12.6. Принцип действия фазового канала азимута с доплеровским арм.
Радикальным
методом повышения точности фазового
канала азимута (ценой усложнения АРМ)
является использование так называемых
доплеровских радиомаяков. Антенная
система такого АРМ (рис. 12.7,а) состоит
из большого числа вибраторов
(например, из 50), размещенных по окружности
с радиусом
,
где
– длина волны несущих колебаний. На
противоположные вибраторы, например,
и
,
подают от передатчика токи с частотами
.
Поочередное подключение пар вибраторов
к источнику высокой частоты имитирует
их вращение по окружности (
и
на рис. 12.5,б). Принимаемый сигнал из-за
вызываемого вращением вибраторов
и
эффекта Доплера становится
частотно-модулированным:
,
(12.6)
а спектр его соответствует рис. 12.1, в.
Рис. 12.7. Антенная система доплеровского АРМ (а) и эквивалентная ей антенная система с вращающимися вибраторами и (б), а также изменения поднесущей частоты сигналов, принимаемых в точках 1 и 2 (в)
Индекс модуляции зависит от угловой скорости вращения вибратора и частоты питающего его тока. Опорный сигнал
(12.7)
при
этом передается ненаправленной антенной
– вибратором
.
Если
в доплеровском АРМ та же, что и в обычном
фазовом АРМ, то для обработки сигнала
пригодны приемные устройства, подобные
показанному на рис. 12.5,г. Только теперь
азимутальный и опорный сигналы меняются
местами. Так как информация об азимуте
заключена в фазе частотной модуляции,
то влияние отраженных сигналов на
точность канала азимута уменьшается и
результирующая погрешность снижается
в 10…15 раз.
12.7. Vhf omnidirectional range (vor).
The VOR system is a navigation aid that enables the flight crew to determine aircraft bearing to a VOR ground station and aircraft position with respect to, and deviation from, a selected VOR course.
The VOR system operates in the frequency range between 108 to 118 MHz, with 50 kHz channel spacing. This frequency band is shared with the localiser frequencies of the Instrument Landing System (ILS), the VOR being allocated 160 of the 200 available channels.
Of the 160 channels, 120 are allocated for use with VOR stations used for en route navigation. Such stations have channels in the frequency band 112 to 118 MHz and have power output of about 200W providing service ranges up to 200 nautical miles.
The remaining 40 channels are allocated to terminal VOR (TVOR) stations which share the frequency band 108 - 112 MHz with the ILS localiser. TVOR stations, being terminal aids, have a power output of about 50W providing service ranges up to 25 nautical miles.
Between 108.00 and 112.00 MHz VOR uses even decimal frequencies, and ILS the odd decimal frequencies.
Principles of VOR
The principle of VOR navigation is a phase comparison of two 30 Hz audio signals transmitted from the VOR ground station. The phase difference between the reference signal and the variable signal, measured in degrees, corresponds to the bearing of the aircraft with respect to magnetic north.
The reference 30 Hz signal is transmitted on a 9960 Hz subcarrier. The sub- carrier is frequency modulated between 10,440 Hz and 9,480 Hz at a rate of 30 Hz.
The variable 30 Hz signal is transmitted as amplitude modulation of the RF carrier. The 30 Hz variable signal is obtained by transferring the RF carrier to a dipole aerial and rotating the aerial at a rate of 30 revolutions per second.
The field strength pattern is a Cardioid that is rotated through 360° at a rate of 1800 revolutions per minute. The phase of the 30 Hz variable signal with respect to the 30 Hz reference signal varies directly with the bearing of the receiver from the VOR station.
NORTH
Fig. 12.8 REFERENCE & VARIABLE SIGNALS AROUND A VOR BEACON
The transmission from a conventional VOR (CVOR) is a horizontally polarised signal, consisting of the reference and variable phase signals and a 1020 Hz station identification signal keyed in Morse code.
A summary of the VOR transmitted signals is given below:
1. Omni directional AM Signals.
a) VOR reference - 30 Hz reference signal which frequency modulates the 9960 Hz ± 480 Hz.
Keyed 1020 Hz identity tone.
Voice audio (if used).
2. Directional Signal
a) 30 Hz variable signal, which has apparent modulation of 30 Hz AM due to the rotation of the radiation pattern at 30 revolutions per second.
VOR Beacon
The heart of the VOR beacon lies in the aerial system. A typical aerial consists of four radiators less than a quarter-wavelength long arranged in a square to form an Alford loop.
At any one instant, the currents in all the radiators are equal and alternately move clockwise and anticlockwise. The result is a doughnut shaped field with zero radiation in the upwards and downwards directions.
Fig. 12.9 THE ALFORD LOOP
A fifth loop was originally used to radiate the omni directional signal, but it was soon realised that by proper phasing the four loops could radiate an omni directional signal by themselves.
The omni directional signal consists of a continuous wave which carries the Morse code ident 1020 Hz tone. Present at all times is the other 9960 Hz tone varied + 480 Hz, sinusoidally, at a rate of 30 Hz.
Figure 13 shows a simplified VOR arrangement.
The transmitter is crystal controlled, with a power output of 200W for CVOR and SOW for TVOR. It is amplitude modulated to a depth of 30% by the output of a mechanically driven tone wheel, which has 332 teeth and is driven by an 1800 rpm motor. The teeth are arranged in a slightly staggered manner and this irregularity imparts a cyclic variation of between 9480 Hz and 10,440 Hz on the output frequency.
On the same shaft is a capacitive goniometer, fed by the same transmitter via a modulation eliminator that strips off the amplitude modulation from a portion of the transmitter output signal to provide the unmodulated RF power for the variable phase signal.
The goniometer feeds the unmodulated transmitter power first to the northwest - southeast pair of Alford loops and then, 90° later, to the northeast - southwest pair of loops, generating a rotating cardioid, when combined with the modulated energy applied simultaneously to all loops.
Each pair of loops is fed via a balanced bridge network. Each bridge has three arms that are each one-quarter wavelength long, the fourth arm being half a wavelength longer. Energy fed into one corner of the bridge does not appear at the diagonally opposite corner. The bridge therefore allows the mixing of two signals and application of the result to two loads without the loads affecting each other and without the signal sources affecting each other.
The phasing between the tone wheel and goniometer and the physical placement of the Alford loops are such that the two 30 Hz signals are exactly in phase when viewed from magnetic north.
Fig. 12.10 VOR BEACON
VOR Receiver
The airborne equipment is comprised of a horizontally polarised receiving aerial and a receiver. The receiver detects the 30 Hz amplitude modulation produced by the rotating pattern and compares it with the 30 Hz frequency modulated reference. A basic receiver is shown in figure 12.11.
BEARING
Fig. 12.11 VOR RECEIVER
At the output of the receiver is an AM detector, the output of which comprises:
- 30 Hz tone produced by the rotating cardioid.
- 9960 Hz tone, frequency modulated ± 480 Hz by the 30 Hz reference tone.
- Morse code 1020 Hz ident tone.
- Voice audio, if used at the transmitter.
The 30 Hz information is filtered to remove the other components and fed to the phase comparator. The 9960 Hz is filtered and limited before being applied to a frequency modulation detector, the output of which is the 30 Hz reference frequency. After filtering this is compared with the variable phase to derive the bearing information.
Two types of display of VOR information are common, the first of which is called a Radio Magnetic Indicator (RMI) and indicates directly relative bearing between the aircraft and the VOR station. The RMI frequently has two indicating pointers which may be operated in conjunction with two VOR, two radio compass receivers or one of each.
The second method of display uses the vertical pointer of an ILS meter. This operates in conjunction with a digital phase shifter on which any bearing may be present. The ILS meter then indicates deviation from the pre-set bearing, allowing a VOR radial to be flown using a "fly left/fly right" indication. The meter sensitivity is normally ±10° for full scale deflection (fsd).
Omni-Bearing Selection
The phase difference between the reference phase and the variable phase at the receiver depends on the bearing of the receiver from the beacon, referenced to magnetic north.
A bearing to a beacon is expressed in whole degrees, and therefore there are a possible 360 bearing angles to a beacon, known as Radials, tUK)
Fig. 12.12 VOR RADIALS
The pilot may select any one of these radials manually by simply rotating the omni-bearing selector knob or course selector knob, as fitted to the associated indicator, which dials up the chosen bearing radial.
Fig. 12.13 "TO - FROM" VOR RADIALS
Every bearing has a reciprocal and therefore for any particular radial selection, there will be a second radial on the opposite side of the transmitter having the same bearing angle. The only difference between the two is one is "TO" the beacon and the other "FROM " the beacon.
Figure 12.13 shows that each radial has two directions, and that each direction appears on two radials, one "TO" and the other "FROM" the station.
Having selected the radial to fly, the manual VOR receiver will inform the pilot of any deviation from the selected radial and whether flying "TO" or "FROM" the beacon.
Consider an aircraft bearing 250° magnetic from a VOR beacon when a course of 060° is selected. The manual VOR receiver will present an output on the associated indicator to indicate that the aircraft is left of its selected course and therefore should fly right to capture the 060° radial "TO" the beacon.
n (m)
Fig. 12.14 FLYING THE VOR RADIAL
Manual VOR Receiver
A typical manual VOR receiver capable of supplying a suitable indicator with deviation from a selected radial, "TO" and "FROM" indications and also a warning flag indication which comes into view when incoming signals are lost or weak, is shown.
The signals are received by a common VOR/ILS localiser aerial and are processed through a normal VHF communication type of receiver up to the detector stage.
Fig. 12.15 MANUAL VOR RECEIVER
After detection, the three amplitude modulations are separated by filters. The three modulations are:
30 Hz variable phase signal.
9960 Hz subcarrier, frequency modulated by a 30 Hz reference phase with a deviation of + 480 Hz.
1020 Hz Morse code ident signal.
The 30 Hz variable phase signal is amplified and fed as an input to a phase comparator circuit for phase comparison with the reference phase signal.
The 9960 Hz subcarrier is filtered to an FM detector strip consisting of amplifiers, limiters and a discriminator for removal of the 30 Hz reference phase. The 30 Hz signal is then passed to the rotor of a resolver synchro located in the indicator and operates as a phase shift device. The phase of the reference signal can therefore be varied by rotating the resolver synchro's rotor, which is connected directly to the OBS knob. A clockwise rotation of the OBS knob through X degrees causes the phase of the reference phase signal to be phase retarded by X degrees. This shifting of the reference phase gives rise to the term "Shifted Reference Phase" where the shift is the angle of the selected course radial.
The 1020 Hz ident is passed into the audio integrating system and to the ASP's.
It can be seen from figure 18 that the output of the phase shift resolver is further advanced by a fixed 90°. This is done to make the phase comparison easier. Phase comparators respond to slight phase variations between signals better when the signals are at 90° to each other.
After the phase shift resolver the reference phase signal, which is still at 30 Hz, is amplified and applied to the phase comparator for phase comparison with the variable phase signal.
The output of the phase comparator is the difference between two positive, or negative, dc voltages for operation of the deviation indicator. The sum of the same dc voltages is for the operation of the failure warning flag.
The left or right deviation of the deviation bar is determined by the phase relationship of the variable phase and the shifted reference phase at the phase comparator input, such that when the inputs are in~phase quadrature the deviation bar is centred indicating an on-course situation.
When the variable phase and the shifted reference phase are less than 90° out of phase, the deviation bar deflects to the right, advising the pilot to fly right. When the signals are more than 90° out of phase, the bar deflects to the left, indicating fly left to capture the selected radial.
The "TO" - "FROM" indication flag is dependent upon the dc difference output from the comparator. The inputs to the "TO" - "FROM" comparator are the variable phase and the shifted reference phase advanced by a total of 180°.
Omnirange Indicator
Several types of indicators are used to display manual VOR information. A typical indicator is shown in figure 12.16.
This type of indicator is also used for ILS approach for which the horizontal pointer, as well as the vertical pointer is used.
The vertical pointer is used for omnirange and indicates by position, left or right, on which side of the selected course the aircraft is located. When in the centre it indicates an on-course heading.
The number at the top of the meter is the course heading to or from the station in degrees from north. Any course can be selected by the course selector knob.
ALARM COURSE VERTICAL