Let’s land an airplane. Shall we?
Now that you are aware of how the orbiter lost communication with the Vikram Lander, you must also be aware that Communication indeed plays an important role in most of our lives from WiFi to a phone call if not in space missions. In this blog, let us see about how an airplane communicates with the ground station to land perfectly.
We all know that an aircraft descends in the altitude gradually before it extends the wheels out to land. Wait a minute, it could even descend towards a barn or a house. How does it spot a runway to land. Did the pilot eyeball the runway as Captain Sully does. No. Here an interesting thing happens, the pilot doesn’t go in search of the runway. The runway invites the aircraft on its landing strip. But how?
There is a communication equipment installed in the runway called the Instrument Landing System (ILS). The ILS provides guidance to the pilot to land the aircraft on the runway. To provide correct approach path information to the pilot, two different signals are required to be transmitted. The first signal gives the information to the pilot indicating the aircraft’s position relative to the center line of the runway. The second signal gives the information indicating the aircraft’s position relative to the required angle of descent. The ILS achieves this feat using two sets of Antennas called the Localizer and GlideScope. Lets see what are they.
Localizer:
The VHF localizer providing horizontal guidance radiates at 108.10 MHz to 111.95 MHz at about 100 W with horizontal polarization. The antennas used for this emission are highly directional array antennas positioned at least 1000 feet from the stop end of the runway. The carrier signals are amplitude modulated where the Modulation frequencies are pure sine waves at frequencies 90 Hz and 150 Hz. If you look at the radiation pattern below, we could see that one lobe on the right is emitted at 90 Hz whereas the other lobe on the left is emitted at 150 Hz. The aircraft receives the signals from both the lobes and compares their signal strength. Now, if an aircraft receives equal amplitudes of 90 Hz and 150 Hz, the aircraft is headed towards the center of the runway. If the aircraft receives signal such that the amplitude of 90 Hz is greater than that of 150 Hz, it means the flight should move in the left till both the amplitudes are equal. It has to move towards right if the Signal strength at 150 Hz is more. The difference in signal strengths is referred in a term called the Difference in depth of modulation (DDM). If this DDM is zero, it means that the pilot is on course towards the center of the runway.
The range of these lobes extends for a few nautical miles (about 10 nm) to be detected by the flight from a distance. Added to the guidance, it provides the Code Identification stating the information about the Airport and runway at 1020 Hz using Morse codes.
GlideScope:
Now, the aircraft has got the guidance to the center of the runway. Up next is when to descend. As you might guess it could even descend early before the runway or descend near the stop end the runway. For descending purpose we have another set of antenna called the GlideScope.
The UHF GlideScope transmitter is slightly offset from the runway positioned at 1000 feet from the approach end of the runway and provides a vertical guidance for the aircraft from 329.8 to 335 MHz. The same modulation frequencies 90 Hz and 150 Hz used in localizer are used here too where the DDM is calculated. The emission takes place at around 5 watts with a Horizontal polarization. As seen in the below picture, there is a broadcast of two lobes centered at 3 degrees from the horizontal plane. Now the aircraft follows the same approach as it followed for the localizer reading the signal strengths of both the lobes. The 3 degree offset is due to that the aircraft avoids and remains above the obstructions and reaches the touch down point.
Receiver:
The receiver has a navigation equipment, radio, two antennas to pick up the VHF and UHF signals. The radio basically does a demodulation to split 90 Hz and 150 Hz from the carrier waves and those signals are sent to the navigation instrument to calculate the DDM. Coming to the bands, like the WiFi which has 14 channels on a 2.4 GHz, the ILS has around 40 channels to choose from. If the localizer frequency in the VHF is known, the corresponding GlideScope frequency is automatically tuned by the radio.
Now, the navigation instrument displays two markers to the pilot. One marker which goes left to right depicting the localizer reading and the other going top to bottom depicting the GlideScope reading. Now the pilot has to maintain both the markers in the center to go on course.
When does the pilot start paying attention to these signals? Of course, during landing but the signals are transmitted miles away (at least 10). Now, the aircraft first detects the localizer signal first positioning itself in parallel to the runway then it goes along detecting the GlideScope signal to gradually descend.
That’s all with the Instrument Landing System Folks. Do try to figure out where these antennas are located if you get a chance to board or deboard a flight.
-73,
