Indicator indicator pointer
Fig. 12.16 OMNIRANGE RECEIVING INDICATOR
The rotatable pointer indicates the angular difference between the selected course and the magnetic heading of the aircraft. A difference in these readings indicates that a cross wind is causing the aircraft to "crab" to stay on course.
The "TO" indication will change to "FROM" when the station has been passed, and the signal is being received from the reverse direction.
The "OFF" indication, over the vertical pointer, is an alarm. The appearance of this indication, which is not normally visible, indicates that some part of the system is malfunctioning and that the meter indications are unreliable.
The movements of both deviation bar and "TO" - "FROM" flag are those of a centre zero micrommeter and will therefore give a central reading when the terminal supplies, from the comparator, are equal in value and polarity.
A difference in supply polarity sufficient to cause 150 jiA to flow through the movement will cause full scale deflection. For the deviation bar, in VOR mode, this will occur when the aircraft is 10° off the selected radial.
The "OFF" warning flag is a simple dc micrommeter arranged so that a current of 125 fiA will begin to lift the flag out of view, while 250 will take it to full scale deflection and totally out of view. The dc voltage applied to the warning flag is the sum of the dc voltage applied to the terminals of the deviation bar movement as shown.
Fig.
12.17 DEVIATION BAR AND WARNING FLAG MOVEMENTS
Auto VOR Bearing
The auto VOR receiver processes the received VOR signals and provides a continuous indication of magnetic bearing of the station from the aircraft.
Since the raw output of the auto VOR is magnetic bearing information, the aircraft heading has no effect on the indicated magnetic bearing.
The basic auto VOR magnetic bearing indicator is called the Omni Bearing Indicator (OBI). But this is rarely used these days, since the output is generally combined with compass information in a differential synchro transmitter for display on an RMI.
Auto VOR Receiver
Auto VOR indicates the bearing of the beacon from the aircraft receiver. In manual VOR the phase difference between the variable phase and the reference phase is the bearing of the receiver from the beacon. Due to this difference auto VOR uses the reciprocal of the reference phase, (~R0 in figure 21).
The 30 Hz -R0 is extracted from the 9960 Hz sub carrier at the FM detector strip consisting of amplifiers, limiters and a discriminator. It is then applied to the rotor of a phase shift resolver and the output is again advanced by 90° to give -SR0 +90°. The 90° phase advance is to facilitate phase comparison and the S indicates that the reciprocal of the reference signal, -R0, has undergone a shift in the resolver synchro.
The chopper is triggered by 400 Hz from the aircraft supply and its output phase is determined by the resulting polarity of the dc input, such that when the input dc difference is zero, the chopper output is zero. With two opposite polarity inputs the output is an antiphase 400 Hz signal.
The 400 Hz output signal from the chopper is applied as the control phase to the two phase induction motor. The fixed phase is supplied from the aircraft 400 Hz supply, which is phase shifted ± 90° for rotation to be possible.
The induction motor will turn the direction determined by the phase of the controlling signal from the chopper, and in so doing will turn the pointer of the RMI and the rotor of the resolver synchro.
As the rotor of the resolver synchro rotates, the -R0 is shifted until the -SR0 +90° is in-phase quadrature with the variable phase, V0. When this occurs, the difference in the comparator dc outputs is zero, as will be the chopper output, and the motor stops. At this point, the needle of the RMI will be indicating the magnetic bearing of the VOR beacon.
Radio Magnetic Indicator (RMI)
The RMI has a rotatable dial face which indicates the direction of flight relative to magnetic north, and a pointer that indicates the bearing of the station being received. The pointer indicates the bearing on the dial face so that it can be compared, at a glance, with aircraft heading.
Modern RMI have two pointers which may be switched to read either ADF or VOR bearings.
Fig. 12.19 RADIO MAGNETIC INDICATOR
VOR Aerial
The VOR aerial is designed to receive very high frequency horizontally polarised radio signals between the frequencies of 108 and 118 MHz. VOR shares its frequency range with the localiser of the instrument landing system, and these shares the aerial system and much of the receiver unit.
A typical VOR/LOC aerial, as fitted to a B737, is shown in figure 12.20.
The aerial is a dual element aerial with a characteristic impedance of 50 ohms. It is composed of two balanced loops enclosed in a fibreglass housing with metallic fin tips on the leading and trailing edges and the whole assembly is located on top of the fin.
VHF Nav Control Panel
/ TUNING STUB
CONNECTORS
Fig. 12.20 B737VOR/LOC AERIAL
Fig. 12.21 TYPICAL VHF NAV CONTROL PANEL
The frequency selector is used to select a frequency in the range of 108 to 118 MHz which is displayed on a frequency indicator above the frequency selector knob. Only VOR-LOC frequencies are displayed. The DME frequencies are paired with the VOR frequencies and are automatically selected with VOR frequencies. When a localiser frequency is selected, the paired glideslope frequency is automatically selected.
The frequency selector knob provides a selector control logic 2-out-of-5 binary wire code which corresponds to the selected frequency, and is used to tune the RF input circuits in the receiver.
Conventional VOR Errors
With conventional VOR (CVOR) equipments the major bearing errors are attributable to siting of the transmitting beacon and ground station error.
The major cause of error in CVOR is multipath reception. Signals arriving at the receiver may include some which arrive after reflection from objects, both natural and man-made, located in the area of the beacon. On less than ideal sites bearing errors due to multipath reception may be as high as 15° in some sectors.
The ground station error results from failure of the 30 Hz reference phase and the 30 Hz variable phase to be exactly in-phase on the beacon's north bearing alignment. The major cause of this error is the spurious vertically polarised signals generated by the aerials resulting in a vertically polarised 30 Hz bearing dependent component. This 30 Hz signal is not in-phase with the 30 Hz variable phase horizontally polarised signal. The horizontally polarised aircraft aerial will pick up some of the vertically polarised signal, when the aircraft banks, so the receiver operates on a combination of the desired horizontal signal and the unwanted spurious vertically polarised signal.
To overcome these problems, Doppler VOR (DVOR) was developed.
Doppler VOR (DVOR)
The DVOR ground beacon consists of a central omni-directional aerial surrounded by a circle of 52 aerials 44 feet in diameter. Aerials are usually Alford loops and are mounted above a large metallic mesh counterpoise.
The central aerial radiates an omni-directional continuous wave that is amplitude modulated at 30 Hz, this forms the reference phase.
The circle of 52 aerials is fed by a capacitive commutator or solid state switching device to simulate the rotation of a single aerial at a radius of 22 feet. Rotation is at 30 revolutions per second, and a carrier frequency 9960 Hz higher than that in the central aerial, is fed to the commutator. This 9960 Hz higher frequency is frequency modulated by the simulated rotation of the aerial, increasing in frequency as the aerial appears to move towards the receiver and decreasing in frequency as it recedes from the receiver. This forms the variable phase.
Fig. 12.22 PRINCIPLE OF DVOR
The dimensions of the outer ring of aerials are critical. A diameter of 44 feet combined with a rotational speed of 30 revolutions per second gives a radial velocity in the order of 4150 feet per second. This causes a maximum Doppler shift of 480 Hz, as required by the specification for the VOR system. The 9960 Hz frequency difference is therefore varied by ± 480 Hz at 30 Hz rate with the phase dependent on the bearing of the aircraft.
In the receiver, the output of the amplitude modulation detector contains all the signals present with the conventional VOR. Phase comparison between the two 30 Hz sine waves is performed as before, the only difference being that the 30 Hz amplitude modulated signal is the reference and the 30 Hz frequency modulated signal is the variable. To maintain the same phase relationships which exist in conventional equipment, the signal is radiated anti-clockwise around the aerial system.
Fig. 12.23 DOUBLE SIDED SIDEBAND DVOR
Some receivers react unfavourably to operation with a single sideband reference signal, and in consequence modern DVOR are designed to radiate double sideband transmissions. This is achieved by providing separate transmitters for each sideband (fc + 9960 and fc - 9960) and applying their transmissions to diagonally opposite aerials.
The central aerial radiates the carrier frequency fc, one of the rotating aerials radiates fc + fs and the other fc - fs, where fs is the frequency of the sub- carrier.
The importance of DVOR lies in the improvement it provides without any change being made to the airborne equipment. Even on bad sites DVOR can be as much as ten times better in accuracy than CVOR.