Radio Navigation Flashcards
Speed of light
300,000 km/s
Relationship between frequency and wavelength
c = f x lambda
c = speed of light
f = frequency
lambda = wavelength
Phase angle
Fraction of a wavelength, expressed in degrees
Increase in power required to get an increase in range
Power needs to increase at square of increase in range (so 4 x power to get a 2 x range signal)
Radio frequency spectrum
Radio frequency mnemonic
Very
Lovely
Maidens
Have
Very
Useful
Sewing
Equipment
Sidebands
When an AM (amplitude modulation) signal is created from intelligence and carrier wave, two sidebands are produced:
eg 400 kHz carrier, 1 to 3 kHz intel.
Upper sideband is 401 to 403kHz
Lower sideband is 397 to 399kHz
SSB transmission
- description
- which bands use it?
Single Sideband transmissions only transmit the upper sideband (which contains the intelligence info) to save space.
Single Sideband Suppressed Carrier also don’t transmit the carrier wave itself.
HF comms use this method.
Frequency Modulation advantages
Higher quality as static has less impact
Frequency Modulation disadvantages
Mixture of frequencies more complex so sidebands can’t be cut out, so twice as much bandwidth and three times as much power needed.
Higher bandwidth means transmission restricted to lower power (lower distance) to avoid clogging the airwaves.
More complex equipment needed.
Modulation of HF, VHF & UHF
All AM
Phase modulation
- known as
- uses
Known as phase-shift keying (PSK)
- GPS
- WLAN
- Bluetooth
Methods of sending binary information
Use amplitude, frequency or phase shift keying.
ASK: Switch carrier wave on and off
FSK: Vary frequency for 1s and 0s
BPSK: Reverse the phase for 1s and 0s
[B stands for binary]
Emission classification
3 digit code
1 - Describes type of modulation (N - unmodulated, A - double sideband)
2 - Type of modulating signal (e.g. digital, analogue)
3 - Type of information carried (e.g. morse, voice, morse & voice)
Polarisation
Vertical vs horizontal matters, aerial has to be oriented same way as transmission.
VHF voice signals vertical (vertical aerial), nav frequencies horizontal so V shaped aerials.
There is a magnetic field at right angles to the electric field we use (designated H and E respectively).
Dipole aerial
The SIMPLEST FORM OF ANTENNA!
Simple straight aerial with two ends, oriented in same way as signal, length should be half of the wavelength required.
Monopole aerial
Half a dipole so a quarter of the wavelength.
Transmits from the sides of the pole, deadzone in area it points to (cone of silence)
Antenna shadowing
If one antenna is in the way of another (e.g. two VHF antenna on top of aircraft) the one further from the signal will be in a shadow. The one ahead absorbs some of the energy of the wave.
Parabolic Antenna
Shaped like a dish. The shape causes transmissions at all angles into the dish to head out in parallel lines. Useful for navigational signals.
Inefficiency leads to loss of signal in sidelobes and also backscatter. This causes loss of energy and confusion to directional signals.
Phase array aerials
Series of dipole aerials next to each other set up with phases to achieve a better directed signal than parabolic antenna. Still get some sidelobes.
Slotted scanners
- aka
- description
AKA slotted planar arrays or Flat Plate
Work similar to phase arrays but use slots instead of multiple vertical dipoles. Used in aircraft.
Tighter beam than phase arrays or parabolic antenna.
Benefits of phase array/flat plate/slotted scanner vs parabolic antenna
Primarily have reduced side-lobes.
Also can have tighter beam, but NOT considerably.
Cause of refraction
Change of speed, either due to the medium waves are passing through, or the surface they are passing over.
Which frequencies refract the most in ionosphere?
Low frequencies
Cause of diffraction
Caused by “sharp objects”, i.e. getting blocked by an edge or wall with an opening. Through a small opening a circular pattern will be created emitting from the hole.
Which frequencies are diffracted the most?
Diffraction greatest at longer wavelengths, so low frequencies
Specular vs diffuse reflection
Specular is like a mirror, i.e. a smooth surface. Diffuse is off a rough surface.
Note that depending on the wavelength a surface that appears rough may produce specular reflection, for example radio waves bouncing off a mountain.
Radar reflection
Depends more on re-radiation than specular reflection.
It is important that the wavelength is compatible with the target size (the same size as the target or smaller).
Attenuation
Reduction in power of a radio wave.
Atmospheric attenuation is due to dust and items in the atmosphere, surface attenuation changes over different surfaces (ice caps and poles worst, sea the best).
Which frequencies most affected by attenuation?
High frequencies most affected by surface and atmospheric attenuation.
However attenuation in the ionosphere due to passing through charged particles, is strongest on low frequencies.
Space waves
- Max range
- Frequencies that use it
Line of Sight waves
Max range (NM) =
1.23 x sqrt(height in feet rec.)
+ 1.23 x sqrt(height in feet trans.)
Used in VHF (and higher freq)
Surface waves
Waves following curvature of the earth due to diffraction and attenuation.
Diffraction strongest at low frequencies so low frequencies give longest surface waves.
HF: 100NM [VERY SMALL - not useful]
MF: 500NM
LF: 1000NM
VLF: 4000NM
Skywaves
- time of day
- layers
Waves that are reflected back from the ionosphere, strongest in day (due to solar radiation). Refract from E and F (not D) layers of ionosphere.
Skip zone
HF skywaves bounce back around 600NM to 1200NM away, whilst ground waves (space & surface) only 150NM, so there is a gap in the middle where no signal is received, called the skip zone or dead zone.
Skip DISTANCE is distance from transmitter to skywave landing point.
At night it is bigger due to less refraction.
Ionospheric refraction of different frequencies
- VHF
- HF
- LF/MF
VHF - Not refracted
HF - Refracted a bit so get a skip zone, thus used for long distance comms
LF/MF - Refracted a lot so ground waves interfere with sky waves
Skywave interference
MF/LF refract more in ionosphere so don’t have a skip zone, but ground signals can interfere with skywaves. Get “fade” as you move and waves go in and out of phase.
This is strongest at night as attenuation in the ionosphere is too strong in the day to let any waves get through.
Thus skywaves are interference for MF/LF, not useful as they are with HF.
Sporadic E
This is the rare circumstance where high levels of solar activity lead to skywaves in VHF, which will get unexpected very long range.
Atmospheric ducting
AKA super-refraction
When atmospheric conditions (inversion) cause VHF to EHF waves to bounce in the layer and get longer than expected range.
Ionospheric ducting
Wavelength of VLF is roughly the distance from earths surface to the ionosphere, so they can be bounced beyond the natural 4000NM range and go around the earth.
Static - 2 sources
Thunderstorms can cause nav equipment to point to the thunder.
Precipitation carries static charge and can also affect radio, especially LF & MF.
Propagation summary chart
Doppler usage
Positive doppler (compression to higher frequency when moving towards receiver) and negative doppler shift can be detected to assess speed.
Used in old systems before GPS.
Civilian VHF range
118MHz to 137 MHz
SELCAL
Aircraft have a 4 letter SELCAL code (item 18 of flight plan) which ATC can call on HF/VHF system which just triggers a visual/aural signal in cockpit. Pilots put on headset to speak to ATC. Avoids keeping headset on constantly.
Need to check SELCAL with every ATC agency contacted before removing headsets.
Audio control panel (ACP)
Allows selection of radios to speak on, listen to, displays SELCAL call indicator on relevant station. Intercom selection etc.
Satcom
- frequency used
- type of satellite
- company who run them
- coverage
Uses UHF, minimal attenuation
Geostationary satellites used (30,000km)
INMARSAT run them
Full coverage up to 80 deg latitude (N & S)
VHF direction finding
- how it works
Uses a series of dipole antenna arranged in a circle which each receive a different phase of the signal.
Can be confused by multiple transmissions on the same frequency or reflections from terrain.
Q codes
QDM: Magnetic TO
QTE: True FROM
QDR: Magnetic FROM
QUJ: True TO
Bearing accuracy classes
A: +/- 2 degrees
B: 5 degrees
C: 10 degrees
D: >10 degrees
VDF letdown
- description
- last phase of approach
Uses direction finding to keep updating you with a QDM which you can use to direct to an airfield.
Not accurate so will have high minimum altitude (airfield not runway approach).
Once you are overhead you will turn away for an outbound track before returning to land.
QGH procedures
Similar to VDF but controller is responsible for giving you headings. More typical in military installations.
VDF fix
“FIX” is the operative word, normal VDF is a Q code (no wind correction) from ANY VHF ground station.
FIX is only on 121.5 as it needs more than one receiver to triangulate a position.
NDB frequencies & ranges
Were for long distances over sea so used MF/LF bands which have surface waves.
Allocation is 190kHz to 1750kHz, in Europe we use 280kHz to 530kHz.
Ranges were 600NM over sea, but generally 300NM over land due to surface attenuation.
NDB Identifiers (typical)
En-route - 3 letters
Locators - 1 or 2 letters
N0N A1A type NDB
“Keyed morse code” signal (A1A) interrupts the N0N unmodulated carrier wave used for direction. Needs beat frequency oscillator to extract the audible morse (BFO/Tone setting).
Carrier wave does NOT get modulated (frequency & amplitude remain constant), gets interrupted instead.
Directional functionality degraded during the interrupted segment.
You are listening to the CARRIER WAVE itself.
N0N A2A type NDB
Amplitude modulate the N0N signal with A2A signal instead of interrupting it. This allows the audible morse to be heard without special equipment. The disruption of direction finding is reduced.
Can be used for all NDB beacon types including short distance (homing, holding and approach).
A3E signal
Amplitude modulated speech signal used for VHF comms
Loop aerial functionality
NDB signal vertically polarised so will be received on vertical parts of the loop.
If the loop is at 90 degrees to the signal both sides get the exact same signal, if parallel with it they get signal out of phase.
But could be in two different directions.
Sense aerial
The sense aerial is monopole and signal added to the loop aerial signals creates a cardioid polar diagram with a single sharp null point.
Loop aerial rotates until the null point is found, which is the NDB direction.
Relative Bearing Indicator (RBI)
AKA radio compass
Simply shows relative direction of an NDB signal.
Moving card ADF
Has a manually rotating card that you can turn to match the aircraft heading, thus the arrow pointing in relative direction now shows you its bearing.
Radio Magnetic Indicator (RMI)
Similar to moving card ADF but with compass card automatically turned (usually by remote compass). This is what we have on modern aircraft, possible as part of EFIS (on the ND).
Often have two arrows for two signals and can feed from ADF or VOR.
ICAO requirement for NDB accuracy
+/- 5 degrees
NDB - Static & thunderstorms
All forms of static affect ADF accuracy. Snow and freezing rain especially cause precipitation static and attenuation.
Thunderstorms are a major source of error and can cause the needle to flail around or point to the thunderstorm directly.
NDB - night effect
Greatest at dawn and dusk, and at over 200NM from beacon. Weak sky waves aren’t vertically polarised so signal is degraded and the needle ‘hunts’.
Can check it by listening to the morse signal and you’ll hear the fading effect.
Range is increased, accuracy is decreased.
THIS IS THE BIGGEST EFFECT FOR ADF ACCURACY
NDB - station interference
In daytime this can be avoided by only using an NDB up to its rated range. At night however ranges can increase due to sky waves, so you may get interference from stations further away than expected.
Listen to carrier wave to detect if this is happening.
NDB - coastal refraction
+ how to combat it
Signals (from land to sea obviously) are bent close to the shore. You plot a position based on the direction it arrives at the aircraft, so will put you closer to the coastline than you really are.
Combat by:
- flying higher
- taking bearings at 90 degrees to coast
- using NDB closer to the shore
NDB - dip & mountain effect
Dip occurs in some systems in a turn, when the loop and sense aerials interfere. Gives large errrors. Strongest on bearing 45 or 135 degrees, left or right.
Mountain effect is disruption from terrain, DIFFRACTION!
NDB - quadrantal error
Incoming information bent by fuselage electric effects, strongest at 45 or 135 degrees
NDB range power formula
Max range (NM) = 3 x sqrt(power in watts)
Types of NDB station
- Locator
- Homing/holding
- En-route/long range
- Commercial
Locator - Aid to FINAL APPROACH, low powered beacon with 10-25NM range. Co-located with ILS outer marker.
Homing & holding - Aid for transition from en-route to destination airfield. Range just less than 50NM.
En-route/long range - Rated coverage over 50NM. Usually LF frequencies to maximise range.
[Note commercial MF/LF stations and marine beacons can be tracked, but not used for navigation]
ADF equipment settings
- test
- ant
- adf
Test - turns to 325 degrees
ANT - for listening to morse (need to select BFO/Tone at the same time)
ADF - to get directional info
NDBs
- Tracking vs homing
Homing is just pointing the nose at the NDB, wind will blow you on indirect course. Can only be used TO the NDB.
Tracking involves adjusting for wind and can be used to go TO or FROM.
NDB failure warnings
NONE - NO FLAGS!
How is BFO activated in modern aircraft?
Automatically
VOR Frequencies
108MHz to 117.975MHz (VHF)
108 to 112 MHz shared with ILS so 0.2MHz spacing (108.0, 108.2) and terminal VORs only (generally)
112 to 117.975 MHz at 0.05 MHz spacing more likely for en-route VORs.
Conventional/standard VOR (CVOR)
- Description
- Modulation
- Rotation direction
Horizontal dipole spins at 30Hz in cylindrical cover.
Slots in the cylinder create a “limacon” shaped polar diagram (similar to cardioid of NDB but no sharp null point.
This variphase signal is AM and an omnidirecional reference signal (FM) is sent out to be compared in phase.
Doppler VOR (DVOR)
- Description
- Modulation
- Rotation direction
- Usage
Combats multipath/reflection issue and moving parts of standard VOR.
Variphase signal comes from a series of omnidirectional dipoles in a circle switched on and off @ 30Hz
Variphase moving to and from means it is FM, so reference is AM.
Anticlockwise (unlike CVOR).
200NM range, used for en-route IFR.
Cone of confusion size
Maximum 50 degrees from vertical.
Radius is therefore at most 1.2 x height.
VOR/DME identification
VOR is 3 letter morse code @ 1020Hz, repeating every 10 seconds.
Can also have voice message.
Linked DME will have higher pitch ident @ 1350Hz every 40 seconds.
VOR types
En-route: 80NM spacing to achieve 10NM wide airways based on 7.5 degree accuracy
Terminal VOR (TVOR): Low powered approach, often shared with ILS frequencies
Broadcast VOR: Terminal aid with airfield info or ATIS on the carrier wave.
Test VOR: In the US, zero degrees in all directions for testing accuracy.
VOR aircraft equipment
Horizontal antenna (as signal is horizontal) a quarter of a wavelength long.
Data displayed on RMI and Horizonal Situation Indicator, or omni-bearing indicator (OBI).
Omni-bearing indicator (OBI)
AKA CDI
VOR indicator which you dial in to a radial and see deviation marks and TO/FROM indicator.
Full scale deviation is 10 degrees (can have 5 dots of 2 dots).
Can have glidescope info which gives vertical and horizontal deviation lines.
Instrument has no concept of heading or direction (i.e. compass card)!
Horizontal Situation Indicator (HSI)
Similar to RMI, both have rotating cards driven by remote compass.
Where RMI simply points to VOR, HSI has a selected radial and then deviation marks (and to/from).
Procedure turn (2)
Head out from NDB/VOR for fixed time period or until a fix position.
Turn off 45 degrees and straight for a period of time (about 1 min) then commence turning circle (in opposite direction) to regain radial.
OR turn off 80 degrees then immediate 260 degree turn in opposite direction to regain radial.
VOR errors (3)
Uses shorter range and line of sight so no sky wave issues, coastal refraction or night/day.
Site Error is due to reflections which causes scalloping - needle flicking back and forward. Doppler less prone due to larger effective aerial.
Multipath signals bounce off terrain (similar to mountain effect).
Atmospheric ducting can carry conflicting signals from far away.
VOR accuracy
Accuracy expected to be within 5 degrees 95% of time.
Need to be within OR AT half of full deflection to be considered on track (equivalent to 5 degrees, but written in these terms).
Transmitters required to be accurate to within 1 degree.
Cause of VOR failure flag
Ground station detecting problems and removing identification or navigation transmissions, which will trigger the failure flag.
Can’t be caused by errors in the aircraft.
ILS localiser frequencies
108MHz to 111.95MHz
Odd 0.1MHz, and 0.05MHz above
e.g. 108.1, 108.15, 108.3, 108.35
[108.2 is VOR, 108.25 not allocated]
AMPLITUDE modulated (otherwise the two frequencies on each side wouldn’t work!)
ILS glidepath frequencies
UHF
Automatically selected along with the localiser
ILS marker beacon frequencies
75 MHz (VHF)
ILS Ident
1020 Hz tone amplitude modulated onto the carrier wave.
Usually 3 letter code, can have “I” infront to identify as ILS
Deviation markings for ILS
Displayed on OBI or HSI
Localiser deviation is 2.5 deg each side (a quarter of VOR deviation).
Glidescope deviation is 0.75deg each side (so 0.15deg per dot) [0.7?]
ILS info on VOR indicator
You can’t select a radial for ILS in same way as VOR. Selecting the correct approach radial for ILS will orient the indicator nicely, but doesn’t affect the deviation or information being displayed.
ILS functionality
- Beam modulation
- How centreline is identified
ILS sends out 2 beams, a 90Hz AMPLITUDE modulated “yellow” beam to left of track, a 150Hz modulated “blue” beam to the right.
Indicator measures “Depth of modulation” to assess which it gets more of, with DDM = 0 (or equal DDM) being the “green” centre line.
[Glidepath uses yellow 90Hz above and blue 150Hz below glidepath]
[Change in depth of modulation linear with angular displacement]
ILS - percentage of modulation
DIFFERENT to depth of modulation, 0% modulation means no modulation in either beam, will trigger warning flags
ILS coverage
Up to 25NM away - 10 degrees either side
Up to 17NM away - 35 degrees either side
[Can be reduced to 18NM and 10NM with terrain blockage]
ILS beam bends
Slight curves in ILS signal due to reflections on permanent obstructions.
They are slight, predictable and “CAN BE FOLLOWED BY LARGER AIRCRAFT”.
Offset localiser
If aerial can’t be placed on runway centreline approach will be at an angle.
Beyond 5 degrees “offset” it can’t be a precision approach and approaches must be flow to MDH/MDA not DH/DA.
ILS aerial locations (ground)
300m “in” from the threshold (“off the upwind end”), glidepath is 120m off centreline
Glidepath signal coverage
- vertical range
- degrees from centreline
From 0.45 x glidepath angle to 1.75 x.
Within 8 degrees of centreline.
ILS marker beacons
- positions
- tone frequencies
- morse pattern
Largely replaced with DME now.
Vertically directed beams at threshold (inner), 0.25 to 0.5NM (central) & 4NM (outer).
Broadcast morse codes:
Outer: “O”, “——-“, low pitch 400Hz
Central: “C”, “-.-.-.-.-.”, med pitch 1300Hz
Inner: “I”, “………”, high pitch 3000Hz
ILS marker beacons cockpit
Along with morse code signals, activate lights:
Outer - blue
Central - amber
Inner - white
Panel can be switched to “high” to be used as an airways marker indicator.
Tracking ILS
Intercept from below (to avoid false lobes above glidepath) at 2000 to 2500 ft (as low as 1500ft).
Don’t descend until you have localiser.
Need to be within half the full deviation to descend safely, otherwise terrain clearance not guaranteed so go around.
Back course approach
- Impact on OBI
- Options for HSI
- Information available
Not approved in Europe but signals are there.
With OBI (light aircraft) the selected course has no impact, so you’ll get a reverse sense deviation.
With HSI, selecting the “front course” (i.e. as if making a normal approach) to see CORRECT deviations, “back course” (i.e. your actual approach track) will show REVERSE deviations.
NO glidescope info
ILS categories and minima
Cat I: 200ft minima (Baro) [100ft accuracy]
Cat II: 100ft minima (Radalt) [50ft accuracy]
Cat IIIA: 0ft, RVR 200m
Cat IIIB: 0ft, RVR 75m
Cat IIIC: 0ft, RVR 0m
Type A and B approach minima
Type A: >= 250ft
Type B: < 250ft
FM Immunity
ILS equipment is AM and is fitted with FM immune filters to prevent commercial radio interfering.
Also applies to VOR equipment.
FM interference doesn’t trigger failure flags, but could affect accuracy.
ILS integrity failure response times
Ident removed and signal stopped if system detects accuracy has dropped:
Cat I: within 6 seconds
Cat II or III: within 2 seconds
Ground areas defined for ILS interference (2)
ILS Critical Area - Vehicles & aircraft excluded when ILS operational
ILS Sensitive Area - Vehicles & aircraft CONTROLLED to prevent interference [kept clear for cat II & III approaches]
Microwave Landing System (MLS) Frequencies & no. channels
5031 MHz to 5090.7 MHz
SHF
200 discrete channels
MLS functionality
Sends out 2 sets of signals, one in azimuth, one in elevation and a precision DME (DME/P).
Sweeps signals from left to right so timing of when “to” and “fro” signals pass tells you where you are in the wave.
FMS/APFDS can be programmed to follow a route.
Impact of loss of DME/P on MLS
Only get straight-in approaches
MLS aircraft equipment
At simplest level the MLS & DME/P receivers and a control panel allow straight in or offset approaches.
With a guidance computer, 3d waypoints can allow programming of curved path.
With FMS integration stored complex flightpaths can be followed.
MLS coverage
20 NM distance, 20,000ft height
40 degrees either side of centre
Elevation 0.9 to 20 degrees
[DME/P 22NM distance]
MLS vs ILS
MLS allows curved flight paths.
Both affected by shadowing but MLS can interrupt signals to take into account stationary object, so LESS sensitive to geographic location.
In reality MLS was rarely implemented and is being replaced with GPS.
Description of MLS presentation and what it represents
A 2D presentation of a 3D segmented (NOT curved) approach
MLS azimuth and glideslope combined ground units or separate?
Separate
Frequency bands for primary radar
UHF/VHF would be best for low attenuation, but we need short wavelengths to give narrow beams and to detect small objects (eg rain).
So typically UHF/SHF, some EHF.
[Can be called Primary SURVEILLANCE radar]
Pulse radar vs continuous wave radar
Pulse radar uses single aerial for transmission and receipt but has max and min range restrictions.
Continuous wave has less range restrictions.
Pulse radar used on ground, continuous for radio altimeter.
[May refer to pulse MODULATION]
Pulse recurrence period (PRP)
Inverse of pulse recurrence frequency (PRF).
In a single recurrence period the pulse radar sends out a pulse then has a silent period waiting for the response.
Max theoretical pulse radar range
c / (2 x PRF) [2 because there & back!]
Need a long PRP (short PRF) to give time for a pulse to travel to target and back again.
Also depends on power.
Pulse radar minimum range
Affected by pulse width/length (amount of TIME pulse transmits for in the recurrence period). Too long and the returning signal arrives during transmission and isn’t recorded.
Note that pulse width can’t be too small otherwise power is limited.
Power measure for primary radar
- Peak, average, continuous?
Peak power is relevant (excludes the waiting periods which would impact an average or continuous measure).
Radar bearing accuracy
Affected by beam width, which is smaller for a large aerial:
Beam width
= 70 x wavelength / antenna diameter
So high frequency and large aerial gives best bearing accuracy.
Radar range resolution
- short or long pulses
Short pulses give the best range resolution.
Radar return quality factors
- Super-refraction (ducting)
- Sub-refraction (reduces range)
- Attenuation with distance
- Nature and size of reflecting surface
ASMR
Aerodrome Surface Movement Radar
Used to control aircraft in manoeuvring area.
Very short wavelength (3.8cm) and high accuracy (can show type/size of aircraft).
Moving Target Indication (MTI)
A mode of primary radar which uses doppler shift to differentiate moving targets from stationary.
Can fail to detect movement of aircraft moving “across”, i.e. not towards or away from the radar aerial.
En-route surveillance radar
Long distance (up to 300NM) using primary and secondary radar.
600MHz (50cm wavelength).
Secondary will be better at the longer distances as signal doesn’t have to travel there and back (signal from transponder just travels one way). Also secondary doesn’t suffer from weather clutter.
Terminal Surveillance Radar
Provide separation between aircraft in terminal area during transit, approach and departure.
Medium distance (60-80NM) using primary and secondary radar.
1000MHz
Approach Surveillance Radar
Cover initial, intermediate and possible final approach.
Can be used to provide Surveillance Radar Approach, which use QFE vertical guidance (radar no vertical info) to guide down to MDH.
3GHz (UHF/SHF boundary)
Precision Approach Radar (PAR)
- description
- min decision height
Uses 2 elements, vertical and horizontal, using sector (rather than rotational) scan at 10GHz.
Will guide down to 200ft decision heights using continuous guidance - pilot doesn’t talk, radar controller continuously describes situation and required changes.
Weather radar
- wavelength
- frequency band
- range
9 to 10 GHz (for 3cm wavelength), SHF band
Up to 320NM
Weather radar aerials
- type
- side to side coverage
- vertical coverage
Scan up to 90 degrees left and right and can be tilted 15 degrees up and down.
Gyro balanced for pitch and roll (use IRS system now).
Modern versions use slotted scanners so beamwidths reduced from 5 to 3 degrees and less power lost to sidelobes.
Which way to tilt weather radar around thunderstorms
DOWN
Ice crystals DO NOT reflect, so need to tilt down to ensure vertical separation
Weather radar on ground
Can damage people and objects so switched on when entering runway (or uses WoW switch).
Engineers doing tests establish 5m perimeter.
Weather radar reflection
Water better reflection than ice, so tops of thunderstorms not very visible.
Returns with strongest at top:
Wet hail
Rain
Wet snow
Dry hail
Dry snow
Drizzle
NOT clouds, turbulence, lightning, fog, sandstorms.
Weather radar doppler mode
A selectable mode that works to about 40NM.
Detects movements in raindrops suggestive of turbulence.
Predictive Wind Shear (PWS)
Can be activated and works below 2300ft even when weather radar is off.
Scans for wind shear ahead and provides a “WIND SHEAR AHEAD” warning if detected.
Distinct from TAWS windshear which is based on IRS.
Detects doppler shift in precipitation only (doesn’t work if zero precipitation).
Weather radar tilt
Can be set auto or manual.
At altitude usually set 0 or -1 degree (which at 80NM scans down 20000ft).
In takeoff set +5 to 15 to avoid ground and view area above, more like +5 in climb.
In descent and landing +5 to avoid looking at the ground.
Weather radar auto-tilt responds to altitude or attitude?
To ALTITUDE.
Gyros take care of attitude (& roll)
Multi-scan weather radar
System that scans upwards (medium range) and downwards (short and long range) to deal with curvature of earth.
Information is processed in a database, not directly displayed.
Thus Ground Clutter Suppression can be carried out in processing.
Mapping radar
This setting gets rid of ground clutter suppression and gain control and gives a picture of terrain to help navigation.
USED to use a cosecant squared beam in old systems but MODERN system use conical beams (same as wx)
DME frequency range
UHF, 960 - 1215MHz
DME functionality
Pairs of pules sent out with 12 (X) or 36 (Y) microsecond gap, timing is unique to each transmission though for identification.
After 50 microsecond display the ground transponder responds.
Time delay (less 50 microseconds) converted to a slant distance.
DME issues
- Differentiating different aircraft signals
- Ignoring ground reflections
- Ignoring other sources or radiowaves
Transmission is a random or “jittered” PRF that is unique enough to be distinguished from other aircraft.
Response signals are sent back on frequency 63MHz different so ground reflections are ignored.
Only pairs of signals separated by 12 microseconds are looked for, so any other random radiowaves are ignored.
DME pulse rate stages
Starts with 150 pulse pairs per second when starting to interrogate [SEARCH]
Drops to 60 pulse pairs per second after 15,000 pulse pairs [TRACK]
Once locked on, drops to 24 per second [MEMORY]
Doesn’t start this process until first transmissions (due to other aircraft or squitters) are picked up.
Reason for pulse pairs in DME
To distinguish signals from other single pulse (e.g. radar) systems
DME Memory
If signal is lost, output will continue counting down distance at same rate for 8 to 10 seconds
DME station limits
Limited to 2700 pulse pairs per second, which in effect limits to about 100 aircraft. Responds to strongest signals as a priority (not necessarily the closest).
DME echo protection
To protect against echoed signal reaching DME station, or aircraft from station, only the first signal received is processed and any echos then ignored.
DME accuracy
0.25NM plus 1.25% for older systems
0.2NM for 95% of the time for systems after 1989.
TACAN
Military aid - civilians can only use the DME functionality.
Called VORTAC if paired with a VOR.
Associated VORs & DMEs
- distance apart
- DME ident
For terminal aids must be within 30m.
For other purposes can be 600m apart.
They have the SAME ident, with DME at higher pitch (1350Hz) every 40 seconds.
ALL DME have a related VHF frequency, whether they are paired with VOR or not.
DME variation on 50 milliseconds
Can achieve zero reading at threshold on a DME at halfway point of runway, by reducing the 50ms delay by appropriate amount.
Which holding pattern entries can DME arcs be used for?
Sector 1 and 3
SSR frequencies
Ground station sends out pulses at 1030 MHz.
Aircraft responds with a longer set of pulses on 1090 MHz.
SSR sidelobe supression
An omnidirectional pulse (P2) is sent out along with the rotating interrogation beam (P1, P3). It has an amplitude less than the main beam, but more than sidelobes.
If transponder picks up a P2 at greater amplitude than the P1 & P3, it must be looking at a sidelobe and discards it.
SSR pulse spacing
i.e. P1 to P3 spacing.
Mode A: 8 micro seconds
Mode C: 21 micro seconds
Spacing between pulses tells transponder whether to respond mode A or C
Mode A & C aircraft reply
Stream of pulses on 1090 MHz, with 2 “Frame” pulses on either end. There are 12 pulses in the middle allowing for 2^12 = 4096 combinations.
Mode C includes altitude in 100ft increments.
Mode C accuracy requirement
300ft
Controller will ask you to confirm if you appear 300ft from the mode C output. May be asked to switch off mode C and squawk 0000.
Special Position Identification
Ident button
Sends an additional 4.35 microsecond pulse AFTER the normal pulse chain, causing you to bloom on display for 25 seconds.
Mode A/C errors
False Replies Unsynchronised to the Interrogator Transmission (FRUITing). Aircraft responding to two separate stations with wrong information. Errors in range and bearing.
Garbling caused by aircraft within 1.7NM sending overlapping replies. So for aircraft in formation, only lead aircraft has transponder on (others standby).
Mode S frequencies
Same as mode A & C
Uses same equipment, all can talk to each other
Mode A/C/S All call
Mode A/C All call
Mode A/C only all call
Mode S only all call
Mode S station sends out P1/2/3 as usual but extra P4 pulse which is long. Mode A/C will ignore the P4 and reply. Mode S will start A/C reply after P3, but once it receives P4 it cancels that and then replies with the mode S detail.
An all-call to mode A/C only can be sent by transmitting a short P4.
Mode S only all call adds a P5 and P6.
Mode S selective interrogation
Send out spaces P1 and P2, then a long P6 data request which includes a small P5 control pulse. P6 includes aircraft identification and parity check.
P6 interrogations are Comm-A (56/112 bit) and Comm-C (1280 bit), which receive Comm-B and Comm-D replies respectively.
Mode S All-Call
Sends a P1/P2/P6 which can request that aircraft already in contact don’t reply.
If looking for all aircraft to reply sends a null (all 1’s) AA (Aircraft Address) signal.
Elementary Surveillance level
SSR level 2, lowest acceptable Europe.
- Mode A [identity hard coded]
- 25 ft pressure altitude
- Aircraft Address [identity hard coded]
- Aircraft identification [set in FMS, e.g. callsign]
- Ground/air status
- Datalink capability
- GICB capability
- ACAS capability
Enhanced Surveillance level
Higher level SSR
- Magnetic heading
- Autopilot set altitude
- IAS
- MN
- Vertical rate
- Roll angle
- Track angle rate
- True track angle
- Groundspeed
Mode S antennae
> 5700kg or TAS>250kt require one on top and one on bottom.
Must be capable of working simultaneously and identifying best one to use for a given signal.
Also need TCAS antennae on top and bottom (top directional, bottom omni or directional), but can actually be combined with the mode S antennae now.
Mode S - squitters
Squitter is information sent out unsolicited.
Sent out by mode S transponders to inform other aircraft and ATC of position and altitude.
Also used to send out ADS-B information.
GNSS
- Notional constellation
The minimum constellation of satellites required for system to function.
In reality could have more or less than this.
Navstar GPS
- Notional # satellites
- Orbital planes
- Orbital height
- Orbit period of satellites
- # satellites in view at any one time
24 notional satellites
6 orbital planes (4 satellites each) at 55 degrees to the equator
Orbit @ 20,200km
Satellites orbit every 12 hours
Should have 5 - 8 in view at any one time
Maximum latitude of GPS orbits
55 degrees N/S (ground track), but this is sufficient for global coverage
GPS coverage
Bad at the poles
Elsewhere - varies by time
NOT necessarily best at equator!
Navstar GPS
- Frequencies
In UHF band, two frequencies called L1 and L2.
L1: Precision (P) code and coarse acquisition (C/A) code (1575.42MHz)
L2: Precision (P) code only (1227.6MHz)
C/A is for civilians
AKA SPS for L1 (standard), PPS for L2 (precision)
Encoding of GPS signals
Binary Phase Shift Keying (BPSK) or
Pseudo Random Noise (PSN) code
satellites for
- 2D fix
- 3D fix
2d fix needs 3 satellites
3d fix needs 4 satellites (or 3 + altitude info)
Receiver clock bias and solution
Receiver clock bias is error due to clock in the GPS receiver not being an atomic clock.
It is resolved using an iterative process where a minor correction to time is introduced and then check if fix is more precise. Keep going until fix is exact, then you have correct fix AND exact time.
GPS Control segment
As opposed to “space segment” (satellites) and “user segment” (receivers).
Consists of:
- Master Control Station
- Monitor Stations
- Ground antennas
Data sets sent by GPS
- Almanac
- Ephemeris
- Satellite clock correction
- UTC correction
- Ionosphere Model
- Satellite health status
Almanac: Rough position info on all satellites, valid for 180 days
Ephemeris: Precise position info for this satellite, valid for 4 hours
Satellite clock correction: obvious
UTC correction: To display UTC to user
Ionosphere model: To correct for errors caused by ionosphere
Satellite health status: Healthy/unhealthy for each satellite
GPS data timing and chunking
Takes 12.5mins to send all data sets.
Split into 30 second data frames in case transmissions are interrupted.
GPS velocity
Old systems simply use change in GPS position over time.
Better method is using doppler shift on the space vehicle (SV) frequency, which gives accuracy down to cm/s.
GPS “All in view”
This means receiver is tracking all visible satellites above the mask angle and using them to compute position.
Reducing GPS time to first fix
Having initial position info helps the receiver know where to look in the sky (based on almanac data) for the satellites and improves fix time.
GPS User Equivalent Range Errors (UERE)
- Ionospheric Propagation Delay
Satellites overhead more accurate than on the horizon.
Error proportional to 1/f^2, so military with 2 GPS signals can compare signal delays to correct for this. Not possible with the single civilian signal.
BIGGEST component of UERE
NOT atmospheric propagation
GPS User Equivalent Range Errors
- Satellite orbital variation
Errors in ephemeris data, but tend to be small (0.5m). Can take 3 hours to spot the error and fix it.
GPS User Equivalent Range Errors
- Instrument/receiver error
Can be 5m for mobile phones, but only 1m for specialist receivers.
GPS User Equivalent Range Errors
- Multipath signals
Signals reflected from terrain can be dealt with via software and aerial design, but could still be an issue in mountainous terrain.
GPS User Equivalent Range Errors
- Satellite clock error
Receiver clock bias is corrected via software at the receiver, but satellite clock error is more serious. It only gets fixed when the satellite crosses the control station.
GPS Dilution of Precision (DOP)
Caused by “shallow cut” of satellite signals, i.e. satellites too close together.
Expressed numerically from 1 to 20, with 1 being the best. DOP figure together with UERE estimate gives total accuracy assessment
GPS Dilution of Precision (DOP)
- dimensions
PDOP - Position DOP, 3d error
HDOP - Horizontal DOP, lat & long
VDOP - Vertical DOP, height
TDOP - Time DOP
How to estimate GPS position error
UERE x GDOP
What time does GPS transmit?
GPS time, data set required to convert this to UTC
GLONASS
- Notional # satellites
- Orbital planes
- Orbital height
- Orbit period of satellites
24 satellites (only has 24 total so just enough)
3 orbital planes at 64.8 degrees to equator
19,100km height (lower than GPS)
(So lower orbit period of) 11hrs 15mins
Notes on GLONASS
EASA doesn’t allow GLONASS to be used in aviation
System works similarly to GPS, but uses PZ-90 earth model instead of WGS-84.
GPS & GLONASS
Use different data for navigation services (i.e. WGS 84 model),
BUT are INTEROPERABLE for the user.
GALILEO
- Notional # satellites
- Orbital planes
- Orbital height
- Orbit period of satellites
27 satellites (PLUS 3 spares)
3 orbital planes @ 56 degrees to equator
23,222km height (higher than GPS)
(So longer orbit period of) 14hrs
GALILEO frequencies
3 frequencies in UHF
[Can also pick up SAR on 406MHz]
GALILEO service levels
Open Service - similar to GPS civilian (C/A) system
Commercial Service - 1m accuracy for a fee, uses the third band
Public Regulated Service - Similar accuracy to open service, but more secure (for security forces and ATC)
Mask angle
Aspect of receivers which indicates how far over horizon satellites need to be before they can be seen. Was lower in older receivers (5 to 15 deg) as there used to be fewer satellites. Tends to be higher now (10 to 30 deg) as more satellites should be closer to overhead.
2 purposes of satellite augmentation systems
- Differential GPS, i.e. increasing accuracy
- Provision of integrity monitoring
Note that systems like RAIM and AAIM provide integrity monitoring but do NOT increase accuracy
Airborne Based Augmentation Systems (ABAS)
- RAIM
With 1 redundant satellite (so 4 for 2d fix, 5 for 3d fix) RAIM can provide Fault Detection (FD) which means it can tell a satellite is faulty, but not which one.
With 2 redundant satellites (5 for 2d, 6 for 3d) can provide Fault Detection and Exclusion (FDE).
Look for “redundant range measurements”
Airborne Based Augmentation Systems (ABAS)
- Aircraft Autonomous Integrity Monitoring (AAIM)
This is where the aircraft uses position data from several sources (IRS & barometric altimeter - NOT VOR/DME in questions) to carry out integrity monitoring.
Ground Based Augmentation Systems (GBAS)
- description
- frequencies
- range
A ground station monitors GPS and identifies errors (satellite clock, ephemeris and ionospheric propagation, NOT receiver, multipath or some atmospheric propagation errors).
Data broadcast over VOR range (108 to 118 MHz) up to 20NM from station.
GBAS
- 2 services provided
GBAS positioning service is just for added accuracy. Can have multiple connected (Ground Regional AS - GRAS) together.
Precision Approach Service (GLS approach) works like ILS down to 200ft, needs to be selected via 5 digit channel number.
Coverage is 15NM at 35 degrees either side of centre line, 20NM at 10 degrees. Not to be used outside these areas.
GBAS
- Number of satellites required
4
Satellite Based Augmentation Systems (SBAS)
Use a separate satellite system with space segment (geostationary satellites) and ground segment.
Errors detected at all reference stations, calculated at master station and transmitted at ground earth station to satellites.
Coverage area where data can be received is bigger than service area for which error data is calculated.
No. of elements in SBAS system
No. of segments in SBAS system
2 elements
- Ground infrastructure
- Satellites
3 segments
- Space
- Ground
- User
Can SBAS signals be used to calculate pseudo ranges for position fixing?
Yes
SBAS systems
- Europe
- US
- India
- Japan
Europe: EGNOS
US: WAAS
India: GAGAN
Japan: MSAS
[These are WIDE AREA systems (WAAS), as opposed to local (LAAS) GBAS systems]
EGNOS
- Error notification time
- Purpose
- Satellites
The European SBAS system which also provides notification for users of an error within 6 seconds, rather than 3 hours for traditional GPS.
It increases accuracy.
Broadcasts error corrections and GPS lookalike signals from geostationary satellites. UP TO 4 satellites, currently 3, only needs 1 to work.
EGNOS channel numbers on approach charts
Used to identify the approach only, not a frequency. EGNOS uses the standard GNSS frequency.
Galileo clock types
Rubidium Frequency standard clock
+ the more accurate Passive Hydrogen Maser clock
[Synchronised to ground based caesium clocks]
PBN definition
NON sensor specific navigation specification system based on ability to meet performance requirements, regardless of sensors or tools used to achieve it.
Components of PBN
- Navaid infrastructure (VOR, DME, GNSS, not NDB!)
- Navigation specification
- Navigation application
RNP vs RNAV
RNP specifications include requirement for on-board performance monitoring and alerting.
RNAV do not.
PBN linear & angular guidance
Angular guidance means guidance down a narrowing path around a centre-line.
En-route & terminal phases are LINEAR ONLY.
APPROACH is the only phase with linear and angular guidance.
RNAV/RNP performance aspects
Accuracy: (AKA conformance) difference between estimated and true position.
Integrity: A measure of trust in the system, e.g. ability to provide timely alerts
Continuity: Capability to continue operating without interruptions
[Functionality possible a fourth]
[Availability: Percentage of time features are available]
3 elements of RNP/RNAV Total System Error
Path Definition Error (PDE) - desired path to defined path
Flight Technical Error (FTE) - defined path to estimated position
Navigation System Error (NSE) - estimated path to actual position
Fly-by vs fly-over turns
RNP/RNAV transition points can be defined as either, fly-over is overfly - start turning only once you’re overhead.
Fly-by radius varies based on aircraft, wind (etc) so not often used for closely spaced routes.
Most SID, STAR or FAF are fly-over as they require you to be over the point.
Strategic Lateral Offset Procedures (SLOP)
Flying an RNP or RNAV route with a specified offset, at 1nm increments up to 20nm max, for collision avoidance.
RNP functionality requirements (2)
Ability to carry out specific manoeuvres.
1) Radius-to-fix (RF) leg: A 180 degree turn at low level, defined by the end point, radius and arc length (not the start point).
2) Fixed Radius Transition (FRT): A fixed radius en-route turn defined to achieve a more accurate transition between route segments.
Basic functionality requirement for RNAV
- Continuous indication of lateral deviation
- Distance/bearing to waypoint
- Groundspeed or time to waypoint
- Navigation data storage (user input or database)
- Failure indication
ARINC 424 path terminator
The 24 different types of path the FMS can follow.
Consist of a path to follow AND a terminator.
Described by a 2 letter code, with first letter for path and 2nd for terminator.
“RF” is an example, radius to fix.
Approval for PBN
Pilots must have PBN endorsement on Instrument Rating.
EASA only require specific operational approval for some procedures (e.g. RNP AR APCH and RNP 0.3)
Advanced RNP (A-RNP) specification
A new RNP specification, the only one covering ALL phases of flight.
Incorporates specs of:
RNAV 5, 2, 1
RNP 2, 1, APCH
RNP APCH
RNP AP APCH
ONLY for approach phases and they are the ONLY specifications (other than advanced RNP) including Final approach.
AP stands for approval/authorisation required.
RNAV spec flight phases
RNP spec flight phases
En-route oceanic PBN specs
Oceanic (e.g RNAV 10) don’t use navaids, either IRS or GPS (2 independent ones). With only IRS is time limited to about 6 hours due to degradation of accuracy over time. Need a navaid fix at some point to get a 5hr boost for long oceanic journey.
Which RNP/RNAV specs can have manual input waypoints?
RNAV 5 & 10
RNP 4
Which RNP/RNAV specs can be used for helicopter IFR routes?
RNAV 1, RNP 1 & 0.3
Which RNAV/RNP specs can be used for STARs and SIDs
RNP 1 and RNAV 1
- Note that APCH specs are for approach phases only, STAR gets you up to approach but mostly covers arrival.
Types of approach (diagram)
RNP approaches
Use GPS primarily.
DO NOT include precision approaches, which are based on ground aids.
Are either:
i) non-precision (LNAV only); or
ii) approaches with vertical guidance (APV) - either barometric (LNAV/VNAV) or SBAS (LPV).
Note: Without vertical guidance need to fly to a MDA, which is higher than the published DA
Approaches with Vertical Guidance (APV)
PBN/RNAV approaches with one of two forms of vertical guidance:
i) LNAV/VNAV or APV Baro - FMS constructs a glide slope and uses barometric data to provide guidance along it
ii) LPV (Localiser Performance with Vertical Guidance) or APV SBAS - Use SBAS to give enough GNSS data to construct geometric vertical approach. Needs an enhanced Final Approach Segment (FAS) data block in the ND.
LNAV/VNAV and air temperature
Might have air temperature compensating equipment, but if not there will be minimum operating temperatures for the VNAV (Baro) approach - defined for the APPROACH not the aircraft (so ATC could tell you to go around).
Could also be maximum temperatures, especially at high altitude (to prevent too steep descent).
APV Baro vertical guidance
- equipment used
Uses “a method certified for the purpose”. Not specific to either barometric altimeter or radalt. Look for this phrase in exam.
Approach minima:
- Non-precision GPS (LNAV)
- APV Baro (LNAV/VNAV)
- APV SBAS (LPV)
- Precision (ILS or GLS [GBAS])
LNAV: 400-600ft MDA
LNAV/VNAV: 350 - 400ft DA
LPV: 200 - 300ft DA
ILS/GLS: 0 - 200ft DA
PBN/RNP navigational aspects for en-route & terminal flight phases
- 3D, LNAV, vertical?
ONLY Lateral performance requirements
Some PBN/RNP approaches use vertical guidance, but this is NOT part of PBN/RNP specifications
Rate of descent calculation from groundspeed
Descent rate (fpm) = 5 x groundspeed (kt)
Holding pattern entries
1 - Parallel (110 degs)
2 - Offset (70 degs)
3 - Direct (180 degs)
Typical ATC display info
- Pressure Altitude
- Flight level
- Flight number or registration
- Ground Speed
How STARs and SIDs must be selected in FMS
By ROUTE NAME
Power increase required to double range:
- One way (e.g. VOR)
- Two way (e.g. primary radar)
One way: 4x
Two way: 16x
Required alert time for GPS errors in SBAS
6 seconds (same as achieved by EGNOS)
RNAV SID overlay
Follows exact same track as the SID but based on waypoints defined for that purpose, rather than navaids
EFIS plan vs map mode
Map - Expanded mode showing the route, oriented to heading
Plan - North oriented view of the entire route, expanded compass rose with heading is disconnected from the plan view contents.
GPS antenna shape and reason
Helical antenna due to circular polarization of the signal
Definition of navigation “accuracy”
This actually refers to the estimated error!
Accuracy exceeding allowed amount is bad!
NANU
Informs of GPS constellation and planned maintenance
