CHAPTER 6. Precision Approach Systems
6.1. Introduction
The word approach is used to describe the phase of flight which immediately precedes the landing. While the ap-proach usually terminates with the landing, defining the start of the apap-proach phase is more difficult since there may be some maneuvering before the aircraft is established on its final descent path. This is usually designed to be at a 3 degree descent angle and on a track aligned with the runway centre line. The final descent path usually starts at 4NM from threshold and intersects the runway at a point 1000 Ft. past the threshold. The 4 NM (approx 2 min-utes) straight section of the approach is required to allow the pilot to stabilize the aircraft at the required rate of descent, airspeed and ground track. In the final descent phase the aircraft is in its full landing configuration i.e. landing gear extended, full flaps.
The ultimate objective is put the aircraft, at the decision height, in such a position, attitude and velocity state, that the pilot can make a visual landing without large changes in attitude or power.
6.1.1 Non Precision Approach
A non precision approach is one in which horizontal guidance is provided but there is no vertical guidance.
6.1.2 Precision Approach
A precision approach is one in which both vertical and horizontal guidance are available.
6.1.2.1 Precision Approach Categories.
To meet varying operational requirements ICAO has defined three basic approach categories: I, II and III. Category III is further divided into subcategories a, b and c. Fundamentally these categories determine under what ceiling and visibility conditions an approach may be attempted.
The limits are as follows:
Cat I: 200 Ft. ceiling, 2600 Ft.visibility
Cat II: 100 Ft. decision height., 1200 Ft. *Runway Visual range (RVR)
* Runway Visual Range is an optical system which is used to measure horizontal visibility. Cat III: 0 Ft. decision height,
a) subcat a, 700 Ft. RVR b) subcat b, 150 Ft. RVR c) subcat c, 0 Ft. RVR Notes:
d) For Cat I, the limits are observations made by a meteorologist; for Cat II and III, the visibility requirement is measured by instruments (RVR). Decision Height is the height above the ground at which the pilot must have sufficient view of the runway environment to make a landing.
to allow the aircraft to taxi to the gate.
While there is an operational advantage to be able to land in any ceiling and visibility conditions, there are considerable costs associated with each improvement in capability. Thus an aircraft operator must weigh the benefits against the incremental costs.
It must be noted that, to start an approach for a given category the ground facility, the aircraft equipment and the crew must be certified as meeting the requirements for that category.
Note: Until 1996, the highest category runways in Canada were Cat II primarily because, for the major airports, the percentage of time that the weather conditions were below Cat II limits did not justify the extra costs of Cat III certification. In 1996 Cat IIIa installations were certified for Vancouver and Toronto Pearson International. Some of the requirements which must be addressed in upgrading the category of a facility are:
- extra airport lighting (e.g. CAT II and above require runway centre line lighting) - increased reliability (more complex backup and monitoring systems)
- higher quality guidance from ILS (this may require more complex antenna systems or the removal of reflecting surfaces such as buildings or terrain)
- more training for aircrews
- higher reliability of aircraft systems
6.1.3 Current and Future Precision Landing Systems
- Instrument Landing System (ILS) - Microwave Landing System (MLS)
- Differential GPS (both Local and Wide Area)
6.1.3.1 ILS
ILS was developed just after the second world war. It is the ICAO standard and the fact that it is still the standard approach aid indicates how well it performs its task as well as the difficulty in making changes to internationally standardized systems. This latter point will be mentioned again when MLS is discussed. There are over 110 ILS installations in Canada and more than 1000 in the United States.
6.1.3.1.1 Frequency
Part of the ILS (Localizer or horizontal guidance) operates on frequencies between 108.0 MHz and 112.0 MHz. This creates a potential problem since this is adjacent to the FM broadcast band. The ILS transmitters radiate relatively little power (about 50 Watts) while FM transmit-ters radiate many kilowatts. In addition, when using the ILS aircraft are close to the ground and usually close to urban areas where FM transmitters are located. Thus FM interference can be a problem.
The vertical guidance part (glide path) operates between 329 MHz and 335 MHz. The glide path and localizer frequencies are paired in accordance with ICAO specification.
6.1.3.2 General
The ILS is made up of three main components: the localizer which provides horizontal guidance, the glide path which provides vertical guidance, and markers which provide along track position fixes. In some ILS installa-tions a DME is installed to provide continuous along track position information. The DME is usually collocated with the glide path antenna.
6.1.3.3 Localizer
The localizer antenna array is located at the “stop” end of the runway, usually about 1000 Ft off the end. It radaitestwo signals: one of which is AM modulated with a 90 Hz signal and the other is AM modulated with a 150 Hz. signal
Figure 39: General Layout of ILS
Figure 40:
ILS Localizer Signal Pattern
LOCALIZER GLIDE PATH MARKER
150 Hz
90Hz
DDM
A simplified drawing of the relative amplitudes is shown in the diagram. Thus when the aircraft is to the left of track the ILS receiver will see a higher 90 Hz signal and when it is to the right of track it will see a higher 150 Hz signal.
Actually the receiver measures the difference in depth of modulation (DDM) of the 90 Hz signal referred to the 150 Hz. The locus of all points at which the DDM is zero is called the course or the course line. (dashed line in above diagram)
Thus, as the aircraft passes from right to left of the course the DDM will vary as follows:
and the pilots left/right indicator will vary accordingly.
The antenna array required to produce this signal is a linear array of elements (up to 26) and is quite large
RIGHT
LEFT
DDM
6.1.3.4 Glide Path
In order to provide vertical guidance, a signal similar to that of the Localizer is generated but rotated 90 de-grees. As mentioned before, the frequency is in the 300 MHz range.
While it would be possible generate the Glide Path signal by using an antenna array similar to the one used for the Localizer but mounted vertically, the height of such an array would be too great to allow it to be installed safely near a runway. (The Glide Path antenna is usually installed about 1000 Ft. from the threshold and 400-500 Ft. from the edge of the runway). Thus a different method is used.
Since the ground is a reasonable conductor, it acts as a reflecting surface for electromagnetic waves. Thus, as was mentioned in the lecture on antennas, only half of the required antenna is needed. Therefore an arrangement such as that shown in the above figure is used. The antenna tower is usually about 20 feet high and has either two or three antenna elements mounted on it.
One drawback of this technique is that any change in the height or shape of the reflecting plane has an effect on the antenna pattern and hence on the position of the course line. Thus ground moisture content and snow affect the position of the Glide Path course line, and must be monitored.
6.1.3.5 Markers
There are three types of marker, all transmit on a frequency of 75 MHz. The outer marker is located about 4 NM from the threshold of the runway and marks the nominal start of final descent. The carrier is modulated with a continuous series of 1020 Hz dashes. The middle marker, located about 1/2 NM from threshold, the de-cision point for Cat I approaches, is modulated with alternating dots and dashes. The inner marker, of which there are none in Canada, is located at 1000 Ft. from threshold, the decision point for Cat II approaches, is mod-ulated with 1020 Hz dots.
The antennas radiate a narrow vertical beam so that the signal can be heard only when the aircraft is directly above the beacon.
90 Hz
Although marker beacons are still included in the ICAO specification for ILS, Canada has removed all Markers from its ILS installations.
6.1.3.6 Airborne Installations
The antenna installation consists of three antennas: one each for the localizer, glide path and marker. The lo-calizer uses the same antenna as the VOR, a loop or horizontal blade mounted on the vertical stabilizer. The Glide Path antenna is a small U-shaped horizontal loop antenna usually installed in the nose, inside the radome. Because it shares this space with the radar antenna there is the possibility of reflections off the radar antenna interfering with the operation of the glide path receiver.
The marker antenna a a loop antenna mounted on the belly usually in the aft area.
The airborne receiver is relatively simple. All it has to do is tune the appropriate frequencies, demodulate the signal and generate the guidance and flag signals:
Figure 41:
Simplified Block Diagram of
One Channel (Loclizer or Glide Path) of an ILS Receiver
Outer
Marker
Middle
Marker
Typical Marker Antenna Patterns
Tuning
Demodulation
90Hz
Filter
150Hz
Filter
+
-+
+
Flag
Guidance
6.1.3.7 Accuracy:
Localizer: The ICAO accuracy requirement for the localizer is defined by the point at which the centre of the course crosses the runway threshold and depends on the category of approach:
Cat I - within 35 Ft. This, for a 6000 Ft. runway is equivalent to 0.29˚ Cat II - within 25 Ft. This, for a 6000 Ft. runway is equivalent to 0.2˚ Cat III - within 10 Ft. This, for a 6000 Ft. runway is equivalent to 0.08˚e
Note: 1000 Ft. was added to runway length in these calculations because Localizer antenna array is usually about 1000 Ft. off the end of the runway.
Glide Path: 0.056θ. Where θ is the nominal angle of the glide path. e.g. for a 3˚ glide path, the maximum error is 3 x 0.056 = 0.168˚.
6.1.3.8 Irregularities:
Because the course lines are determined mainly by the antenna patterns of the Localizer and Glide Path they are susceptible to reflections from buildings, terrain and foliage. Such reflections cause unwanted deviations of the course away from a straight line. These irregularities are called “structure”.
ICAO specifies the maximum levels for these deviations for the various sections of the approach and for the three approach categories: The three main sections of the approach are: outside of the 4 NM point (usually the outer marker), between 4NM and 2500Ft. and between 2500 Ft. and runway threshold. The tolerance decreases with decreasing distance to threshold and with higher category.
6.1.3.9 Integrity
The airborne receiver monitors the sum of the modulation depths, and if this sum decreases below as set level, a warning indicator on the pilot’s indicator is activated. Note: this also detects low RF signal levels.
The ground station includes monitors which detect out of tolerance conditions.
6.1.3.10 Availability
High rates availability are achieved by using two transmitters. If the operational transmitter fails or goes out of tolerance, the second is switched into the system. The time for switchover is a function of category with the higher categories requiring shorter switchover times.
6.1.3.11 The Future of ILS
Because of the large number of aircraft and ground installations and the huge financial investment they repre-sent, it appears that ILS, although it is rather archaic, will be in operational use for the foreseeable future (at least until 2015). It is a classic case of “if it ain’t broke, don’t fix it”. The major threat to ILS is the increase of permissible FM broadcast power since the ILS frequencies are adjacent to the FM broadcast band.
6.1.4 MLS
6.1.4.1 Introduction and history
In the mid 1970s the FAA felt that ILS technology had been extended as far as possible and that it couldn’t be made to meet the growing requirements for precision landing systems. In particular they had problems with frequency congestion in the North East corridor. i.e. ILS is limited to 40 channels and, with the density of air-ports increasing, they were starting to interfere with one another. Also, The ILS systems of the day had diffi-culty meeting the requirements of category II and III operations.
To alleviate this problem, they started a program to develop a new international standard system which would operate in the microwave region (200 channels spaced 300 kHz apart between 5.031 and 5.0907GHz). Two signal formats were proposed; one using a doppler technique and the other using a scanning beam. The latter (which was finally adopted) was called officially a Time Referenced Scanning Beam (TRSB). The USA and Australia supported TRSB while the UK supported Doppler.
After much intense lobbying, ICAO selected TRSB.
6.1.4.2 Principle of Operation
MLS works on what is called a time multiplexing technique; that is the time available is divided up into slots, and the various functions of the system are assigned to these slots as necessary.
The horizontal guidance function is provided by generating a fan-shaped beam as shown below:
This beam is scanned (swept) from one side of the runway to the other (the amplitude of the deflection angles depends on the design of the particular system and site but 40˚ is typical.)
MLS Azimuth Beam Pattern
Top View
Since the beam is swung from one extreme to the other and back again, the airborne receiver sees two pulses. The period of scan is known (defined by ICAO) thus the receiver azimuth angle can be determined by measur-ing the time between pulses
Figure 42:
MLS pulses seen by receiver
Figure 43: MLS Beam Angle vs time
The angle to the aircraft may be computed from the equation
where V is the scan rate of 0.02˚/μs
and T0 is the time between zero degree passages
t
time
angle
0˚
t
T
0-40˚
+40˚
ϕ
V
2
----
(
T
0–
t
)
=
Figure 44:
MLS TRSB Multiplex Format The entire MLS message takes 115 ms as shown in the diagram. The MLS is designed to be a modular system with the modules being:
a) Approach Azimuth (equivalent to ILS localizer) b) Elevation (equivalent to ILS glide path)
c) Flare guidance (short range, high accuracy vertical guidance)
d) Missed Approach or Back Azimuth (for guidance on takeoff or overshoot)
A minimal system would consist of an Approach Azimuth and an Elevation unit with the Approach azimuth providing± 40 ˚ guidance
Thus not all of the data blocks are necessarily used.
Notice that some of the blocks are repeated. This is to give a higher update rate for time critical items. The Basic Data words provide such information as:
a) Identity of facility b) Minimum elevation angle c) Azimuth limits
d) Runway Length
6.1.4.3 Accuracy/Integrity/Availability
Typical accuracies at threshold for a 6000Ft. runway are 20 Ft. in azimuth and 2 Ft. in elevation
Integrity is provided, as in ILS, by ground monitors which measure the system performance and take action if a failure occurs
The Availability is expressed by the MTBF which is in the order of 4000 hours.
Elev Flare ApprAz Flare Elev Elev Elev
Missed Appr Az Aux Data Aux Data Aux Data Aux Data
Flare Flare Appr
Az Flare 0 115ms Preamble ‘to’ scan ‘fro’ scan 10.2ms 26ms
6.1.4.4 Airborne installation
Since the MLS signal have a wavelength of about 6 cm, the aircraft antenna is very small, a quarter wave stub being about 1.5 cm long. Receivers are small but relatively expensive ($25,000)
6.1.4.5 MLS Advantages and Disadvantages
6.1.4.5.1 Advantages (over ILS) approach azimuth:
- less susceptible to siting problems (buildings, terrain) - selectable glide path angle and azimuth approach angle - possibility of curved approaches
- much less susceptibility to interference - many more channels available
6.1.4.5.2 Disadvantages
- expensive to buy
- requires expensive installation in aircraft
6.1.4.6 MLS - Current Situation and Future Prospects
In June of 1993 the USA and Canada decided to terminate all development of MLS in favour of differential GPS for future precision approach requirements. MLS has been installed at several airports where ILS could not be. Notably where high glide path angles are needed to provide obstacle clearance (maximum ILS glide path angle is about 5˚). There were two MLS installed at Toronto Island airport primarily because the runways terminate at the lakeshore and there is no room to install a localizer antenna.
There is still some interest in MLS in Europe to fulfill near term needs for Cat III requirements. In particular the Europeans are reluctant to place complete reliance on an American military system. In the longer term they are looking for an international civil satellite system.
Thus MLS will not be the ILS replacement that it was intended to be but will probably survive in special niche situations.
6.1.5 Differential GPS
This was discussed under GPS however there is a potential problem with DGPS vs. ILS and MLS. This is due to the fact that DGPS operates primarily in a Cartesian coordinate system and ILS/MLS operate in an angular (spherical) coordinate system.
The vertical accuracy requirement for DGPS was derived from the equivalent ILS angular error, however, with this definition the same system error willresult in different aircraft paths as is shown in the diagram. At the present time it is not known if this will be an operational problem but the potential difficulty is as follows:
Figure 45:
Effects of Path Error for ILS and DGPS
Path Error for ILS/MLS
In the ILS/MLS case the path angle is in error but the point of touchdown (or aiming point) is still the same, so that, when visual contact is made, there is little or no aircraft attitude change required to accomplish the land-ing.
In the GPS case, however, the aiming points are different and the pilot would have to adjust the flight path in order to avoid landing short or long depending on the sign of the error.