Avionics (a term that combines aviation and electronics) applications have highly demanding and rigorous requirements due to their operational environment. The failure of an aircraft avionics component may place lives at immediate risk. As such, it is vital that all aspects of avionics equipment be monitored and measured closely for installation and servicing defects.
As shown in Figure 1, avionics is broadly classified as covering the categories of navigation, communications, sensors, and displays and data recorder. With the exception of fly-by-wire electronic control flight systems, the classification above remains valid for most modern aircraft, both civil and military.
1. Spectrum allocation
Table 1: RF spectrum allocation (typical) for aeronautical radio navigation signals
High frequency (HF) communications, spanning from 3 MHz to 30 MHz, utilizes single sideband, suppressed carrier modulation with a bandwidth of about 2.5 kHz, typically at several hundred watts of transmitted power. However, HF propagation varies with frequency, weather, time of day, and ionospheric conditions. Very high frequency (VHF) communications span two different bands: 30 MHz to 88 MHz exclusively for military users, and 118 MHz to 156 MHz for both civil and military users, with standard double sideband AM modulation at 40 dBm to 45 dBm of transmitted power. Ultra high frequency (UHF) communications encompasses both VHF and UHF operating from 225 MHz to 400 MHz. FM-modulated schemes employ 40 dBm to 50 dBm of transmitted power, and AM-modulated schemes employ 40 dBm to 44 dBm of transmitted power. This band is often used by military users for various pulsed, frequency hopping and electronic counter-counter measures (ECCM), such as antijamming.
Long range enroute radio navigation such as the global positioning system (GPS) operates at slightly higher ranges of the spectrum band. Line-of-sight (LOS) approach radio navigation such as very high frequency omnidirectional radio range (VOR), instrument landing systems - glide slope (ILS-GS), instrument landing systems -localizer (ILS-LOC), and marker beacon (MB) operate at HF and VHF bands. Distance measurement equipment (DME) is allocated for operation within UHF bands.
Navigational and landing instrumentation
From the perspective of the pilot in the cockpit of an aircraft, VOR/ILS and MB are analog-based displays and GPS forms the digital-based displays. The licensed aircraft engineer (LAE) is responsible for ensuring the accuracy and reliability of the instrumentation before the pilot takes over. Figure 2 focuses on the aeronautical radio navigation aspect, not the auxiliary control and power instrumentation.
The aeronautical radio navigation equipment antenna transceivers are typically located on the nose, fuselage, and vertical stabilizer, depending on the most likely signal direction of arrival (DOA). For example, GPS transceivers are located at the uppermost portion for better satellite reception and the ILS glide slope and localizer are located below for better reception during landing approaches.
Enroute navigation includes the use of GPS, RADAR, and VHF with air traffic control (ATC), distance measurement equipment (DME), and automatic direction finder (ADF). For example, when using the GPS and ADF, pilots will know the aircraft position and will request landing clearance from ATC. For heavy air traffic, ATC sequences the aircraft arrivals, with pilots executing a "holding stack" - usually based on a "racetrack" pattern (refer to Figure 4). VOR/DME stations serve as waypoints before the final descent and landing approach, guided by the distance from the runway as provided by marker beacons (MB) and the instrument landing system (ILS).
As a method of approach navigation, the instrument landing system (ILS) includes three functions: Glide slope (GS), localizer (LOC or LLZ), and marker beacons (MB). The glide slope ensures that the vertical descent path is aligned to the ideal descent path in tandem with the distance from the runway based on the marker beacons. The localizer ensures that the lateral left-right approach is aligned with the center of the runway. MB in landing approach is gradually being replaced by DME, but is essential nonetheless for existing airport infrastructure.
2. Aeronautical radio navigation
2.1 VHF omnidirectional radio range (VOR)
VHF omnidirectional radio range (VOR) operates at VHF frequencies of 108 MHz to 118 MHz to provide aircraft with a bearing to the ground station location. By demodulating the signal of a VOR transmitter station, the VOR receiver of an aircraft is able to provide bearing information relative to the transmitter station . The aircraft position can be obtained by triangulating two or more stations. VOR stations give a relative bearing with respect to ground stations.
A VOR instrument in the cockpit can be set to either a FROM or a TO configuration. In the FROM convention, the beacon is made the reference point and the bearing angle is between magnetic north and the beacon-to-aircraft line. For the TO convention, the aircraft is made the reference point and the bearing angle is between magnetic north and the aircraft-to-beacon line. (pTO = 180° - (pFROM (see Figure 7).
VHF omnidirectional radio range (VOR) operates continuously at carrier frequencies of 108 MHz to 118 MHz, with the code identification, or COM/ID, using up to four letters in Morse code, transmitted on a modulation tone of 1.020 kHz. Figure 8 shows an example VOR spectrum with a 108 MHz center frequency. The spectrum is displayed in logarithmic scale using the R&S®FSV.
The 30 Hz reference (REF) signal is frequency modulated with a peak deviation of 480 Hz on a 9.96 kHz carrier. This frequency modulated subcarrier then undergoes amplitude modulation on the VOR carrier frequency. The variable (VAR) phase signal is amplitude modulated directly on the VOR carrier frequency using an antenna array that establishes a rotating "cardioid" shaped antenna pattern rotating at a 30 Hz rate, or 1800 rpm. The REF signal is transmitted from a fixed omnidirectional antenna, and thus contains no time-varying spatial modulation signals. The relative phase comparison between both 30 Hz signals is proportional to the bearing of the transmitter due to this rotation.
The two signals are set to be in phase at magnetic north, 90 degrees out of phase at magnetic east, 180 degrees out of phase at magnetic south, and 270 degrees out of phase at magnetic west. VOR receivers function by receiving both VAR and REF signals, comparing their phase, and displaying the bearing to the station to the pilot for the FROM convention.
2.2 Instrument landing system (ILS)
The instrument landing system (ILS) provides aircraft pilots with landing approach data relative to the ideal landing course. This is especially critical when visibility is poor due to bad weather, night landings, and crosswind approaches.
ILS-GS enables the pilot to steer up or down correctly during landing. This vertical correction is performed via two AM carriers with an AM depth of 40 % operating at a frequency range between 329 MHz and 335 MHz. For the aircraft approach, the upper tone is modulated at a frequency of 90 Hz by default, and the lower tone at a frequency of 150 Hz . Vertical axis phased antenna arrays are utilized for beamforming.
ILS-LOC/LLZ enables the pilot to steer left or right correctly during landing. This lateral correction is performed via two AM carriers with an AM depth of 20 %, operating at a
frequency range between 108 MHz and 112 MHz. For the aircraft approach, the left tone is modulated at a frequency of 90 Hz by default, and the right tone at a frequency of 150 Hz . Horizontal axis phased antenna arrays are utilized for beamforming.
"Difference in depth of modulation" (DDM) is the relative difference between the two AM carriers of 90 Hz and 150 Hz. By demodulating the received glide slope signal and calculating the difference in depth of modulation (DDM) between the two tones, the ILS-GS provides the pilot with vertical course profile data. Likewise, by demodulating the received localizer signal and calculating the difference in depth of modulation (DDM) between the two tones, the ILS-LOC/LLZ provides the pilot with lateral course data. Marker beacons indicate the distance from the start of runway at different audible tones.
Simultaneous ILS-GS, ILS-LOC/LLZ, and marker beacons provide aircraft with a descent approach path that is reliable and approved by ICAO.
2.2.1 Instrument landing system - glide slope (ILS-GS)
The glide slope transmitter is located near the end of the runway (nearest to the start of the aircraft approach). Typically, vertically aligned antennas transmit two intersecting main beams on top of one another at carrier frequencies between 329 MHz and 335 MHz. The top beam is usually modulated at 90 Hz and the beam below at 150 Hz. With careful field installation and maintenance, the received signal will be modulated equally along the centerline of the glide slope.
The total beam width is approximately 1.4 degrees and the angle of the desired approach glide slope is 3 degrees. By demodulating the received glide slope signal and calculating the difference in depth of modulation (DDM) between the two tones, the ILS-GS provides the pilot with vertical course data .
Difference in depth of modulation (DDM) is the relative difference between the two AM carriers of 90 Hz and 150 Hz. If the DDM is a positive value, the upper 90 Hz beam is predominant, whereas if the DDM is a negative value, the lower 150 Hz beam is predominant. A pilot would need to exercise motor judgment to maintain enough power and approach angle for ideal vertical descent (DDM=0). The landing approach angle must also be corrected so that the aircraft landing will be "cushioned" by air upon touchdown and be within tolerances of the landing gear's mechanical integrity.
2.2.2 Instrument landing system- localizer (ILS-LOC/LLZ)
The localizer transmitter is located near the end of the runway (nearest to the start of the aircraft approach). Typically, horizontally aligned antennas transmit two intersecting main beams beside one another at carrier frequencies between 108 MHz and 112 MHz. As seen from the approaching aircraft coming in for a landing, the left beam is usually modulated at 90 Hz and the right beam at 150 Hz . With careful field installation and maintenance, the received signal will be modulated equally along the centerline of the runway.
The total beam width is approximately 5 degrees, and the localizer receiver uses this modulation to determine the correct descent approach path. By demodulating the received localizer signal and calculating the difference in depth of modulation (DDM) between the two tones, the ILS-LOC/LLZ provides the pilot with lateral course data.
Difference in depth of modulation (DDM) is the relative difference between the two AM carriers of 90 Hz and 150 Hz. If the DDM is a positive value, the left 90 Hz beam is predominant, whereas if the DDM is a negative value, the right 150 Hz beam is predominant. A pilot would need good psychomotor skills to maintain the aircraft at an ideal lateral correction of DDM=0, especially when visibility is poor or in crosswinds. In some instances, adverse weather conditions brought about by crosswinds would require the pilot to bank the aircraft's nose towards the incoming wind, while keeping the undercarriage aligned towards the runway. This process is called "crab landing". Fortunately, accurate and reliable ILS-LOC/LLZ and ILS-GS systems are able to guide pilots even under poor visibility conditions or in adverse weather.
2.2.3 Marker beacon (MB)
Marker beacon (MB) receivers decode audio and provide signaling output to identify one of three marker beacons installed near the runway. The markers are placed as shown in Figure 14 above, in accordance with International Civil Aviation Organization (ICAO) Annex 10 Volume I Radio Navigation Aids.
Marker beacons transmit a narrow beam width at 75 MHz carrier frequency in a vertical direction, and each has a different distinct modulation code to allow the receiver to identify which one it is flying over. The pilot can determine which marker beacon was flown over either by visual identifying the color of the marker beacon or by listening to the audio tone. The outer marker beacon is modulated at 400 Hz, the middle marker beacon at 1300 Hz, and the inner marker beacon at 3000 Hz. The audio/visual pairing of marker beacons is as follows:
• Outer marker flashes BLUE in the cockpit at 400 Hz ("relaxed" tone).
• Middle marker flashes AMBER in the cockpit at 1300 Hz ("hurried" tone).
• Inner marker flashes WHITE in the cockpit at 3000 Hz ("urgent" tone).
Original by Rohde & Schwarz Aeronautical radio navigation