4.3 Instrument Landing System
4.3.4 Twin-Channel Radio Beacons
“Zero-referenced” radio beacons ensure higher accuracy and stability of the course (glide path) line, thus providing CAT II and sometimes CAT III approaches. Besides, they have good possibilities of adjustment which is important under operation con- ditions.
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Fig. 4.21 Influence of LOC antenna parameters in the form of antenna radiation pattern
Fig. 4.22 Antenna patterns of a twin-channel LOC
N=6,d/λ=0.5;b−N=20,d/λ=0.5;c−N =6,d/λ=5). The more emitters are used, the narrower the lobes are. Violation of condition d/λ≤0.5 causes the multilobe character of antenna radiation pattern.
A twin-channel LOC has two channels: a “narrow” and a “wide” one, each using its own antenna system (Fig.4.22). The antenna system of a “narrow” channel (at least 20 dipoles) forms rather narrow antenna patterns 6–12° wide in horizontal plane.
The antenna system of a “wide” channel (6–8 dipoles) creates wide antenna patterns which provide the desired width of coverage (±35°).
In twin-channel radio beacons, the influence of the “wide” channel on the “narrow”
one must be minimal. For this purpose, the following variants of two-channel radio beacons can be constructed:
twin-channel radio beacons with the “narrow” and “wide” channels radiating at different carrier frequencies;
radio beacons with quadrature clearance, that is their “narrow” and “wide” chan- nels use the same frequency with modulating oscillations phase-shifted by 90° (in quadrature);
integrated radio beacons using combination of both methods.
Two-frequency radio beacons are most widely used. They are a little more compli- cated than radio beacons with quadrature clearance [two transmitters, stricter require- ments for the stability of frequency (2×10−5for a two-frequency beacon and 5× 10−5for quadrature clearance)], but they do not need thorough adjustment and are simple in operation. Moreover, two-frequency radio beacons suppress a re-reflected signal of a clearance channel to a greater extent.
Carrier spacing in a two-frequency LOC is from 5 to 14 kHz relative to the nominal frequency specified by ICAO for ILS. Both carriers are within the pass band of an airborne ILS receiver and the channels have hardly any effect on each other.
The “narrow” channel makes up an almost straight course line as its coverage does not include any re-reflecting ground features and topographic inequalities. This channel is used to manage aircraft at final approach with small deviations from the final course plane.
During the initial approach when considerable deviations from the final course plane are possible, the “wide” channel is used. It is vulnerable to the effects of re- reflected signals, but, for its coverage, there are no strict requirements for accuracy as large deviations from the final course plane are admissible.
In the coverage of LOC “narrow” channel, there is a radiation pattern null for the
“wide” channel. It reduces the influence of “wide” channel signals upon “narrow”
channel operation.
The power of “wide”-channel re-reflected signals is much lower than that of the main “narrow”-channel signal. The linear detection in the airborne receiver causes their suppression by the stronger “narrow”-channel signal. So, the “wide”-channel signals are not available on the course line and around it, thus providing high stability of radiation parameters of the LOC antenna system.
In G/S, radiation compensation at small elevation angles (up to 1.5°) is accom- plished to reduce glide path distortions caused by earth re-reflections and an addi- tional channel is used to obtain information about aircraft position in the area (Fig.4.23).
In addition to the lower (A1) and upper (A2) antennas, a supplementary antenna A3 is installed three times as high as the lower antenna mounting in a twin-channel G/S.
All three G/S antennas are used to form the antenna radiation pattern of the
“narrow” channel, antennas A1 and A3 participate in formation of the “wide”-channel radiation pattern.
Phases and amplitudes of currents for feeding the antennas [from the outputs of sum-and-difference bridge (SDB)] are such as to reduce the field level at small angles to the horizon. This reduces the power of signals reflected by topographic inequalities and, hence, leads to reduction of glide path distortions.
In the area above the glide path, “wide”-channel signals are suppressed by a stronger “narrow”-channel signal.
Via the “narrow” channel, the radio beacon forms two antenna radiation patterns in vertical plane. One of them is a sum pattern created by the carrier and side modulation frequencies of 90 and 150 Hz (antenna A1), and the other is a difference pattern where side modulation frequencies in adjacent lobes are anti-phased (antenna A2).
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Fig. 4.23 Block diagram of a twin-channel G/S
Fig. 4.24 Formation of radiation patterns of a twin-channel G/S
Combining the A1 and A2 antenna radiation fields gives a resultant antenna radiation pattern of the carrier and side modulation frequencies ECSB “N” shown in Fig.4.24a.
By signals of side modulation frequencies, antennas A1 and A3 are fed in-phase between each other and anti-phase relative to antenna A2. Antenna A2 is anti-phased
Fig. 4.25 Antenna pattern and the antenna system of a twin-channel G/S
relative to the lower antenna A1 with the purpose of suppressing the sum signal field up to angles about 1/3β.
Combining the radiation fields of all three antennas gives a resultant antenna radiation pattern of the “narrow” channelESBO “N”shown in Fig.4.24b.
Via the “wide” channel, antennas A1 and A3 are excited in-phase by signals of the carrier and side frequencies of the “wide” channel (modulation of the “wide”- channel carrier with 90 and 150 Hz signals with excessive 150 Hz modulation by 30%).
Combining the fields of both antennas gives a resultant antenna radiation pattern of the “wide” channelECSB “W”shown in Fig.4.24c.
On the glide path line (“narrow”-channel coverage), depths of 90 and 150 Hz modulation of the carrier are equal.
In the area above the glide path, “wide”-channel signals are suppressed by much stronger “narrow”-channel signals. At small angles to the horizon, prevailing is the
“wide”-channel signal which contains information only about the deviation side.
In Fig.4.24d resultant radiation patterns of a twin-channel G/S are shown.
The form of the antenna pattern of a twin-channel G/S antenna system is shown in Fig.4.25.
To reduce the influence of the additional channel on the main channel operation, their carriers differ by 18–20 kHz.
The heights of antenna mountings are rather high in comparison with the wave- length, so to provide desired changes of DDM in the near-field, the antennas must be displaced to the runway in such a way that the line of their placement has the form of an arc with radiusRin the plane perpendicular to the runway centerline.
4.3.4.1 Processing of ILS Signals by Airborne Equipment
The ILS airborne equipment is designed to receive and process signals of radio bea- cons to form signals containing information about DDM. In an equisignal direction DDM is equal to zero, when diverting from it DDM increases and the sign of dif- ference depends on the side of aircraft deviation from the equisignal direction. The signal containing information of DDM is supplied to an indicating device and FMS.
154 4 Radio-technical Landing Systems The block diagrams of localizer and glide path receivers are equal. They are also equal for equisignal and “zero-referenced” (CSB/SBO) radio beacons. A simplified block diagram of an onboard analog receiver (localizer or glide path one) is shown in Fig.4.26.
Signals received by the antenna are amplified, converted to an intermediate fre- quency in the VHF module of the receiver and detected by an amplitude detector AD.
Low-frequency oscillations at the AD output are separated by filters F1 and F2 tuned to the frequencies of 150 and 90 Hz. The oscillations are rectified by rectifiers R1 and R2 and enter a subtract circuit SC with a pointer-type indicator (HSI or CDI) at the output. The indicated signals are proportional to DDM, and their polarity shows the side of aircraft deviation from the equisignal direction (Fig.4.27).
Output voltages of rectifiers are also supplied to add circuit AC whose output signal controls the flags of the indicating device (“Course readiness” and “Glide
Fig. 4.26 Simplified block diagram of an onboard receiver
Fig. 4.27 Diagrams of signals in the receiver units
Fig. 4.28 ILS receiver as part of MMR
path readiness” signals or ILS FAILURE flag). The flags get out of sight if there are oscillations of both low frequencies 90 and 150 Hz at the rectifier outputs.
If even one of these oscillations is missing, the flag system does not come into action and the flags fit into the pilot’s field of view, thus indicating that there is no signal from the localizer (glide slope) radio beacon.
On modern aircraft, the ILS receivers are included into multifunction receiving devices.
The ILS receiver (Fig.4.28) includes three functional units: RF module, main ILS unit, and monitor ILS unit.
RF module converts (filters, mixes, amplifies, and demodulates) the sig- nals received by VHF receiver (LOC signals) and UHF receiver (G/P signals).
Analog–digital converter (ADC) transforms output analog signals from RF mod- ule into digital signals for processing by the digital signal processor (DSP).
Main unit controls ILS mode of operation, generates the audio and deviation outputs, controls the aircraft interfaces, and performs the maintenance tasks (for CFDIU interface).
The main unit is divided into five sections:
analog–digital converter (ADC);
DSP section which formats and sends to the precision approach navigator (PAN) the deviations computed with the DoM of the 90 and 150 Hz;
PAN section which processes the data from the DSP section to provide the LOC and G/S deviation information to the input/output (I/O) section. The PAN compares information from monitor unit with the information it has calculated itself. The PAN sends to the maintenance section (MAINT) the in-line test results from all the units installed in the ILS receiver;
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Fig. 4.29 Antenna patterns and appearance of a marker beacon antenna
MAINT which provides the interface with the centralized fault display interface unit (CFDIU);
I/O section which provides the interface with aircraft systems.
The monitor unit provides a redundant dissimilar signal processing path for the LOC and G/S signals from the ILS RF module. It also includes an ADC, a DSP, a PAN, and an I/O section (as main unit).
The monitor unit performs a validity check of the primary instrumentation pro- cessor deviation output and disables the outputs if the check fails.
The ILS receiver applies its audio output (identification signals transmitted by LOC) to the audio integrating system via audio management unit (AMU).