Radio Appreciation for PPLs John Linford

What’s it all about?  Students have to learn about aircraft  But they don’t have to learn about radio  Yet radio is critically important  This seminar aims to bridge the gap!

What we’ll cover  Introduction to the electromagnetic spectrum  Characteristics of radio signals  Radio systems used in aviation –Air/Ground RT –Airfield navaids –En-route navaids –Radar systems (in some detail) –GPS

You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat. Albert Einstein

Electromagnetic Spectrum  An infinite continuum of frequencies…  …from sub-audio to light (and beyond)  The radio spectrum is in the middle Audio 30Hz 30kHz LF 30kHz 300kHz MF 300Hz 3MHz HF 3MHz 30MHz VHF 30MHz 300MHz UHF 300MHz 3GHz Microwave 3GHz 30GHz NDBs R/T VOR Localiser ILS marker ADF DME Transponders Glide slope Radars

Characteristics of radio spectrum  Radio… –Is a form of electromagnetic radiation –Travels at 300x10 6 m/s (186,000 miles/sec) –Travels in straight lines only –Tends to be absorbed by mountains, etc. –Is reflected from conductive surfaces –Can be extensively affected by solar events

Radio Telephony  Civil aviation uses VHF –118.0MHz to 137.0MHz –Amplitude Modulation (AM) –25kHz spacing (8.33kHz in the Airways) –Simple Tx and Rx equipment –Does the job very well  Military aviation uses UHF –300MHz region –Also carry VHF

Factors affecting RT  Line of sight –Bear in mind curvature of the earth! –Range rule of thumb: 1.25 x √(height in feet) nm  Mountains absorb signals  Transmit power determines DOC  Anomalous propagation effects –Ducting –Es –Reflections from other aircraft

The Two at once problem  Two stations transmit simultaneously –Listeners hear a squeal and distorted voice –Why?  Transmitters are never on exactly the same frequency  The difference between the two TXs is in the audio spectrum  The two frequencies hetrodyne in the receiver, producing sum and difference products  Assume TX-1 on MHz, TX-2 on Mhz –The sum is at MHz and has no effect –The difference is 1kHz, an audio tone (squeal) that you hear

RT recording systems  Most airports and all en-route centres  Multi-track recording systems –Record all radio channels, including ATIS –Record important telephone lines –Retained for minimum of 30 days  In case of incident –Recordings are impounded by CAA  Digital recorders increasingly in use

Carlisle recorders

Airfield navaids  Non-Directional Beacon (NDB)  Automatic Direction Finding (ADF)  Distance Measuring Equipment (DME)  Instrument Landing System (ILS)

NDB  One of the oldest radio navigation systems –Supposedly obsolete for the past 20 years!  Simple ground equipment –Low power LF transmitter –190kHz - 535kHz (1600kHz) –Vertical antenna (omni-directional)  More complex aircraft equipment (the ADF) –= expensive!

NDB/ADF principles of operation  It’s all in the antenna system! –Simple antenna has figure of eight response (A) –By adding a sense antenna, a sharp null is created in one direction (B) –Using a Goniometer the cardioid pattern is presented to the ADF instrument  The ADF needle connects to the Goniometer and is rotated by a servo, seeking the null A B

NDB problems  Alignment of aircraft affects readings –Bearings are relative to aircraft direction –Banking alters the apparent bearing  NDBs are prone to propagation errors –Night effect (ionospheric reflections) –Coastal refraction –Electromagnetic storms

Carlisle NDB

Future of NDBs  NDBs are an anachronism –Better navigation systems have existed since WWII –Technically obsolete since the 1980s  Very cheap for airfields to implement  Almost all GA aircraft have an ADF  There are many NDB/DME procedures  GPS procedures are still far far away

Morse code!  How strange that our navaids use Morse code in this day and age!  Morse code is still alive and well  It’s still used on navaids because –It’s there. It works. –Some navaids cannot carry voice –Life expectancy of navaids has been extended  It will eventually be superseded

ADF  Gives ATC the bearing to/from an aircraft  It’s how you get a QDM  Very similar in operation to the NDB –The aircraft’s RT transmission is the “NDB” –Multiple antennas switched at high speed to create a rotating cardioid –The phase of the null represents the bearing of the aircraft

Carlisle ADF

Carlisle ADF (tower)

ILS  Four principal components –Localiser –Glide slope –Marker beacons –Taxiway systems

ILS - Localiser  Gets aircraft on runway extended centre-line – MHz –Transmits two beams centred on the runway centre-line –Typical range ~25nm (always more than the Glide-Slope) 150Hz 90Hz

ILS - Localiser Localiser antenna About 400m beyond stop-end Localiser azimuthal profile Within 17nm valid indications should be obtained up to 35° either side of runway centreline 17nm 25nm

ILS – Glide Slope  Places aircraft on the standard glide slope – MHz –Transmits two beams centred on the glide slope –Typical range 10nm 90Hz 150Hz

ILS – Glide Slope A typical glide slope antenna Located alongside the runway, close to the normal touch-down point

ILS – Marker Beacons  75MHz “fan markers”  Outer marker, 3.5-6nm, blue  Middle Marker, ~1nm, amber  Inner Marker, Cat II minima, white  Middle/inner rarely installed  Becoming obsolete  Replaced by T-DME

ILS categories  Cat I = 200ft DH  Cat II = 100ft DH, 1200ft RVR  Cat III = 0ft DH, 0ft RVR A – lands the aircraft B – gets it off the runway C – gets it to the stand

DME  A type of radar –Aircraft sends a continuous stream of pulses –Ground equipment receives and sends back after a fixed delay –Aircraft times the round trip delay and computes distance: 1nm=12.36µs –Displayed distance is slant range –Can handle ~100 aircraft simultaneously

DME  Characteristics –Theoretical range 100nm –Accuracy ±0.1nm –Aircraft Tx frequencies 1025MHz MHz –Ground reply frequency ± 63MHz –Ground “main delay” nominally 50µs  Carlisle is set to 43µs (why?)

DME  Frequency pairing –There is no apparent way to select the DME frequency –DMEs are frequency paired with VOR & ILS –Pairing concept is used even when no VOR/ILS  Carlisle is channel 44X  Loc: 110.7MHz, G/S: 330.2MHs, MLS: MHz  DME: Aircraft Tx on 1068MHz, ground on 1005MHz –Hence, we think of DME as being on VHF!

DME  Additional techy stuff –Pulse pairs are used to reduce interference –Pulse pair spacing is fixed  X channels: 12µs  Y channels: 36µs  Aircraft handles either automatically –PRF is randomised to form a unique signature  Aircraft gates returns using the signature  Enters tracking mode when returns match the gate

Carlisle DME

En-route navaids  VHF Omni Range (VOR)  NDB  DME

VOR  108MHz to 118MHz  Always collocated with DME  Generally at airways intersections  Occasionally on airfields  Typical DOC range 100 miles

VOR principles of operation  Transmits two separate modulations –An omni-directional 30 Hz reference modulation –A rotating phase modulation on a 9960Hz sub carrier rotating at 1800rpm = 30rps  Arranged that the two modulations are exactly in phase at magnetic north  VOR receiver compares phases and uses difference to drive the VOR display

VOR characteristics  Everything based on radials from the VOR  Aircraft orientation is irrelevant (cf. NDB)  Generally unaffected by propagation issues  Often very well sited, with excellent range –E.g. Talla VOR on top of a 2700ft mountain

Turnberry VOR

RNAV systems  The problem with VOR/DME –Fixed locations, radial navigation system –Only easy to use if flying between them  RNAV helps to fix this problem –Offset a VOR/DME to a convenient waypoint –Fly to that waypoint as normal  Know what you’re doing! –Needs careful setting up –Risk of misinterpretation

FM Immunity  Localiser and VOR now start at 108MHz  Band-2 FM from 88MHz to 108MHz  FM broadcasts are very high power –Theoretical risk of interference with navaids –Does not affect voice com (starts at 118MHz)  Aircraft in IFR require one FM immune Nav –External filters (very expensive!) –New Nav receivers (also very expensive!)

Radar systems  Radio Detection And Ranging  Two fundamental types –Primary radar –Secondary radar  Come in all shapes and sizes –High power area radars (e.g. Great Dun Fell) –Lower airspace radars (e.g. Warton) –Airfield surveillance radars (e.g. Prestwick) –PARs, ASMIs (e.g. LHR) –Weather radars

Basic principles  Radio signals travel at fixed velocity  Signals are reflected by metallic objects  So range can be obtained by –Transmitting a stream of pulses –Listening for the echo –Time taken = distance (1nm = 12.36µs)  Bearing is determined by –Transmitting a narrow rotating beam

Basic principles  Fire a pulse off into space –Simultaneously start a spot moving out from the centre of the screen (PPI) –When the return arrives, brighten the spot  The PPI –Centre of screen is the radar head –Edge of screen is radar range (or less) –Spot moves in same direction as radar head is pointing

Primary radar  Pulse width determines resolution –1us pulse width (150 yards resolution)  PRF determines range –1ms space (80 miles range) 1µs 1ms

Advantages of primary radar  Simplicity  No equipment required on board aircraft

Limitations of primary radar  Fixed objects give radar returns! –Rain –Buildings –Large vehicles –This clutter obscures real returns, especially close to the head (and the airfield)  There is no identification information –ATC has to ask aircraft to turn  Poor returns from small aircraft

The fixed targets problem  What do we know about fixed targets?  What might we want to do with them? –Why would we ever want to see fixed targets? –Do we want to see rain clutter? –How might we reduce fixed target clutter

Moving Target Indicator  Fixed returns don’t move! –So every return will take exactly the same time –Each target is illuminated with many pulses and each one will take the same time to come back  MTI cancels out fixed targets –Send return pulses into a delay line with same delay as pulse repetition –Invert the delayed signal and add it to the undelayed signal

Moving Target Indicator  Some disadvantages of MTI –Not all relevant targets move  Hovering helicopters “disappear”  Slow moving aircraft such as microlights, likewise –Aircraft flying at a tangent disappear  Tangential fading  MTI is generally only applied for 10 miles  Controller can turn off MTI on his screen

Secondary radar  Originally developed as IFF during WWII  Relies upon a transponder in the aircraft  Eliminates fixed target clutter –Buildings and clouds don’t have transponders!  Provides information about the aircraft  Is used in conjunction with primary radar  Is becoming essential in CAS/TMAs

Secondary radar  Principles of operation –Single pair of frequencies used worldwide  1030MHz TX  1090MHz RX –Ground station transmits interrogation pulses –Transponder in aircraft responds with coded information (squawk code, height, other info) –Plotted on PPI with attached information box

Advantages of secondary radar  Positive returns, only from wanted targets  Additional information on aircraft  Improved performance for small aircraft  Improved resolution  Augments (does not replace) primary radar  Universally deployed

Disadvantages of secondary radar  Requires a transponder in the aircraft

Secondary radar modes  Ground transmits interleaved interrogations –Mode A (squawk code request) –Mode C (altitude request) –Mode S (detailed aircraft information)  Aircraft responds with four octal digits –Actual squawk code –Encoded pressure altitude –Optional “squawk ident” (SPI) pulse appended

Mode S  Every aircraft uniquely identified –“Hex code” linked to aircraft callsign –Additional operational information can be sent –Selective interrogation facility  Mandated for class-A airspace  Will be progressively introduced elsewhere –Major class D in the south initially –Unclear if mandated for GA  Confers no benefit on the GA operator!

Code/callsign conversion  Mode A sends a four digit octal code  Controller is more interested in your callsign –Codes allocated when FPL filed –Ad hoc codes allocated by controllers  Code and callsign paired in radar database  Aircraft callsign displayed on radar screen

TCAS  Traffic Collision Avoidance System –An aircraft-based secondary radar system –Interrogates your transponder –Computes range, azimuth –Obtains your height from the Mode-C –Performs collision risk assessment –Provides audible risk avoidance instructions  Mandated for CAT/GA over 5700kg MAUW  A strong argument for transponding A+C!

Radar antennas

GPS  At last! A non-radial navigation system –Triangulates between multiple satellites –Provides an accurate 3-dimensional fix –Navigate between any set of waypoints with ease –Upload your route and away you go!  Moving map GPS –Terrain mapping –Jeppesen database (CAS, danger areas, etc.)

GPS – some cautions  GPS not considered a reliable system(!)  GPS is easy to jam  GPS is owned by the US Government –“Dubbya” can turn it off any time he pleases  As a result –GPS not approved for primary navigation –GPS-based procedures still a distant vision  Use GPS in conjunction with other navaids

Summary  We’ve looked at –Radio communications –Airfield systems –En-route systems –Radar systems  Knowledge of these systems is important and makes us better pilots  Research the Internet for more!