Lecture 14

Radio Navigation Systems

• Major Topic: Hyperbolic Navigation (LORAN, DECCA & OMEGA)

Hyperbolic Navigation Overview

• Definition: Hyperbolic radio navigation is a medium to long-range navigation system primarily used in oceans and unpopulated areas where manned radio stations are impractical.

• History: Several systems developed since the 1940s, with Decca and Omega ceasing operations by 2000. Current reliance on LORAN has diminished in commercial aviation.

• Basic Principle: The principle relies on two stations (master and slave) transmitting pulses at pre-determined intervals so that a receiver measuring the timing of these pulses can calculate its relative position from the stations.

Technical Details

• Pulse Transmission:

  • Master station transmits a series of pulses at $1 \text{ ms}$ interval.

  • The slave station, after an interval of $8 \text{ ms}$ (depending on the distance of the baseline), receives the first pulse and transmits its own pulse after a $1 \text{ ms}$ delay.

  • All stations operate on the same frequency. Their identities are encoded in the signal (M for master, W, X, Y or Z for slave).

Hyperbolae Formation

• Lines joining locations with $2 \text{ mS}$, $4 \text{ mS}$ and $6 \text{ mS}$ delays, etc., formed Hyperbolae. With three stations or more, navigators can calculate their locations:

  • $1 \mu\text{S} = 300 \text{ m}$

  • $1 \text{ mS} = 300 \text{ km}$

  • $8 \text{ mS} = 2400 \text{ km}$

Loran Navigation

• Introduction to Loran: Long Range Navigation is the only surviving hyperbolic navigation system. Although it was thought that it will be made obsolete by GPS, there are some talks about reviving it as a back-up to GNSS which have inherent vulnerability.

• System Specifications:

  • Loran-C operates on LF band (90KHz to 110KHz).

  • Range: up to 1,000 nautical miles ($1 \text{ nm} \approx 1.852 \text{ km}$).

  • Accuracy: better than $0.25 \text{ nautical miles}$ ($460 \text{ m}$).

  • Pulse duration: $270 \mu\text{S}$ (27 cycles of the $100 \text{ KHz}$ carrier wave).

  • Pulses are formed as variable amplitude sine waves at a fixed frequency to keep it within the 90 KHz to 110KHz bandwidth.

  • At $100 \text{ KHz}$, wavelength $\lambda = 3 \text{ km}$ .

  • Transmitter towers are typically 700 to 1,300 ft and power ranges from 400W to 1,600W.

  • Baseline distances vary from 175 nm to 1,000 nm.

  • Transmitters are grouped in chains with each Master having 2 to 5 slave transmitters.

Technical Challenges

• Interference Issues:

  • Electromagnetic interference from power lines and electrical storms.

  • Loss of any one transmitter affects the area of coverage.

• e-Loran Development:

  • Enhanced Loran (e-Loran) was first tested by the FAA and US Coast Guard in 2005.

  • e-Loran carries additional data such as UTC, corrections, warnings, and signal integrity.

  • The system treats each Loran station as an individual and does not relate to any specific chain.

• Other Systems: Other hyperbolic navigation systems operated by Russia have all ceased operation.

Decca Navigator System

• Station Configuration:

  • Consisted of individual groups of land-based radio transmitters organised into chains of three or four stations: a master station and three (occasionally two) secondary stations, termed Red, Green, and Purple.

  • Ideally, secondaries would be positioned at the vertices of an equilateral triangle with the master at the centre.

  • Baseline (master-secondary distance): typically $60-120 \text{ nmi}$ ($110-220 \text{ km}$).

  • Each station transmitted a continuous wave signal that, by comparing the phase difference of the signals from the master and one of the secondaries, produced a relative phase measure that was presented on a clock-like display.

• Operational Details:

  • Dectra receivers with Omnitrac computers and a lightweight version of the Flight Log were used on commercial airliners (notably a BOAC Vickers VC10) before the advent of INS.

  • The Omnitrac could take inputs from Decca (and Dectra), Loran-C, VOR/DME, ADC, and Doppler Radars and combine them to produce Lat/Long output, bearing, distance-to-go, and autopilot coupling.

  • As the receiver moves, distances change, and those changes are represented by the movement of the hands on the displays.

  • At $100 \text{ KHz}$, wavelength $\lambda = 3 \text{ km}$ (= $360 \text{ deg}$), meaning $1 \text{ deg} = 8.3 \text{ m}$.

OMEGA System

• Overview:

  • First global radio navigation system, operational from 1971-1997, later overtaken by GPS.

  • Used very low frequency (VLF) radio signals in the range $10 \text{ to } 14 \text{ kHz}$, transmitted by eight fixed terrestrial radio beacons.

  • Approved for development in 1968 with eight transmitters.

• Technical Details:

  • Operational frequencies: $10 \text{ kHz}$, $13.6 \text{ kHz}$, $11.333 \text{ kHz}$ plus a unique fourth frequency for each of the eight stations.

  • Achieved position accuracy of $4 \text{ miles}$ ($6.4 \text{ km}$).

  • At $10 \text{ KHz}$, wavelength $\lambda = 30 \text{ km}$. Omega stations used very extensive antennas.

• Broadcast Pattern:

  • Each Omega station transmitted a sequence of three VLF signals (and a fourth unique frequency) differing in a fixed pattern, repeating every 10 seconds.

  • The duration of each pulse ranged from 0.9 to 1.2 seconds, with 0.2-second blank intervals between pulses.

  • The 10-second pattern was common to all 8 stations and synchronized with the carrier phase angle.

  • The envelope of the individual pulses could be used to establish a receiver's internal timing within the 10-second pattern.

  • The phase of the received signals within each pulse was used to determine the transit time from transmitter to receiver.

  • Using hyperbolic geometry and radionavigation principles, a position fix with an accuracy on the order of $5-10 \text{ km}$ was realizable over the entire globe at any time of the day.

Automatic Direction Finder (ADF)

• Operational Frequency: $190 \text{ KHz}$ to $1.750 \text{ MHz}$, operating in the Medium Wave commercial AM Radio Broadcast Band.

• Accuracy: $\pm 5 \text{ degrees for approach}$; $\pm 10 \text{ degrees for enroute}$ .

• History: One of the earliest methods of using the directional property of radio waves for navigation. Early ADF made extensive use of commercial radio stations as their Non-Directional Beacon (NDB) for navigation.

ADF System Components

• Typical System Includes:

  • Two antennas, a receiver, a control panel, and an indicator.

  • Most aircraft has dual systems for redundancy.

Principle of Operation

• Directional Detection:

  • The directional property of a dipole antenna enabled it to scan for the highest signal strength of a transmitter. However, this resulted in two possible directions.

  • By adding a sense antenna (omnidirectional), the combined radiation pattern forms a cardioid, which has a clear direction of highest reception.

  • In practice, it is easier to detect the direction of lowest reception (known as Null Point) as the rate of change of signal strength near the null point is much higher.

• Manual to Automatic Transition:

  • Early ADF systems required the radio operator to manually rotate the loop antenna to find the direction.

  • This was improved as servo motors were used to drive the loop antenna.

  • Later versions used two loop antennas placed in a quadrangle and resolving the x and y components to detect direction.

Control Panel and Receiver Details

• Control Panel Functions:

  • Allows the crew to select frequencies, volume of audio output, and display of frequency selected.

  • Some panels have a BFO (Beat Frequency Oscillator) which creates an audio frequency for NDBs that are not modulated with an audio signal.

  • An ADF/ANT selection allows normal operation or sense antenna only for checking station existence within range.

  • Many aircraft integrate ADF controls with other systems in a combined Control Panel.

• Receiver Inputs:

  • The sense signal.

  • A loop cosine signal.

  • A loop sine signal.

  • The sense signal is processed to recover the audio ID, then summed with loop signals and further processed to recover the bearing information.

  • In analogue aircraft, this signal is sent directly to a synchro motor to drive a pointer in the cockpit.

  • In digital aircraft, the signal is sent to a digital bus such as the ARINC 429.

  • In some aircraft, the ADF Receiver is incorporated into a multimode unit together with other radio navigation receivers.

Information Display

• The ADF output from the receiver is transmitted to the pilot through an indicator displaying magnetic heading and bearing to the NDB station.

• Indicators:

  • Earlier aircraft had dedicated ADF indicators, some with fixed compass card, but mostly with movable compass card driven by a gyrocompass.

  • Later aircraft use Radio Magnetic Indicators (RMI) for ADF display. These RMI are multipurpose indicators which can display VOR information too.

  • RMIs can display two navigation information, and are selectable: ADF and VOR or both ADF or both VOR.

  • In modern digital aircraft, the information is sent through the ARINC bus to the EFIS (Electronic Flight Information System).

System Performance and Errors

• Range and Limitations:

  • ADF signals are transmitted by ground waves and sky waves. The range is limited by Line-Of-Sight (LOS).

  • Some waves may be reflected by ionospheric layers which can cause errors. Other reflections and refractions can also add to the errors.

  • At $30 \text{ m}$ wavelength, it is about the same magnitude as the metal aircraft structure which can absorb and re-radiate causing interference and errors.

  • This frequency band is also susceptible to static discharges and other electrical equipment.

  • The accuracy of ADF is about $\pm 5 \text{ degrees}$ for locator beacon and $\pm 10 \text{ degrees}$ for en route beacons.

  • ADF is not suitable for precision navigation due to various errors.

• Class of NDB Transmission Power and Effective Range:

  • Locator: below $25 \text{ W}$ -> $15 \text{ NM}$

  • MH: below $50 \text{ W}$ -> $25 \text{ NM}$

  • H: $50 \text{ to } 1,999 \text{ W}$ -> $50 \text{ NM}$

  • HH: $2,000+ \text{ W}$ -> $75 \text{ NM}$

• Common Errors Include:

  • Ionospheric Error: Specifically, during periods of sunset and sunrise, the ionosphere reflects NDB signals back to Earth, causing fluctuations in the ADF needle (fading).

  • Electrical Interference: In areas of high electrical activity, such as a thunderstorm, the ADF needle will deflect toward the source of electrical activity, causing erroneous readings.

  • Terrain Errors: Mountains or steep cliffs can cause bending or reflecting of signals. The pilot should disregard erroneous readings in these areas.

  • Bank Error: When an aircraft is in a turn, the loop antenna position is compromised, causing the ADF instrument to be off balance.

  • Shoreline Error: Ground waves change direction as they pass from land to water and vice versa; they are bent slightly. Pilots should note potential bearing indication errors when flying in the vicinity of coastal areas. NDBs used primarily for oceanic navigation have been designed to minimize this error.

VHF Omnidirectional Range (VOR)

• Development Background:

  • Developed in the 1940s, following the successful use of VHF communication, as a reliable navigation system using VHF frequencies.

  • It is still in widespread use, serving as the primary short-range aircraft navigation system for more than 60 years.

  • Operates in the VHF frequency range from $108 \text{ to } 117.95 \text{ MHz}$. At this frequency, only the sky waves are usable, which eliminated many of the problems faced by LF/MF navigation systems.

• Identification:

  • VOR stations are identified by Morse code which is modulated into the carrier wave as a $1020 \text{ Hz}$ audio tone.

  • Some stations have a voice ID announcement alternating with the standard Morse Code ID.

• Frequency Coordination:

  • VOR operates in the same frequency range as ILS (Instrument Landing System) and in coordination with a DME (Distance Measuring Equipment) station.

  • ILS frequencies are odd tenths of each $0.5 \text{ MHz}$ increment (e.g., 108.10, 108.15 and 108.30 are ILS channels).

  • VOR channels occupy the even tenths (e.g., 108.20 and 108.40 are VOR channels).

VOR Operational Principle

• Signal Structure:

  • VOR beacon transmits two signals: a reference phase and a variable phase.

  • The two signals are in phase at Magnetic North. For the rest of the circle, the phase difference is equal to the number of degrees from Magnetic North.

  • The omnidirectional carrier is amplitude modulated to $30 \text{ Hz}$. The directional signal is radiated as a cardioid pattern rotating at $30 \text{ revolutions per second}$ .

  • A $9960 \text{ Hz}$ sub-carrier is frequency modulated in the range $9960 \pm 480 \text{ Hz}$ to $30 \text{ Hz}$.

• Indication Mechanism:

  • Aircraft displays magnetic bearing to/from the station using a Course Deviation Indicator (CDI).

  • The VOR receiver senses the phase difference between the two frequencies and the difference identifies 360 different directions or "radials" from the VOR.

  • The system does not provide distance information; it only informs the pilot on which radial the aircraft is on.

  • The CDI on the VOR indicator shows whether the aircraft is right or left of the chosen course.

  • A "To/From/Off" indicator indicates whether the aircraft is on the "to" or "from" side. An "off" indication is given if the aircraft is directly above the station.

  • To fly toward the station, the Omni Bearing Selector (OBS) is turned until the CDI is centred with a "to" indication. The pilot then flies that heading.

  • To find out where the aircraft is located from that station, centre the needle with a "from" indication.

  • VOR enables the pilot to obtain: magnetic bearing of the aircraft to or from the VOR station, and position of the aircraft with respect to the selected radial (left or right).

System Operation Details

• Active Range: VORs provide radials which form airways. They are generally accurate within one degree.

• Position Fixing: When two VOR radials intersect, they provide a unique navigation position fix. The accuracy is best when they intersect at right angles.

• Reliability: The VOR system is relatively simple and therefore extremely reliable.

• Range Limitation: Since it operates on VHF, the range is limited to line-of-sight (LOS) given by the relationship $d = 1.1\sqrt{h}$ (where $d$ is the range in nautical miles and $h$ is the altitude of the aircraft in feet).

  • At $10,000 \text{ ft}$, the range is $110 \text{ nm}$.

  • At $20,000 \text{ ft}$, the range will be about $154 \text{ nm}$.

  • At $40,000 \text{ ft}$, it will be $220 \text{ nm}$ (about $400 \text{ km}$).

• DME Pairing: Most VOR stations are paired with a DME station. Whenever a VOR station frequency is selected, the aircraft system will automatically tune to the corresponding DME station.

Distance Measuring Equipment (DME)

• Functionality: Measures the "slant range" from the DME station. Slant range is a measure of an aircraft's position relative to the DME station that incorporates the height of the aircraft, its angle from the ground station, and its unknown ground range based upon a $90^\circ$ angle.

• Accuracy: The farther the aircraft is from the station and the lower the aircraft's altitude, the more accurate the distance reading. An aircraft directly over the DME station at an altitude of $10,500 \text{ feet}$ above ground level (AGL) would correctly indicate two miles from the station (due to slant range).

• Measurement Principle: Airborne DME measures the elapsed time taken for the exchange of signals and converts it into distance.

• Display: DME displays distance in nm, ground speed (GS) in knots, and time-to-station in minutes. These are only accurate when the aircraft is flying directly to or from the ground station.

• Identification: DME has its own identification. When a VOR and DME are co-located (as in a VORTAC or VOR-DME station), the DME transmits the same coded ident as the VOR but sends it during the pause between successive VOR idents. The DME ident is also higher-pitched ( $1350 \text{ Hz}$ ) compared with the VOR ident ( $1020 \text{ Hz}$ ).

• System Configuration: Commercial aircraft are usually fitted with two DME systems. The antennas are L-Band blades located on the underside of the fuselage.

• Operational Principle: The aircraft DME sends randomly spaced interrogation pulses to create a unique "signature". It then examines the ground station replies for a sequence with the same randomly jittered signature.

Transmission Frequency Ranges

• DME Frequency Range (L-Band, within UHF):

  • Transmitting: $1025 \text{ to } 1150 \text{ MHz}$

  • Receiving: $962 \text{ to } 1215 \text{ MHz}$.

Overall Operational Principles

• Accuracy: Accurate slant range readings emphasize direct routing to/from the ground station. This technique is crucial for precise navigation in aviation.