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We take it pretty much for granted that almost 400 tonnes of 747-400 screaming along at the best part of 1000km/h manages to find and land at the right point on the right runway, every time . . . but how do they do it so well? Instrument Landing Systems. . . how they work I t was a drizzly, miserable day with a low, grey blanket of cloud, heavy and oppressive. Looking out the airport terminal window, the world seemed to end barely a kilometre away. Vague silhouettes of aircraft moved silently about in the misty rain. I looked up at the arrivals screen to check my friend’s flight from Perth. It had already arrived, five minutes early, of course! Best get to the gate quickly . . . As I walked to the arrivals gate I reflected on what I had just taken for granted. Just a few minutes ago, my friend’s plane had been hurtling towards the ground at 250km/hr with nothing but solid murky grey out all of the windows – including those of the cockpit. In fact, the plane may have descended to as low as sixty metres above runway height with little, perhaps no, outside visibility. We expect planes to land in all 8  Silicon Chip sorts of weather. In Europe and North America, a pilot can land an aeroplane on some runways without seeing the ground at all. Automatic landing and taxying systems are continually becoming more capable. Today it is technically possible to safely and reliably land, slow down and taxi to the correct gate with no outside visibility and hardly any human intervention. tral challenge is navigation. In this article we will look at the Instrument Landing System (ILS), which still provides precision landing guidance 65 years after its invention. We will also see the impact of newer technologies such as GPS, Microwave Landing Systems and ever-increasing on-board digital processing power. But first, some background. The issue is guidance Most readers would be familiar with basic radio direction finding using a loop antenna. You rotate the loop until you find a “null” in the signal. If you point with your hand straight through the loop, you are pointing either directly towards or directly away from the transmitter. Direction finding is the basis of radio navigation. Instead of plotting a visual “fix” to a landmark on a chart, you plot a radio “fix” to a known transmitter. Radio navigation today is quite sophisticated, even before you include So how does all of this work? First, let’s keep in mind what we’re looking at. A modern aeroplane can fly equally well through clear day or foggy night. The challenge with all weather landings is to provide some form of guidance so that the pilot (or an autopilot) can stay lined up with the runway and descend on the correct path without actually seeing the runway. The cen- by Daniel Field Radio Navigation siliconchip.com.au Somewhat stylised diagram of a typical runway with ILS. Not shown here are the inner, middle and outer “markers”. A little trivia: one runway number subtracted from its opposite end number will aways equal 18! GPS. Apart from Instrument Landing Systems, three radio navigation systems have been central to aircraft navigation for several decades. They are: 1. ADF (Automatic Direction Finder). Gives the bearing (direction) to the transmitter relative to the nose of your aircraft. Combined with “magnetic heading” information from the aircraft compass system, it gives the magnetic bearing to the transmitter. 2. VOR (VHF Omnidirectional Range). Tells you what your bearing is from the transmitter, no matter which way you are pointed at the time. These fixed lines of bearing from the transmitter are called “radials” and are commonly followed as “roadways in the sky”. 3. DME (Distance Measuring Equipment). Uses pulse timing techniques to tell your distance in a straight line from the ground equipment. It is usual to co-locate DME and VOR ground equipment to provide range and bearing from the same location. Pilots use these systems to find their position over the Earth’s surface without any visual references. None of them can determine height above the ground: they are all lateral navigation (LNAV) techniques. The problem of landing and approach Using lateral navigation techniques an aircraft can fly clear of obstacles and line up with a runway. This procedure is called a “Non Precision Approach” (NPA). During an NPA the pilot uses an altimeter (which is based on air pressure) for height information. For a variety of reasons, pressure altimeters can theoretically be inaccurate by up to 100 feet (30 metres). So a published NPA must allow plenty of margin for error if pilots of varying skill levels in a wide variety of aircraft are to follow it thousands of times per year. A typical NPA in an area clear of obstacles can get the aircraft down to about 400 feet (120 metres) above the ground. If the pilot cannot clearly see the runway then he can safely “go around” – pulling out of the landing approach and flying in a predetermined pattern to return to the approach path and again line up with the runway. Therefore, without some form of precise vertical guidance, an aeroplane cannot land unless the cloud base is at least 400 feet above the ground. The Instrument Landing System (ILS) As early as 1928 (which, by the way, is only a year after Lindbergh first crossed the Atlantic), engineers and scientists were giving careful attention to the problem of vertical guidance for landing. Teams in different countries developed various solutions. By 1940 a working “Instrument Landing System” had been installed at Indianapolis airport in the United States. The Instrument Landing System (ILS) was a new radio navigation system that provided precise vertical guidance (referred to as the “Glide The heart of the ILS system: at one end of the runway (far end from approach) is the antenna system for the “localiser” beam. This gives the aircraft its left and right guidance signals to help it line up with the runway. Alongside the runway, roughly at the touchdown point (ie the approach end), is the guideslope (or glidepath) antenna which helps the aircraft approach the runway at the right angle and hit the tarmac at the right place. siliconchip.com.au June 2004  9 Path” or “Glide Slope”) as well as precise lateral guidance (referred to as the “Localiser”). Both the glide slope and the localiser worked on the same technique: a radio technique that is still in use today at thousands of ILS-equipped runways around the world. The glide slope carrier signal is in the range of 329-335MHz. The localiser carrier is in the range of 108-112MHz. The glide slope and localizer carriers are each directional radio beams radiated in two parts: one amplitude modulated at 90 Hz, and the other at 150Hz. In the case of the glide slope, a directional antenna array radiates the 90Hz signal just above the correct approach path and the 150Hz signal below it. Right on the approach path, the modulation of both components is 40%. An aircraft anywhere along the correct glide path will receive both the 90Hz and 150Hz components equally. If the aircraft moves above the correct path it moves toward the centre of the 90Hz beam and away from the 150Hz beam. Because they are both on the same carrier, the detected depth of modulation of the two signals is no longer equal. The 90Hz signal will seem to have deeper modulation than the 150Hz signal. While every glide path is adapted for each particular airport, a typical path is at an angle of about 2.5 to 3°. The aircraft’s receiver detects the 90Hz and 150Hz components then separates them using a simple filter network. The two components are full-wave rectified to produce two DC signals: one representing the strength Pilot’s-eye-view of a 747-400, lined up on runway 34R at Sydney International Airport. This amazingly realistic view is actually taken from Microsoft Flight Simulator IV, which we’ll have more to say about shortly . . . of the 90Hz component, and the other the 150Hz. The difference between these DC signals drives a moving coil meter. If the 90 and 150Hz components are equal then there is no difference, so the meter stays in its “at rest” position in the centre of the indicator. If the 90Hz component is stronger, the meter drives down to indicate “fly down”. A deviation of half a degree above or below the glide path gives full-scale deflection of the meter. This corresponds to a difference in depth of modulation (ddm) of 0.175, or 17.5%. In the case of the localiser, a directional antenna array transmits the 90 Hz signal to the left of the runway cen- tre line (from the point of view of the approaching aircraft), and the 150Hz signal to the right. The modulation of both signals is 20% on the correct path. Again, the receiver rectifies the two components and drives a meter movement. Full-scale deflection indicates about three degrees deviation from the centre line, with a ddm of 0.155. How far to go? Assuming that the pilot has no outside visual cues, the Instrument Landing System that I have described still relies heavily on the altimeter. Sure, the pilot knows that he is approaching the runway on the correct path. The beam pattern set up by the glideslope (or glidepath) radio signals. It really is quite simple – fly too high and the 90Hz signal is received; fly below it and the 150Hz signal is received. 10  Silicon Chip siliconchip.com.au Again from Flight Simulator IV, compare the clean, modern instrumentation of the 747-400 to the instrument panel of a Beechcraft King Air, here lined up on runway 29C at Bankstown airport, Sydney (incidentally, the busiest airport in Australia for aircraft movements). But what is to stop him from staying nicely on path until he crashes into the runway? The fact that he crashed right on the touch down point is unlikely to be much consolation. The designers of the Instrument Landing System decided to place various “Markers” along the approach path so that the pilot knows what stage of the approach he is up to. These markers are low power transmitters that radiate in a narrow beam straight up. The carrier is always 75MHz. The AM signal depends on the function of the marker. On a normal instrument approach, the pilot initially uses his altimeter to fly at a particular altitude (say, 2,500 feet above the ground) and various radio navigation aids to intercept the Localiser. The aircraft then flies along the localiser toward the runway, maintaining a particular altitude (using the altimeter). As the aircraft flies along, it is actually below the plane of the glide slope. If you were in the cockpit, you could say that the glide slope is in front of you, slanting down toward the runway, and you are flying level towards it. Imagine for a minute how this works in the cockpit: As the aircraft moves into the lower part of the glide slope signal the indicator shows “fly up”. The pilot continues to hold the same altitude. The glide slope indicator starts to show that the aircraft is coming up to the centre of the glide path. The pilot then initiates a descent to capture the glide slope. As long as the aircraft stays on the glide path, it is safe to descend. This is where the designers used the first marker: The Outer Marker. Right at the point where the pilot should intercept the glide slope the aircraft flies through the outer marker beam. The pilot hears a 400Hz tone (a moderately low pitch) which also causes a blue indicator light to illuminate in the cockpit. The tone and light make a continuous stream of Morse code “dashes” at the slow rate of two dashes per second. If a pilot passes the outer marker and still does not have a glide slope signal then he knows that there is a problem. Australian approaches actually use the Outer Marker at some point along the descent rather than at the glide slope intercept. The intercept point may be directly over some other radio beacon that does not normally form part of an ILS. Sometimes it is not marked at all but can be anticipated a certain distance from the airport using radio distance measuring equipment. Whether or not the outer marker coincides with the glide slope intercept, it is an important indication of the aircraft’s progress along the ILS. The next marker is the “Middle Marker”. This is usually about a kilometre from the runway. The pilot hears alternate dots and dashes at 1300Hz, illuminating an amber light in the cockpit. The middle marker normally And here’s the way the middle and outer markers are set up. They are very narrow beams which are received in a very specific location, telling the pilot the plane has passed through the marker. siliconchip.com.au June 2004  11 indicates that the aircraft is 200 feet (60 metres) above the ground. On basic Instrument Landing Systems (including most systems currently in use around Australia), 200 feet is the “decision height”. The pilot may continue to descend beyond the middle marker only if he sees the runway. Once again, there is some variation from one approach to another. For example, there is no middle marker at Nowra, NSW. The pilot must use DME to determine decision height (at a distance of 0.8 nautical miles from the runway). Perth’s ILS runway 03 has no markers at all. Three of Sydney’s six ILS approaches also have no marker beacons (because of possible confusion with markers for parallel runways). They all use DME distances instead. On more advanced instrument landing systems the decision height can be either 100 feet or zero feet. Those systems can include an “Inner Marker” which gives the sound of rapid dots at 3kHz (high pitch) and causes a white light to flash in the cockpit. The inner marker normally indicates a height of 100 feet above the ground. Note that at this point, the aircraft altimeter could possibly indicate anything from zero to 200 feet above the ground (though, in 12  Silicon Chip siliconchip.com.au reality, almost all altimeters in instrument rated aircraft are likely to be within ten feet of the actual altitude). Three levels of accuracy Instrument Landing Systems are theoretically capable of guiding an aircraft all the way to the ground. But the very high accuracy and reliability required for this task comes at a cost. Installations that can guide an aircraft right down to the ground must be tested and proven over a period of years, then continually monitored, tested and maintained to exacting standards. There is also a paradox that causes more accurate systems to be less capable than less accurate systems: as an aircraft travels through the directional localiser and glide slope beams, it warps them. The signals received by a following aircraft might not be accurate. The most precise instrument landing systems depend on much larger spaces between approaching aircraft than the less precise systems. As a result, less precise systems can handle more than double the number of landings per hour. None of the systems installed in Australia can guide an aircraft all the way to the ground. The majority are “Category One” Instrument Landing Systems (ILS Cat I), with a decision height of 200 feet. A runway will only be open for Cat I approaches if the “Runway Visual Range” is at least 800 metres. The next level of precision is a Category two ILS. In a Cat II system the decision height and visibility (“Runway Visual Range”) requirements are half those of Cat I. That is a decision height of 100 feet and a visibility of 400 metres. Category three systems are installed primarily in North America and Europe. For example, there are 31 Cat III systems installed at 15 airports in Germany. Nearly all of these are “Cat IIIB” (see table). Europe is currently moving away from Instrument Landing Systems in favour of the more capable “Microwave Landing Systems” (MLS). Improving the system The basic ILS with moving coil meters is still used to-day in some private aircraft, older charter planes, and many instrument-training planes. But these systems are practically obsolete. Modern business aircraft and airliners only use mechanical instruments as back-ups, if at all. LCD screens, modular digital computers and data-links are standard fare in today’s new aircraft. Autopilot coupling One of the first refinements of the basic Instrument Landing System was autopilot coupling. A traditional ILS receiver puts out a DC analogue signal that drives a meter movement. If you think of the signal as a command to “fly up/fly down” or “fly left/fly right” then you can use it as an input to an autopilot. The human pilot may manually select “ILS” mode siliconchip.com.au June 2004  13 HF Radio Antenna “NAV” Antenna “NAV” Antenna Above: typical “rabbit ears” VOR/ILS antenna near on the fin of a Piper Navajo Chieftain, with the longer wire HF antenna at top. The ILS receiver normally has separate transmission line inputs, one for LOC (~110MHz) and one for GS (~330MHz). Some aircraft have a separate GS antenna (typically dipole, mounted inside the nose), while many have just one antenna with a splitter. Right: a more aerodynamic nav antenna used on aircraft above about 250km/h. on the autopilot, telling the autopilot to follow the ILS output commands. Future displays Several companies are experimenting with new ways to display ILS information. One major source of inspiration is the video game industry. Current prototypes display a 3D graphic of the actual surrounding landforms and hazards such as masts and towers. The colours on the display indicate potential hazard, from red (land at or above the level of the aircraft) through yellow to green (land far below). Contours are shaded to give an easy to interpret depiction of the actual surrounding area. The display is based on a computer model of the actual terrain (yes, every aircraft might carry a detailed digital model of the entire world in the near future). Developers and promoters of these systems often call them “Synthetic Vision Systems”. During a landing in cloudy or foggy Aircraft and video games designers might seem to be strange bedfellows but they have a lot in common. One tries to make games simulate the real thing as much as possible, the other is incorporating much of the graphics of the games into the real thing, as this “Synthetic Vision” screen grab shows. 14  Silicon Chip conditions, a Synthetic Vision System can display the surrounding area as if it was a clear day. With suitable overlays (such as markers showing the correct approach path, and an aeroplane graphic displaying pitch and roll attitude as well as actual position), an approach and landing in poor weather could become very much like a computer game. On-board digital processing power The rise and rise of digital technology has hugely impacted the field of aircraft avionics. One of the first tasks given to digital processors was to process flight and navigation data using algorithms designed to make the most efficient use of resources. In the area of navigation, this meant keeping the aircraft right on the most direct track, and manoeuvring through standard terminal approach routes as accurately as possible. It wasn’t long before manufacturers started to integrate avionics systems that had previously been independent. As technology developed through to the late 80s, various researchers experimented with the idea of having one central navigation computer. By the mid 90s, the World’s major avionics producers all offered some variation on a central navigation and performance computer: the “Flight Management System” (FMS). Today, practically all new jets and an increassiliconchip.com.au Microwave Landing Systems Europe is moving rapidly away from the Instrument Landing System in favour of its newer rival, the Microwave Landing System (MLS). MLS is not simply an ILS using different carrier frequencies; in fact the operating principles of MLS are completely different. The purpose, though, is the same: to give precise lateral and vertical guidance, as well as distance from the runway. The basic technique used in the Microwave Landing System is a “Time Referenced Scanning Beam”. Without going into too much detail, MLS transmits a narrow beam at around 5GHz that sweeps across the approach area in a set pattern. The aircraft receiver measures the time intervals between sweeps and calculates its lateral position (azimuth) and vertical position (elevation). The “to and fro” azimuth and “up and down” elevation beams both occupy the same carrier frequency, although they are transmitted from two different antenna arrays located similarly to an ILS. The third, essential component of MLS is a precision DME (distance measuring equipment) which gives range accurate to within 30 metres (compared to 360 metres for regular DME). MLS also transmits data to the aircraft by modulating the azimuth signal. Data can include information about the approach, weather, runway condition, etc. There are several advantages of MLS over ILS. Perhaps the greatest advantage is its flexibility – ILS has only one correct path (where the difference in depth of modulation is zero), so its output must always be an error signal: “fly right”, “fly up”, etc. MLS is designed to tell the receiver its precise angle from the runway centre line (to about 40° either side) its elevation above the horizon, as seen from the touch down point (to about 15° up), and range from the runway. The receiver’s output is a position rather than an error. The MLS computer in an aircraft can be programmed with a desired approach path and then guide the pilot or autopilot along that path, comparing the actual position with the desired position to give the standard “fly right”, “fly up” signals. That means that the one MLS installation can precisely guide many different approaches at any glide path angle as well as manoeuvres such as dog-legs or curves around obstacles out to a distance of about 35km. It is reasonable to expect that MLS will completely replace ILS in Europe by about 2020, with only a few ILS installations surviving beyond 2015. The UK has purchased over 40 MLS installations (including options) over the past year alone. However, the rest of the World is likely to stick with ILS for several decades more. ing number of propeller planes come with a Flight Management System as part of an integrated, modular digital system. While different levels of integration are available, a fully functional FMS will have inputs from all of the on-board navigation and flight data systems and outputs to the autopilot computers, digital engine control computers, and various cockpit displays. During an approach in low visibility conditions the FMS can handle many tasks like selecting the frequency on the ILS receiver, continually monitoring how well the aircraft is performing, and commanding the autopilot and engines so that the aircraft follows a pre-defined “Standard Terminal Approach Route” (that’s right: when an aircraft is within about 30km of its destination it is usually “following a STAR”). This centralisation of control and monitoring functions has allowed automation to move into the part of flight that uses practically every system on the aircraft: the approach and landing. system is not too hard to imagine. A computer selects the right navigation inputs and autopilot modes so that the aircraft follows the ILS. A radar altimeter input (giving actual height above the ground, potentially accurate to a few feet), a precision DME (as in a Microwave Landing System), or a suitably augmented GPS controls the timing of the “flare” (the deliberate loss of lift as the plane lands). Weight-on-wheels switches detect the actual landing, and the computer controls deployment of spoilers, reverse thrust and brakes as required to slow the aircraft. GPS combined with a database of the airport layout provides for the aircraft to automatically taxi to its gate. (This ignores taxi clearances and other aircraft – a data link from the airport surface move- ment controllers could provide the required information.) However, technical possibility is not the whole story: If planes full of people are to routinely “autoland” in all sorts of conditions then technically possible is not enough. A reasonable margin of safety must be a part of the system. When a plane is landing itself, the Autopilot system has control of the aircraft. All autoland-equipped aircraft must have a “triplex” autopilot. That means that there are actually three separate autopilot systems installed in the aircraft. There are various ways Automatic landing Technically, an automatic landing siliconchip.com.au This cockpit almost looks like a video game –but it’s not. It’s from the Eclipse Aviation E500, a new mini jet scheduled for release in 2006. June 2004  15 GPS and all-weather landings... “What about GPS?” I can hear you asking. “Doesn’t GPS make the whole instrument landing system obsolete?” The answer is a resounding “not really”. In Europe, ILS should be obsolete by 2015. But that is due to a rival system (Microwave Landing System), not GPS. Outside Europe, ILS will still be around for a few more decades. So why hasn’t GPS taken over the precision approach scene? Since selective availability was switched off isn’t the accuracy down to millimetres? There are several good reasons why aviation has not relied on GPS for precision approaches. The first is political: the U.S. Department of Defence owns and operates the GPS constellation. Five years ago when the Clinton administration announced that they would switch off selective availability, they reserved the right to switch it back on again at any time. Despite formal agreements between the US Dept of Defense and the US Federal Aviation Administration, there has always been a tacit understanding that civil aviation should never rely too heavily on GPS without extra in-built safety. There are other issues with GPS. Accuracy is excellent, but still variable. For just a few minutes per day in any location, the various random errors combine to significantly degrade the accuracy. In the big picture it is hardly significant. But it does mean that you cannot solely rely on GPS if you require very high precision on demand. Another issue is signal availability. The receiver needs at least five satellites to verify the integrity of its position solution. Along the south coast of Australia, for example, there may be fewer than five satellites in view for a total of around 30 minutes out of every 24 hours. Having said all of that, satellite navigation systems are continually improving. Several developments are making GPS more available and accurate. A European consortium is developing a rival system called “Galileo”, which 16  Silicon Chip will have the political advantage of civilian control. The International Civil Aviation Organisation, which sees all of the major Western countries jointly determining policies, has decided that satellite navigation systems will be the basis for future aeronautical navigation systems. So does GPS have a role in all weather landings? Well, yes, it definitely does but not by itself. GPS will be available for precision approaches once suitable methods of “Augmentation” have been developed and tested. One ground-based system, called “Local Area Augmentation System” (LAAS) is basically a GPS receiver fixed to a precisely surveyed point on the ground. A computer compares the actual, known location of the receiver with its “GPS location”. It instantly detects any error. The system then broadcasts information about the error over a data link to all aircraft within a radius of, say, 50 kilometres. If the LAAS site is located near an airport, an aircraft can make a precision approach using GPS data corrected by the LAAS data link. Australia is likely to adopt this system. In the United States a satellitebased “Wide Area Augmentation System” (WAAS) is under development. This is similar in principle to LAAS but on a different scale: In version one of the system there are 25 surveyed “Reference Stations” across the country and two “Master Stations”. The data link to aircraft is via communication satellites. The advantage of covering the entire country is offset by the necessary compromises in accuracy and integrity. As a result, WAAS will only be good for the equivalent of ILS Cat I approaches (Decision Height of 200 feet). Even that will only be under ideal conditions. This limitation ensures the continued use of ILS in North America, at least for a few more decades. Europe is set to adopt a combination of ground based augmentation and microwave landing systems, with the Galileo global navigation satellite system likely to take over from GPS for essentially political reasons. of making a triplex system work: Normally, all three work together. They each gather their data (such as airspeed, attitude, deviation from intended path, etc) and then “vote” on the action (for example, to roll left at a certain rate). As long as all three systems agree, the autopilot is working in its full triplex mode. International standards allow automatic landings only when the autopilot is working in triplex. If one of the three systems fails or produces an error, the aircraft can still fly under the command of the other two autopilots but the safety of the triplex voting system is lost. In that case, the pilot must abandon the automatic landing, but may continue with a regular instrument landing. Instead of having three complete autopilot systems, it is possible to have a “pseudo-triplex” system. A computer model that votes according to the aircraft’s expected movements replaces one of the three autopilot systems. Conclusion Every year around the world, aircraft of all sizes safely make millions of landings in conditions that make a visual landing impossible. The Instrument Landing System has provided precise guidance for landing in these conditions for over sixty years. According to some authorities, ILS is likely to be in use for another fifty years yet. But in aircraft systems, like so many other things, technology is continually advancing as individuals look for better ways and companies look for a competitive edge. Improvements in GPS-related technologies, new capabilities of Microwave Landing Systems, and the almost limitless memory and processing capabilities of digital computers are turning our heads toward the future. Stanley Kubrik’s film “2001, A Space Odyssey” may have proven to be a tad optimistic in its setting. But anyone from that sci-fi mad era transported to the flight deck of a modern airliner as it approaches and lands in cloud and fog would surely think that he could be in a space craft landing on another planet. A crew of two, calmly watching the large, clear, uncluttered displays and checking altitude and system parameters out aloud while the plane lands itself: surely this was the stuff of science fiction not so long ago. siliconchip.com.au Ever looked at an aircraft instrument panel? What are all those meters and things for? For the uninitiated (ie, non-pilots!) an aircraft instrument panel can be a pretty confusing place. To make matters worse, every aircraft is different. But once you recognise what each is for and what it does, it’s not so daunting after all. . . B A C D G E F H This is just a tiny section of what a pilot has to keep his/her eyes on. But apart from radio systems, these are arguably the most important instruments as far as the pilot is concerned. (A) Clock (yep, to tell the time) (B) Airspeed indicator – in knots (C) Attitude Indicator (the plane’s, not the pilots!) (D) Altimeter – how high you are above sea level (E) Turn Co-ordinator (also called “turn & bank” or “turn & slip” indicators). (F) Radio Magnetic Indicator – displays both magnetic and radio compass. (G) Horizontal Situation Indicator – shows the localiser beam (the vertical yellow line) and the glideslope (the two yellow triangles on the edges). That’s the one that this article is most concerned with! (H) Rate of Climb Indicator – tells you how fast you are going up or down. And here’s how the loc/glideslope indicator helps you land... If the receiver is not receiving a strong enough signal, or if the signal is not valid, then a red “NAV” flag on the indicator warns the pilot not to follow the indications. Having used other radio navigation aids for lateral guidance and the altimeter for height, the aircraft is now lined up with the runway centre line, about 25km from the runway. The localiser indicates “on localiser” but there is no glideslope signal yet, so the plane does not descend. Having maintained altitude and followed the LOC, the plane is approaching the GS intercept. There is now a GS signal, indicating that the plane is below the glide path (ie, a “fly up” indication). “Fly Down” half scale ~ 0.25° above glidepath (the two yellow indicators are below the horizontal reference). “Fly Right” one dot (on 5-dot scale) ~ 0.5° left of path (the vertical yellow line is to the right of the vertical reference). “Fly Up”, “Fly Left” “Fly Down”, On LOC Continuing to hold the same altitude and follwing the localiser, the plane is now only about 0.2° below the glide path. The pilot (or autopilot) will start initiating a descent soon. “Fly Up”, On LOC The plane descends along the ILS and simply follows any “fly up”, “fly down”, “left” or “right” indications. By doing so it is flying precisely along the correct approach path. On glidepath, On LOC SC siliconchip.com.au June 2004  17