fig 4-1: DF Aerial Systems
3. This system is not efficient, requiring time and a dedicated operator. More modern systems use a number of fixed receiver directional elements arranged in a circle, pointing outwards (Fig 4-2).
Each element will receive a slightly different Fig 4-2: VHF Direction Finder
strength of signal from the aircraft. Electronic switching samples the signal strength of each of these elements in turn around the circle, very rapidly. This produces a sine curve of signal strength, one cycle per revolution.
4. At the same time, a generator in the system produces a reference signal at the same frequency as the sampling. The phases of the sampled signal and the reference signal are compared, and the phase difference equates to the direction from which the signal is coming, compared to the datum direction where the reference signal started. The display can be arranged to give either true or magnetic bearings, or both.
5. Older displays use a cathode-ray tube for indicating bearings. With this type of equipment, the direction of the received wave is displayed on the tube instantaneously as a trace and the bearing is read off against a scale. Some more modern units use liquid crystal digital displays, showing the required bearing in the form of numbers. This can be either in a separate display, or as a part of a larger display unit. Bearings can also be displayed as lines on the same display as a radar screen, and this is used by radar controllers on the ground to assist in identification of aircraft (displays are described in chapter No. 12).
Services
6. A ground VHF direction finding station can give true or magnetic bearings when requested by a pilot. For brevity, the requests can be expressed either in plain language or in what we call the 'Q code', as follows;
(a) QTE = The aircraft's true bearing from the station.
(b) QUJ = The aircraft's true track to the station (the opposite to QTE).
(c) QDR = The aircraft's magnetic bearing from the station.
(d) QDM = The aircraft's magnetic track to the station (the opposite to QDR).
QTEs and QDRs are normally used in en-route navigation as position lines. QDMs are requested when a pilot wishes to home to the station. QUJ is generally only used as a stage in navigation calculations.
7. Bearings from three or more stations are used to give triangulation fixes. VDF stations are plotted on a central map. QTEs from each of them to a particular transmitting aircraft can be plotted on that map, and where the QTEs intersect, is the position of the aircraft. The QTEs can be transmitted either verbally, or automatically by electronic means. The more QTEs that can be plotted, the more accurate the resulting fix. The fix can be transmitted to the pilot by an operator looking at that central map. Two bearings can give a poor quality fix, but three is regarded as the minimum acceptable for safety.
8. Direction finding stations do not guarantee their service and they can refuse to pass bearings to pilots if the conditions are poor or the bearings do not fall within the classification limits of the station. They will of course give the reasons for any refusal.
9. Classification of Bearings. According to the judgement of the operator, bearings are classified according to their accuracy as follows:
(a) Class A - Accurate to within + 2°.
(b) Class B - Accurate to within + 5°.
(c) Class C - Accurate to within + 10°.
(d) Class D - Less accurate than class C.
The controller will pass the bearing with its classification in the form 'Your true bearing 247°, class alpha'.
10. Scope of the Service. There are many automatic VDF stations intended purely to assist in radar identification for ATC purposes. These stations are not listed in the FLlP for the obvious reason that they do not provide a normal DF service to aircraft. The stations that are listed in the FLlP provide a normal 'homer' service. Generally the class of bearing is not better than class B. Automatic VDF stations should not be used as en-route navigation aid but their service is always available in an emergency situation or when other essential navigation aids have failed.
Factors Affecting Range
11. Being a VHF emission, the range will primarily depend on the height of the transmitter and receiver i.e. the line-of-sight range. As we have seen from equation (2.2), the maximum range may be found by the formula D = 1.25 √HT + 1.25 √HR Under normal circumstances, using a station close to sea level, an approximation may be calculated quickly by a simpler formula:
D = 12√F (4.1)
where D is the maximum range in nautical miles F is the aircraft's flight level.
This simpler formula is recommended for practical use.
12. Other factors, for example the power of the transmitter, intervening high ground, etc. will also affect the range. In addition, as explained earlier, the station may receive both the direct wave and a ground-reflected wave, in which case fading might be experienced or the signals may be lost completely for a time, until the aircraft changes its position.
13. The aircraft's attitude when transmitting may also affect the results. In general aviation, VHF communication transmission is vertically polarised. The best reception will occur when the signal
arriving at the ground aerial is vertically polarised. If the aircraft attitude places the transmitter antenna in the horizontal plane, no signal will be received at the ground station. Normally, such an extreme will not occur, but range can be reduced as the polarisation alters from vertical.
Factors Affecting Accuracy
14. If an aircraft's transmission has been reflected by either uneven terrain or obstacles during its travel to the receiver, the aerial will receive the signals from a direction other than the original transmission. A similar effect occurs if the signals arrive at the aerial site directly, but suffer reflection from the ground or obstructions before entering the aerial itself. Therefore two factors affecting the accuracy of VDF are:
(a) Propagation error.
(b) Site error - This is by far the most significant error in most systems.
In addition, when the aircraft is nearly overhead the station, the signal will not be received at the station's aerial. Where this occurs we call it the 'cone of no bearing'. A problem can also occur when two aircraft transmit at the same time. The VDF station receives signals from both at the same time, which changes the shape of the sine curve received, and the resultant calculated bearing, in proportion to the relative strengths of the two signals.
VHF Let-Down Service
15. The VHF let-down service, available throughout the world, has the primary advantage that the aircraft does not require any specialist equipment to carry out a let-down (descent and approach to land). It is therefore useful if airborne navigation aids fail. The stations, which provide this service, are listed in the FLlP with their frequencies and callsigns. Details of the actual procedures are also published in the FLlP, and extracts appear also in various commercial publications, such as Jeppesen charts.
16. Two types of procedure are in current use, the VDF procedure and the QGH procedure.
Generally, the VDF procedure is available at all airfields with published VDF stations, but QGH is only available at airfields annotated as such in the FLlP. Where both procedures are available, they will follow the same let-down pattern.
17. VDF Let-Down (Free Let-Down). For a VDF let-down, the pilot calls the station and requests 'VDF let-down'. The station gives the pilot a series of QDMs (also termed QDL) in reply to his frequent transmissions. The pilot uses these QDMs to orientate himself, making his own calculations, including allowance for wind, to achieve the published approach pattern for landing.
18. QGH Let-Down (Controlled Descent). With the QGH procedure, the pilot is given a series of headings to steer by the controller, who assumes responsibility for directing the aircraft in the pattern using QDMs from frequent transmissions. The aircraft is first homed-in to the overhead at a safe altitude, and then directed along an outbound leg. When safely 'established' on the outbound leg, the controller instructs the pilot to descend. After a calculated time, the aircraft is turned back 'inbound' towards the airfield.
19. On the inbound leg, the controller will give the pilot headings to steer to the airfield, and clearance to descend to minimum descent altitude, at which altitude the aircraft can continue to home to the overhead until the airfield is in sight. If the pilot reaches the overhead again without seeing the airfield, he must go-around and climb away for another attempt or divert.
CHAPTER 5