lnterscan MLS on Trial

Some 4,000 MLS systems will be required worldwide, probably between 50 and 60 of them at both civil and military airfields in Australia. The leading system candidate is lnterscan’s MLS, which is designed by Australians and manufactured at a factory in western Sydney.

ILS operates in radio communications bands, some of them among the radio and TV signals we tune into for news and entertainment. Consequently, the number of channels is limited. The 40 that are available are too few for congested parts of countries like the US which, although it needs more ILS approaches, simply can’t get the radio frequency allocations to install them. ILS also requires careful and often expensive site preparation. The signals can also reflect off buildings or surface features and provide misleading paths for aircraft users. And for the crowded skies of today, let alone for the years after 2000, it is too inflexible.

Here’s how ILS works. Transmitters operating in the very high frequency and ultra high frequency bands (VHF/UHF} project a radio beam that denotes the runway centreline at an angle of three degrees. The beam is ten degrees wide, divided into two that are each five degrees wide. On the left, the signal operates at a distinctive frequency; on the right, at another frequency. In the centre, like the little bear’s porridge, the signals a user aircraft receives are just right. Both signals merge and the airborne instrument identifies this as the track to follow down to the runway.

At intervals along this narrow fan of radio signals are markers that trigger other visual and audible signals in the receiver. The first is known as the outer locator or locator outer marker and the second as the middle marker. Once established on the beam, the aircraft following the glide path will descend at a steady angle of three degrees (on some equipment it can be two degrees} until the pilots sight the runway.

Over the years, the system has been developed to an astonishing degree of accuracy. British Airways, for instance, is certificated to fly the beam right on to the ground in completely blind

conditions. The landing and the rollout are completely automatic. So if the system is that good, why change it?

Apart from the inherent site and frequency congestion problems described earlier, the new MLS have an outstanding advantage.

With ILS, every aeroplane using the system (and that means almost every landing at some airports} has to fly an identical track. From the point where they arrive in the airport control area, they are vectored to intercept the ILS beam. Then each one has to fly down the same slope, maintaining a safe separation If you want to see how it works, stand on a point overlooking the arrivals end of a working runway of a busy airport during the rush hour on a busy evening. You can see the landing lights of the approaching aircraft stepped back for miles, all following the same track. From miles out, the noise from the arrivals harries the same footprint on the ground as every flight passes overhead. And the capacity of the system is necessarily limited.

MLS operates in a completely different manner (see figure 2}. For the first time it will allow aircraft using advanced systems to use a number of different approaches into a major airport and furthermore approaches which can be curved in both the horizontal and the vertical planes (see box and diagram}. Providing aircraft are certificated accordingly, users are no longer limited by the constant three degrees of descent down an identical path. They can plan a curving arrival that will take them into the 120 degree coverage area of the MLS signals and their airborne computer controlled systems will then use the MLS information to steer them to a point close to the end of the runway, where they join the runway centreline for a conventional landing.

How does this happen?

Instead of using a single flat and relatively narrow radio beam to delineate the arrival’s path, MLS uses three radio signals. The first is a vertical fan shaped beam which sweeps to and fro through the approach area. This is the azimuth beam which tells the aircraft its bearing relative to the extended runway centreline. The second provides the glide slope angle. It does this by producing a flat, fan shaped beam and scanning it up and down. The third is simple. It is a precision DME (distance measuring equipment) that continuously tells the airborne equipment how far away the plane is from the runway.

In some installations, the equipment is duplicated at the reverse end to give accurate departure or missed approach guidance, or to permit landings on the opposite end of the runway.

If you know your bearing to the runway horizontally (in azimuth) and ver tical angle (elevation) and also your distance, a computer can quickly calculate the exact position of the aeroplane and display the information to the pilot or pass it on to the automated flying controls.

Normally the azimuth beams scan some 13 times a second (or twice as fast for even more precise positioning information}, which means the computers are updating the aircraft’s position every thirteenth of a second. Even at the approach speeds jets operate at, the computers are providing position information along about every 10 metres or so of track, or for every five metres or less as they slow down and flare for landing. The position accuracy at touchdown is also highly accurate, to within a twentieth of a degree of arc. Because vertical position is critical during approach, the elevation beam scans up and down at three times the normal rate of the azimuth beam, or 39 times a second.

The ICAO-adopted system is called time-referenced scanning beam (or TRSB} and it is basically an Australian idea, which was called lnterscan MLS. Here’s how it works.

Visualise a spotlight swinging to and fro, as ififwere fastened to an electric fan. You are standing in a position where you can see it sweep across and back again. As the light passes you from left to right, you start your stopwatch and punch the timer again when the beam returns in the other direction. You know the speed of the scan so the elapsed time while the beam was tracking to the end of a scan and back again will tell you where you are on the arc and therefore your position from the spotlight. You could determine the vertical angle in the same way if the scan were up and down.

The original idea came from Australia’s Commonwealth Scientific and Industrial Organisation (CSIRO). Time-referencing a fast scanning beam is a technique used in radio astronomy. But instead of using a stopwatch calibrated in hundredths of a second, TRSB does its sums using milliseconds.

US aviation experts favoured the TRSB system over a British proposal, which used measurements of frequency changes caused by movement, or the doppler effect (the classic illustration is the change in pitch of a car horn or train whistle as it passes an observer). Basically both need advanced and complicated electronics. TRSB uses it at the ground end; doppler requires more complex equipment in the aeroplane itself.

The Australian government decided to back the Australian idea by establishing a company called interscan (derived from time INTERval SCANning beam). The problem the company ran into was that the original CSIRO TRSB transmitting equipment used an electronic scanning method called an electro-magnetic lens to shape the beams and move them in space. Unfortunately the concept proved to be sensitive to temperature variations, with expansion of the metal in the antennas causing beam drift. It also had only a limited azimuthal coverage.

Taking on an American partner, Wilcox Electrics of Kansas City, interscan moved to the US for a period and began extensive work developing new antenna systems to meet the proposed US standards. With Wilcox, it tendered successfully for some key installations at US airports for both demonstration and operational systems. The main change from the original concept was a new set of completely different transmitting antennas, which consist of arrays of radiating columns and patches. The beams are formed and scanned by electronic phasing. The same principle is used to transmit radar pulses in phased array systems, such as on the latest US Navy Aegis air defence cruisers.

One advantage of moving to electronically phased arrays is that they are applicable to other uses. While waiting for the world MLS market to emerge, lnterscan was able to translate its antenna expertise into developing a new electronically modulated Tacan (tactical air navigation) antenna for the RAAF (these are the military equivalent of civilian ground based aids) and also forcivil VHF omni range beacons (VOR). lnterscanhas gone on to develop and manufacture a secondary surveillance radar antenna for Italy’s Selenia, which that company is marketing worldwide as part of its approach radars. These secondary surveillance radar antenna contracts have so far earned about $30 million for Australia and lnterscan.

The first Australian MLS installation was recently installed at Canberra Airport, where both the Australian Civil Aviation Auth ority and the Royal Australian Air Force will test it this year. Interscan has also been successful against strong competition in selling the first European MLS installation, which will go into Salamanca in Spain and has signed an agreement with Italy’s leading electronics and telecommunications group to market MLS in southern Europe. lnterscan is also building MLS receivers as part of its product range and the company is wholly owned by the Australian Industry Development Corporation (AIDC).

”. At a time when Australians are concerned with the need to develop high technology industries, based on Australian science and engineering, lnterscan has shown that it can be done,” managing director Barry Atmore said. “At interscan, we are obviously ideally positioned to build many of the new MLS installations the world is going to need in the coming eight years. ”

How Interscan Allows Varied Approach Tracks

The microwave landing system (MLS) scans an area up to 60 degrees in arc for a distance of up to 20 nautical miles (36km) with two fan shaped radio beams, one of them vertical and the other horizontal. The timed beams allow a receiver in an aircraft to measure its vertical angle (or elevation) to the transmitter, which is located next to the runway of the target airport, and also its horizontal (or azimuth) bearing. A third device, a precision DME (distance measuring equipment – described as DME/P) gives a readout of the distance to the runway. The elevation angle, bearing and distance provide an exact position.

As with traditional instrument landing systems, the information can provide either a flight deck display showing the track along which the aircraft must be flown to reach the end of the runway, or it can feed navigation information directly into the flight control computers which fly the aeroplane.

A modern flight control system allows a number of intermediate points to be set in the computer, known as waypoints, and will fly the aircraft from one waypoint to the next. It can also be programmed to descend through certain preset heights.

As interscan MLS develops and airborne equipment is improved, it will allow pilots to fly a landing approach curved in both altitude and in track over the ground. The pilot will be able to set up waypoints and heights in a predetermined pattern. The waypoints will be able to track an aeroplane around residential areas to minimise noise or to avoid a hazard, such as a high tower or high ground. But eventually the onboard receivers will feed into aircraft flight control systems which will steer it along a predetermined track into the extended centreline and down to a runway.

For instance, a controller in the future will be able to instruct a pilot using MLS to operate on Alpha or Bravo approach – or many others. The pilot would be able to call up these on his navigation computer and preset the appropriate one (figure 2). The aircraft will then enter at a designated waypoint and fly a curving track towards a point where it will enter the extended centreline path down to the runway, perhaps initially with a steep descent that will flatten out for the final landing phases.

Eventually, MLS will be used for precision automatic landings, positioning the pilot so he is at a safe height and lined up with the centreline of the runway. The extent to which the aircraft is cleared to descend without the crew being able to see the runway are divided into categories.

Instrument landing categories use two measurements, vertically in feet or in metres for forward visibility (technically, RVR for runway visibility range).

In the earlier years, MLS will be cleared for Category I landings. This means a pilot can descend to 200ft but must be able to see 800 metres ahead (or RVR) so that he can complete his landing. A move is under consideration to lower the Category I decision height to 150ft. Category II clearance will allow the pilot to descend to 100ft with 400 metres RVR. There are three stages in Category Ill. The first allows a descent to zero height but still requires 200m RVR, lb is zero height and 50m RVR and Ille is the ultimate – ‘zero-zero’