Background:
Aviation has always been dependant on weather and climatic condition and still is to a certain extent but the invention of the ILS has meant that certain aircraft can still operate in complete fog bound conditions.
The British RAF invented the ILS approach equipment during the Second World War to help the aircraft to get back to their airfields after bombing missions. Although very basic and very expensive to install the potential of the system caused it to be developed after the war. It is now a system that has been incorporated into nearly every international airport in the world. As well as the fixed ILS the RAF have developed the ILS to make it mobile for use abroad and when at war. This allows aircraft to complete missions and return to bases safely in near zero visibility. Newer systems that are still under design use microwave beams that makes them incredibly accurate and reliable.
The picture to the left is a mobile ILS that is used by the RAF when they move around the world.
How Does It Work?
The ILS is a device that is created from many different applications of technology. In order to fully explain how the device works I am going to split the explanation into four topics, the localiser transmitter, the glide path transmitter and the marker beacons and the equipment used. These are the three components needed to make the ILS work.
UHF and VHF Equipment:
The ILS uses UHF and VHF equipment to transmit to the aircrafts instruments.
The transmitter takes some sort of message (it could be the sound of someone's voice, pictures for a , data for a radio modem or whatever), encodes it onto a sine wave and transmits it with radio waves. The receiver receives the radio waves and decodes the message from the sine wave it receives. Both the transmitter and receiver use antennas to radiate and capture the radio signal.
You can see below the frequencies of radio waves; The ILS uses frequencies in both the UHF and VHF region to transmit data.
Below I have described two different types of radio waves and how they are transmitted.
Amplitude Modulation - In amplitude modulation, the amplitude of the sine wave (its peak-to-peak voltage) changes. So, for example, the sine wave produced by a person's voice is overlaid onto the transmitter's sine wave to vary its amplitude.
Frequency Modulation - FM radio stations and hundreds of other wireless technologies (including the sound portion of a , cordless phones, cell phones, etc.) use frequency modulation. The advantage to FM is that it is largely immune to static which is very important when dealing with very accurate electronic devices. In FM, the transmitter's sine wave frequency changes very slightly based on the information signal.
Once you modulate a sine wave with information, you can transmit the information.
The receiver is then simply tuned in to the specific frequency that the transmitter is transmitting on and then the information is fed into the instruments where it is decoded. Once the instruments decode the information that is sent out by the transmitter it is displayed on the instrument face, which can be viewed by the pilot.
The Aerials:
All transmitters of any kind require an aerial to transmit the signal through the air. There are quite a few different types of aerial including the ferrite aerial that is used for AM radio and consists of a coil, the multi-element aerial which are used to pick up VHF signals and TV signals and the car aerial which is a half dipole. Then there is the dipole aerial which is designed to be a certain length for a certain range of frequencies. In my case the frequency of the ILS transmitting signal. A dipole aerial consists of two pieces of wire, which together are half the wavelength of the station. VHF stations such as the ILS require aerials of about 3 metres in size. This aerial in conjunction with the transmitting circuit make up the ILS transmitter.
The Localiser Transmitter:
The localiser transmitter works in conjunction with the aerial system and gives direction to the pilots’ instruments along the horizontal plane. The transmitter transmits using two different frequencies, a higher frequency of about 150Hz and a lower frequency of about 90Hz. The system is then calibrated regularly to align the centre of the two frequencies with the runway centre line. This signal is sent out and the aircrafts ILS receiving equipment detects the signal. It utilises the signals sent out by the transmitter and will give a display on the instruments how far left or right the aircraft is of the centre line. The signal that is send out is sent out in two fan shapes of the two frequencies that over lap. This overlap area is the centre of the runway line. The diagram shows the localiser signal pattern and what the instruments look like in the aircraft at different positions on the localizer. The width of the navigational beam may be varied from approximately 3º to 6º, with 5º being normal. It is adjusted To Whom It May Concern: provide a track signal approximately 215 meters wide at the runway threshold (very end start of runway). The width of the beam increases so that at 10 NM from the transmitter, the beam is approximately one mile wide.
Glide Path Transmitter:
The glide slope provides vertical guidance to the pilot during the approach. A ground based UHF radio transmitter and antenna system, operating at a range of 329.30 MHz to 335.00 MHz, with 50 kHz spacing between each channel, produce the ILS glide slope.
The picture above illustrates the glide-path component of the Kelowna ILS.
As we know the frequency of the signal and the speed at which the signal is transmitted it is reasonably straightforward to work out the wavelength of the signal.
λ=0.911m at the low end of the frequency
λ=0.895m at the high frequency end
The transmitter is located 230 to 381 meters down the runway from the threshold and offset 122 to 183 meters from the runway centreline.
The picture above illustrates the localizer component of the Kelowna ILS.
Like the localizer, the glide slope signal consists of two overlapping beams modulated at 90 Hz and 150 Hz. Unlike the localizer, however, these signals are aligned above each other in line with the runway centre line.
Again the image shows how the signal is sent out and how it is displayed on the instruments.
Autoland Capabilities:
Each runway ILS Radio Signal is given a category depending on its quality. The better the quality the higher the category. The best being Cat 3C. The worst Cat 1 that is not Autoland approved. Only Cat 2 and 3 allow aircraft to Autoland.
For a pilot to be able to make an approach to a runway the weather has to be at or above certain conditions depending on the CAT number of the ILS.
The table below gives these conditions:
For example if the ILS was a Cat 1 ILS and the visibility was 400 metres and the base 30 meters the pilot would have to hold or divert. This is what causes so many problems at smaller airports where the budget airlines fly, as they do not pay to have their aircraft fitted with CAT 3 equipment.
This all basically means that aircraft can fly themselves following the ILS signal onto the runway as long as the CAT 3 equipment has been purchased and installed into the aircraft, which brings us on to the next section, problems.
Problems Of The ILS:
Although now the ILS is an almost essential piece of technology on aircraft it didn’t use to be so accurate and many aircraft would attempt a landing in zero visibility only to find that the runway wasn’t where the ILS said that it was.
When the company Airbus came into the aviation market they thought that it would be a good idea to incorporate the ILS system into their new aircraft, which meant that the computer expected the aircraft to land on the runway that the ILS was tuned into. This worked very well until the aircraft wasn’t actually landing. On the 26th June 1988 a new airbus A320 was displaying at an air show, when the onboard computer decided to land. The pilots attempted to climb but the computer kept pushing the nose down. The consequences of which were the aircraft overran the runway into the near by forest.
Bibliography:
http://www.allstar.fiu.edu/aero/ILS.htm - A site dedicated to Navigation Systems and avionics for aircraft.
Advanced Physics Text Book; S. Adams - Used as a resource for information about antenna arrays.
Ground Studies For Pilots Book; S.Taylor - A very detailed book dedicated to explain radio aids in aircraft and on the ground.
Avionics, Past & Present; Honeywell – This book gave some good information about avionics in aircraft.
