Jeppesen-Radio-Navigation

Ground Radar Chapter 12

RANGE, ACCURACY, AND LIMITATIONS OF SURVEILLANCE RADAR

Surveillance Radar is capable of providing coverage to ranges greater than 200 nm. However,
both primary and secondary elements are strictly “line of sight” so if an aircraft is below the radar
horizon it will not be detected.

If the aircraft is above the horizon, there are still other considerations:

¾ The size of the aircraft
¾ If the aircraft is heading straight toward or away from the radar aerial
¾ If the aeroplane is made of glass reinforced plastic

Any of the above make the aircraft a poor target for primary radar. SSR provides information that
is not size or aspect related, but depends on:

¾ The ground unit being equipped with an interrogator
¾ The aeroplane having a transponder
¾ The pilot using the transponder correctly

The accuracy of Surveillance Radar depends on the type of unit used but is sufficiently effective
to allow for a traffic separation of 5 miles. This may decrease to 3 nm within a range of 40 nm of
the radar aerial.

SURVEILLANCE RADAR PROCEDURES

ENROUTE

Procedures are limited to those required for identification. This may involve:

¾ Carrying out turns as directed by the controller
¾ Identifying the aircraft’s position as a radial and range from a VOR/DME beacon at

the request of the controller
¾ Identifying the aircraft’s position as a geographical point at the request of the

controller

In European regions, identification occurs more frequently using the SSR element. In the event of
a primary radar failure, the controller introduces non-radar separation standards using SSR to
assist.

APPROACH

Surveillance radar may be used to provide the pilot with approach guidance including azimuth
information and altitude advisories.

The success of such an approach depends upon:

¾ The skill of the controller
¾ The ability/willingness of the pilot to carry out the controller’s instructions.

Remember that this type of radar has no height-finding capability, so all height information is
advisory. This is the height the aircraft should be flying at the range and bearing observed by the
controller. Also remember that if the pilot cannot see the runway upon reaching Minimum Descent
Height (MDH), the pilot must not descend below this height. When the aircraft reaches the Missed
Approach Point (MAPt) a go-around is necessary.

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Chapter 12 Ground Radar

PRECISION APPROACH RADAR (PAR)

Many military airfields have PAR installations. These are primary radar units designed to provide
guidance during final approach to landing.

The PAR consists of two elements:

¾ One providing azimuth and range information
¾ The other providing elevation/range information

Each element utilises 3 cm wavelength (10 GHz) radar with high PRF and short pulse length (less
than 1 µs). Both elements must be capable of providing detection to:

¾ Range of 9 nm
¾ Elevation of 7°
¾ Azimuth sector of 20° (10° each side of the extended runway centre line)

Within this volume of airspace, the PAR must be capable of detecting a target with a radar cross
section of 15 m2 or greater. The maximum allowable error is ± 30 ft on azimuth and ± 20 ft in
elevation.

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The two elements are sited at the approach end of the runway to the side of the landing
threshold.

Azimuth
The azimuth element scans a very narrow beam (0.6°) backward and forward over a
sector covering the required minimum azimuth sector.

Elevation
The elevation element has a narrow vertical beam width (0.6°) but a broader azimuth
beam width (up to 30°). It sector-scans vertically from an elevation of about 0.5° up to 8°.

Both scans are at a rapid rate in order to ensure that the target information is refreshed quickly.
Target information is presented to the controller on two screens mounted one above the other.
The upper screen shows the range to the target and its position relative to the nominal glide path.
The lower screen shows the range and the position relative to the extended runway centre line.

PAR PROCEDURE

A typical procedure is detailed below:

Prior to commencing the approach, the controller advises the pilot of the Aerodrome QFE. All
heights refer to this datum. Instructions are designed to help pilots on the glidepath and centreline
by providing regular azimuth and glidepath correction information to the pilot.

Remember it is the pilot’s responsibility to ensure that the runway is in sight before DH. Initiate a
missed approach procedure if the runway is not in sight at DH.

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AIRFIELD SURFACE MOVEMENT INDICATOR (ASMI)

This is a highly specialised primary radar unit designed to assist controllers in maintaining safe
separation between aircraft and vehicles on the ground and to monitor all ground movements.
The maximum range is approximately 2.5 nm.

It requires only short range but must be capable of:

¾ Very low minimum range
¾ 360° cover
¾ Very high level of accuracy
¾ Excellent target discrimination

For some time it was necessary to use a radar operating in the “Q” band (35 000 MHz) as this
provided:

¾ A very narrow beam width with a small aerial
¾ Excellent bearing discrimination

Modern signal processing techniques have led to the achievement of equivalent definition with a
cheaper “X” band (10 000 MHz) radar. This is now the favoured option.

The following are the typical specifications for an ASMI:

¾ Frequency 10 000 MHz
¾ PRF 15 000 pps
¾ Pulse length 0.05 µs
¾ Beam width 0.4°
¾ Aerial rotation 60 to 75 rpm

Here is a typical ASMI picture:

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Ground Radar Chapter 12

WEATHER RADAR

The weather radar found at an airfield is not an ATC radar but is for the use of the meteorological
services to supplement their forecast information.

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12-8 Radio Navigation

INTRODUCTION

The primary radar element of the ATC Surveillance Radar System provides detection of suitable
targets with good accuracy in bearing and range measurement, but with certain limitations:
Targets that are too small, built of materials that reflect radar energy poorly, or have a poor
aspect may not be detected.

¾ Targets cannot be identified directly
¾ Radar energy suffers attenuation (losses) both on the path out to the target and on

the return path of the reflections

To overcome these problems, a Surveillance Radar installation often consists of both a primary
and a secondary radar, the latter being the Secondary Surveillance Radar (SSR). The role of the
SSR is to complement the primary radar element.

PRINCIPLES OF OPERATION

SSR operates on secondary radar principles. An SSR “link” uses one ground-based transmitter
and receiver, called the interrogator, and one airborne transmitter and receiver, referred to as the
ATC transponder, or simply transponder. The interrogator transmits pulses. A receiver within the
interrogator’s beam receives these pulses and decodes them. The transponder then responds by
transmitting a pulse train (many pulses in a stream) back to the interrogator. The pulse train
contains information according to what the interrogator requested.

All interrogations transmit at a frequency of 1030 MHz and all transponders respond at a
frequency of 1090 MHz.

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Chapter 13 Secondary Surveillance Radar (SSR)

The SSR aerial consists of a radiator and reflector similar to that used in the primary radar. Since
the return is much stronger than that of a primary radar reflection, it is much smaller. As a result
of this small size and the frequencies used, the beam width tends to be large. This results in the
transmission of a considerable number of side lobes.

PULSE SPACING

There are four modes and their applications and pulse intervals are as follows:

Mode Use Pulse spacing
A ATC 8 µs
B ATC 17 µs
C Auto Pressure Altitude report 21 µs
D Not Assigned 25 µs

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Secondary Surveillance Radar (SSR) Chapter 13

Mode A is known to the military as Mode 3.

Modes B and D are not currently in use and the conventional aeroplane transponder is designed
to use only modes A and C.

SIDE LOBE SUPPRESSION

The large beam width reduces the bearing accuracy and increases the chances of reflections
from buildings and other obstacles generating false interrogations. The side lobes can cause
aircraft responses from any direction. Aerial design has minimised this problem.

Side lobes still create a problem since aeroplane receivers, especially at close range, detect
them. This triggers a false response outside the main beam and can result in the effect of side
lobe clutter, illustrated below. A process known as side lobe suppression (SLS) can counteract
this by modifying the method of interrogation so as to electronically cancel the effect of side lobe
radiation.

MEDIUM AIRCRAFT AT
RANGE LONG RANGE

SHORT SIDE LOBE
RANGE CLUTTER

RING-A-ROUND

Interrogations are sent in the form of a group of three pulses identified as P1, P2, and P3. P2 is a
control pattern placed over the main beam.

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Chapter 13 Secondary Surveillance Radar (SSR)

2µs

The spacing between P1 and P2 is constant at 2.0 µs. Pulse P2 is used in the electronic side
lobe suppression. The spacing between P1 and P3 is set at a value dependent upon the mode
required from the aeroplane transponder as shown earlier.

Position 1
The control pattern is omni-directional except in the direction of the main beam. This
means that an aircraft painted by the main beam receives a P2 pulse that is lower in
amplitude than the P1 and P3 pulse, as in the diagram shown above.
Position 2
If an aircraft is painted by one of the side lobes, the P2 pulse is of greater amplitude than
the P1 and P3 pulse. The side lobes only have 50% of the power of the main beam and
thus P2 appears as the pulse with the greater amplitude (see diagram below).

2µs

Whenever the transponder receives P1 and P3 at greater amplitude than P2, it transmits a reply.
Should P2 be greater than P1 and P3, it indicates that the aircraft is being painted by side lobes,
and the transponder suppresses any reply transmission.

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Secondary Surveillance Radar (SSR) Chapter 13

OPERATION

The pilot sets the transponder to the mode and code as instructed by ATC.

OFF ATC ALT RPTG TRANSPONDER SELECTOR
LR - SELECTS DESIRED TRANSPONDER
0767 OFF ON
ATC CODE INDICATOR
FAULT - DISPLAYS CODE SET BY ATC CODE SELECTOR

IDENT ALTITUDE REPORTING SELECTOR
ON - ENABLE ALTITUDE REPORTING

ATC FAULT LIGHT (AMBER)
- ILLUMINATES WHEN A TRANSPONDER

MALFUNCTION HAS BEEN DETECTED

ATC CODE SELECTORS
ROTATE - SETS CODE IN ATC CODE INDICATOR

AND BOTH TRANSPONDERS

ATC IDENT SWITCH
PUSH - TRANSMITS IDENT SIGNAL

TYPICAL TRANSPONDER CONTROL

If the transponder is set to the “ON” position, the unit responds to Mode A interrogations. If set to
ALT, the transponder responds to Mode A and C interrogations, sending identification and
automatic altitude information. The transponder’s response occurs in the form of a pulse train.
This consists of two framing pulses separated by 20.3 µs. Between the framing pulses is the
facility for transmitting up to 13 coding pulses. Pulse ‘X’ is not used at this time, utilising only 12
pulses. The pulses are used as follows:

¾ A pulses form the first digit of the four-figure code
¾ B the second
¾ C the third
¾ D the fourth

The diagram below shows the arrangement of A, B, C, and D pulses for sending the digits. Note
that for each digit, there are 8 possibilities ranging from 0 to 7. This leads to a total of 4096
selectable codes (8 x 8 x 8 x 8).

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Chapter 13 Secondary Surveillance Radar (SSR)

After the main framing pulses, a special position indicator (SPI) pulse is transmitted 4.35 µs after
the last pulse when the ident button is used on the main control panel. The SPI code paragraph
discusses this.

The system uses a form of binary numbering to identify the code. The altitude information is
relative to the 1013.2 hPA level no matter what pressure setting is on the altimeter.

SPI CODE

Utilising a special “IDENT” feature allows ATC to confirm an aeroplane’s identity. The pilot
activates this when instructed by ATC. When the IDENT button is pushed, an additional pulse is
transmitted 4.35 µs after the second framing pulse. At the controller’s display, the ident pulse
causes the particular aeroplane’s echo to brighten or flash. This lasts for approximately 15 – 30
seconds.

USE OF TRANSPONDER

Pre-departure the transponder is set to stand by. This allows the equipment to warm up but not
transmit. The test function is then activated to establish the operational status of the equipment.
When instructed, the pilot sets the mode and code given by ATC, and when told to “Squawk” sets
the controller to “ON” or “ALT” as appropriate. Normally ALT is selected, as the altitude encoding
has to be selected in all cases unless ATC instructs otherwise. In order to avoid causing
interference, do not change the mode or code without first selecting “STBY” on the controller.
When in an abnormal situation, there are three codes to alert the controller. These codes have a
predefined meaning and, with one of these selected, triggering a signal indicating a “special
condition” on the controller’s screen. The aircraft symbol may change colour to attract more
attention. On some radar systems, an aural alarm sounds together with the target on the screen
either flashing or brightening.

¾ Code 7500 Hijack
¾ Code 7600 Radio failure
¾ Code 7700 Emergency

From time to time, the ATC controller may ask the pilot to “SQUAWK IDENT”. Pushing the
“IDENT” button activates the transponder to transmit the additional pulse. This appears on the
radar display as a flashing target. This function, when first enabled, continues for approximately
15 - 30 seconds. Do not press the “IDENT” button unless instructed by air traffic.

PRESENTATION AND INTERPRETATION

SSR information is presented together with the primary radar information. The difference between
the two is that the primary information is very accurate in bearing and range, but does not consist
of any extra information. The secondary radar information is inaccurate in bearing and range,
although it can serve as a back up, but provides reliable information that can identify every
aircraft and provide altitude information.

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Secondary Surveillance Radar (SSR) Chapter 13

The primary radar element provides the necessary bearing and range. The use of computer-
generated displays allows calculated information, such as course and ground speed, to be
shown. Here is a common style of displaying combined (primary and secondary surveillance
radar) information on the air traffic controller’s radar screen.

LIMITATIONS

SSR complements the primary radar system and, although effective, is likely to be replaced by
Mode S in the near future. Previously noted is the fact the bearing and range information is not as
good as that of the primary radar. In addition there are two other major problems:

FRUITING

Although ground based interrogators have a nominal range of approximately 200 nm, the
propagation is “line of sight” and it is not unusual for aeroplanes, especially at cruising altitudes
over well-developed ATC regions, to be within range of two or even more ATC interrogators.
Since all SSR units operate at the same frequency, this can result in detection of an aeroplane’s
response to one interrogator by other ground units. Such responses occur out of synchronisation
causing random responses to appear. This is called fruiting. Electronic circuits are employed
(de-fruiters) to remove this effect but they do not remove all random responses and the situation
becomes worse as traffic density increases.

GARBLING

Another problem is garbling, which occurs when targets are close to one another (e.g. in a
holding pattern or progressing along an airway one above the other). If both aircraft are in the
interrogation beam at the same time, and are close enough to each other, the ground interrogator
cannot differentiate between them and records only one confused return.

Although fruiting and garbling effects are manageable at this time, future traffic growth places
more and more stress on the system and the controllers.

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Chapter 13 Secondary Surveillance Radar (SSR)

MODE S

This is a development of the basic SSR, which is being introduced. The Mode S ground
interrogators and airborne transponders are fully compatible with the conventional Mode A and C
units and use the same basic frequencies of 1030 MHz and 1090 MHz. Mode S units working
together have much greater capabilities.

OPERATION OF MODE S

The Mode S interrogator and receiver operate on the same frequency as standard SSR. The
initial part of the interrogation signal is such that a normal airborne transponder unit recognises
the standard SSR modes. The second part of the Mode S interrogation consists of a message of
up to 112 data bits of which 24 bits are allocated to aircraft address. This permits the controller to
interrogate a specific aircraft. The 24 bits allocated are sufficient to provide for over 16 million
individual addresses. This is considered sufficient for the registration of all aircraft in the world.

In order to detect further Mode S transponders, a special feature known as SSR/Mode S “ALL
CALL” is broadcast at intervals. Normal SSR transponders respond to this in Mode A or C
(dependent on the P1/P3 relationship). Mode S transponders recognise the special character of
the “ALL CALL” interrogation as a roll call request and transmit a response that includes the
aircraft’s identity/address along with details of the capability of the relevant on-board equipment.
This is a 56-bit message. The other interrogations and responses are as follows:

INTERROGATION RESPONSE

Type Content Type Content

Surveillance 16 control bits Surveillance 13 SSR identify & altitude
16 altitude echo 19 control purposes
24 address 24 address

Comm A As for Surveillance + 56 bits Comm B As above + 56 bits air to
ground to air data ground data

Comm C 112 bits for data Comm D 112 bits data transmission
transmission of long of long air to ground
messages messages

The altitude echo function in the Surveillance interrogation indicates (to the pilot) the flight level
that the aeroplane’s transponder is providing ATC. Comm A and Comm C interrogations can
send longer messages by breaking the messages up into suitable sized blocks and transmitting
on successive cycles. Comm D, from the airborne transponder, has a similar capability. Comm D
cannot be used for position update, as the messages contain no altitude information.

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Secondary Surveillance Radar (SSR) Chapter 13

The expanding use of Mode S has the following benefits over standard SSR:

¾ Elimination of synchronous garbling
¾ Elimination of ‘fruiting’
¾ Increased traffic capacity
¾ Improved accuracy

In addition, the ability to send messages allows for a reduction of congestion on the current R/T
communication frequencies. Transmitted data can be presented to the pilot on a CDU either
integral with the Mode S transponder or on the FMS screen.

Mode S information, transponder to transponder, can also be integrated with the airborne
Collision Avoidance system allowing the systems of conflicting aircraft to communicate and
resolve convergent situations.

ATC SERVICES

Mode S data link can serve as a back-up to many ATC services provided today by VHF voice
communications. This data link back-up improves system safety by reducing communications-
related errors within the ATC system. Many types of messages are potential candidates for data
link back up and other ATC services. These include:

¾ Flight identification
¾ Altitude clearance confirmation
¾ Take-off clearance confirmation
¾ New communication frequency for sector hand-off
¾ Pilot acknowledgement of ATC clearance
¾ Transmission to the ground of aircraft flight parameters
¾ Minimum safe altitude warning

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Chapter 13 Secondary Surveillance Radar (SSR)

13-10 Radio Navigation

INTRODUCTION

All transport aircraft, and some small general aviation aircraft, are equipped with weather radar.
The weather radar has four main functions:

¾ Locating clouds ahead of the aircraft
¾ Assisting the pilot to avoid turbulent clouds
¾ Determining the location and height of cloud tops
¾ Mapping of the terrain ahead

Weather radar is an airborne pulse radar designed to locate turbulent clouds ahead of an aircraft.
Some weather radars have a secondary ground mapping capability.

PRINCIPLE OF OPERATION

Cumuliform clouds are associated with both rising and descending currents of air, which lead to
turbulence. In the case of towering cumulus and cumulonimbus cloud, this turbulence can be
severe. The turbulence within the cloud retains the water droplets within the cloud until they are of
a size to fall as precipitation. It is the precipitation, and in particular, the large water droplets, that
reflect the radar energy. The reflection depends upon the form and size of the droplets within the
cloud:

Hail is covered by a film of water and gives the strongest return.

Light Rain and Snow give the weakest returns.

Weather radar does not detect non-turbulent clouds, as the water droplet size is too small.

Frequency
9 to 12 GHz

Frequency Range
The frequency range above provides good returns from droplets of water, rain, or hail in clouds.

¾ Using a frequency that is too high causes the smaller wavelength to reflect off the
smaller water droplets of non-turbulent clouds.

¾ Using a frequency that is too low causes the radar to penetrate the cloud and to
provide inadequate returns from turbulent clouds.

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Chapter 14 Airborne Weather Radar (AWR)

AWR AERIAL

The aerial consists of either a parabolic dish or, in modern units, a flat-plate phased array. It is
mounted in the nose of the aeroplane and scans ahead of the aircraft from side to side with a
narrow beam. The sweep can be up to 90° to each side of the aircraft nose. The beam can be
tilted manually ± 15° in the vertical plane. A gyro stabilises the scan horizontally to prevent loss of
target if the aircraft rolls or pitches. This stabilisation is effective up to ± 20° of combined roll,
pitch, and tilt.

The radar can be selected to either “weather” or “mapping” beam. The weather beam is used for
detecting clouds and is a conical pencil beam with a width of 5°.

The directional properties of the radar produce side lobes. One side lobe goes vertically down to
the ground and is received back by the radar receiver. This received signal produces a height ring
on the display. The ring indicates that the radar is working and appears at the approximate height
of the aircraft above the ground. An aircraft flying at 12 000 ft will have a permanent return at
approximately 2 nm.

CONTROL PANEL

POWER SWITCH MARKER BRILLIANCE

OFF UP TILT MARK
POWER 15 10 BRILL

ON 5 CONT
STAB 0 WEA
OFF MAN
STANDBY 5 MAP
20 CONTRAST
15 10
50 DOWN

150

TIMEBASE CONTRAST TILT MANUAL FUNCTION
RANGE CONTROL CONTROL GAIN SWITCH
SWITCH
CONTROL

POWER SWITCH AND TIMEBASE RANGE SWITCH

These two switches are used together. On the ground, the timebase range switch is set to
standby. The power switch is then set to either STAB ON or STAB OFF (the significance of these
positions will be explained later in this chapter). The radar warms up but does not transmit.

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Airborne Weather Radar (AWR) Chapter 14

When the aircraft is airborne, the pilot makes the appropriate range selection, in this case 20 nm,
50 nm, or 150 nm.

30 30 20 nm range scale
45 45

60 60

30 0 5 10 15 50 nm range scale

45 30
60 45
60

30 0 10 20 30 40 150 nm range scale
45
30
60 45
60

0 50 100

The angle markings are permanently etched on the radar screen every 15°. The angles represent
the relative bearing of a return left or right of the nose. The range markers are electronically
produced depending on the range selected. The marker brilliance control determines the
brightness of these settings. It has no other function.

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Chapter 14 Airborne Weather Radar (AWR)

FUNCTION SWITCH

The function switch selects the mode of the radar:

WEA/CON
This mode shows Weather and Contour. Both of these selections assist in cloud
detection. These selections have automatic gain control.

MAN/MAP
This mode shows Manual and Mapping. Both selections assist with ground mapping.
Gain is controlled manually in these selections.

WEATHER FUNCTION (WEA)

When WEA is selected a conical beam is produced by the radar. A form of “swept gain” is used.
Swept gain is an automatic gain setting that adjusts the returns within a radius of 25 nm. A short-
range non-hazardous return at 5 nm may show up on the radar while a hazardous return at
25 nm may only just be painted. The swept gain adjusts these returns to remove the non-
hazardous cloud off the screen but enhances and amplifies the hazardous cloud return.

CONTOUR FUNCTION (CON)

With a monochrome screen it is difficult to distinguish between the severities of turbulent clouds.
When CON is selected an ISO-ECHO circuit is activated. An ISO-ECHO level supplements the
Automatic Gain Visual Threshold set in WEA mode. The shape of the return on the radar
depends upon the gain level of the cloud:

Automatic Gain Visual Threshold
When a cloud return breaks this threshold, it still appears on the radar scope but only as
a normal cloud return.

Iso-Echo Level
When this gain level is broken, the effect is to create a black hole in the cloud by
reversing and amplifying the signal. This reversal brings the signal below the Automatic
Gain Visual Threshold and so no return appears in the middle of the cloud.

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INPUT SIG NAL - INVERTED
ABO VE THE ISO -ECHO LEVEL

ISO -ECHO LEVEL

AUTO M ATIC G AIN
VISUAL THRESHO LD

30 30
45
45
60 60

R E S U L T IN G P A IN T
T U R B U L E N T C L O U D P A IN T - C O N T O U R M O D E

Turbulence severity is assessable using the above picture:

¾ The hole shows an area of high turbulence.
¾ The outer ring shows the severity of the turbulence gradient within the cloud.

• The narrower the retaining ring, the steeper the gradient.
• The wider the outer ring, the more slack the gradient.

MAPPING FUNCTION (MAP)

For MAP functioning, the conical beam used in the WEA and CON modes is modified into a fan-
shaped or cosecant2 beam. To spread the beam, a system of spoilers is placed on the aerial.
When MAP is selected, these spoilers and the aerial position change the conical beam into the
one shown in the diagram below. In MAP, the power is spread across the beam so that the
maximum range usage is approximately 60 nm.

Aircraft Parasitic
Elements

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Chapter 14 Airborne Weather Radar (AWR)

MANUAL FUNCTION (MAN)

This mode uses the conical beam. Because of the concentration of energy within the beam,
ranges of up to 150 nm can be seen on the radar.

CONTRAST

The contrast rotary switch determines the degree of amplification to the video pulse. It influences
the brightness of the display.

MANUAL GAIN

Signal strength can vary with altitude and the type of terrain the aircraft is flying over. This control
varies the amount by which the returning echoes are amplified by the video processing unit.

TILT CONTROL

The dish aerial sweeps in azimuth and the setting of the tilt control, normally 15° up or down,
determines the variation in elevation. In association with the tilt control, the aerial settings STAB
ON and STAB OFF are used:

STAB OFF: The beam is fixed to the aircraft nose.

EARTH HORIZONTAL

15 10 STAB OFF
UP
PENCIL BEAM LOCKED IN
5
AIRCRAFT PITCHING PLANE
TILT 0

DOWN 5
15
10

ZERO TILT SELECTED

STAB ON: The beam is gyroscopically-fixed to the Earth horizontal. The beam always
looks at the tilt angle selected regardless of the aircraft’s pitch angle.

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COLOUR DISPLAYS

The airborne weather radar displays echoes from clouds having a sufficient concentration of
liquid water. It cannot discriminate between clouds that are likely to produce turbulence and those
that are not likely to do so. Introducing an iso-echo contour circuit overcomes this. Any clouds
having heavy concentrations of large water drops will be associated with turbulence and will also
be good reflectors of radar, especially at 3 to 5 cm. When the contour circuit has been selected
and echoes return above a pre-set level, they are processed to show as black holes on a
monochrome (single colour) screen. Most modern weather radars use colour, and the processing
of the returns produces graded colour paints as follows:

Green Light concentration, slight turbulence
Yellow Moderate concentration, light turbulence
Red Heavy concentration, moderate/severe turbulence
Magenta Heavy concentration of large drops, severe turbulence

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Chapter 14 Airborne Weather Radar (AWR)

CLOUD HEIGHT DETERMINATION

On detecting an active cloud, it is possible to determine the height of the top of the cloud by using
the tilt control. Tilt the beam upward until the cloud echo disappears, then note the tilt angle and
range. Finally, using the 1/60 rule, determine the height above the aeroplane level. Remember to
allow for beam width.

4° BEAMWIDTH
3°
5°
EARTH HORIZONTAL AIRCRAFT
(STAB ON) ALTITUDE

5000 FT

RANGE 30 NM

Echo range 60 nm
Tilt angle at disappearance 4°
Beam width 4° = 4 nm at 60 nm

Beam bottom 2° above horizontal.
Range 60 nm
Beam height 2 nm = 12 000 ft above aeroplane

The simple formula to use is:

Cloud height = (tilt angle – ½ beamwidth) x 100 x range
Cloud height = (4 – 2) x 100 x 60
Cloud height = 12 000 ft above the aircraft

A negative figure means that the cloud is below the aircraft.

At lower altitudes, tilting the aerial down may increase ground returns, and pilots may find it
impossible to differentiate between ground and cloud returns. This may require an upward tilt of
the aerial.

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SHADOW

Water surfaces at any reasonable range reflect the radar energy away from the aeroplane and
give little or no return, causing the screen to remain quite clear. Beware, shadow areas appear
behind hills and could easily be mistaken for water.

Adjusting the gain control can often make a substantial improvement to the radar picture. In order
to interpret the bearing and range information, the pilot must interpret between the bearing marks
to obtain the relative bearing, and between the range markers for the range (which are a slant
range for ground targets).

Contamination of the radome with ice, snow, or rain causes degradation of the radar
performance.

TEST

Most modern radars have a built-in test equipment (BITE) selection. This enables checks of the
display colour patterns and the internal workings of the radar.

HOLD

The hold selection is a method of assessing cloud movement. Once selected, a cloud position is
frozen on the radar screen. Deselecting hold releases the cloud paint to its current position,
allowing the pilot to assess the mean track of the cloud relative to the aircraft.

TARGET ALERT

When the target alert is activated, a yellow T in a red square appears at the top of the screen. If
the system detects a turbulent cloud beyond the selected range, the T changes to TGT and the
symbol flashes.

USE OF THE RADAR ON THE GROUND

The following is a short checklist for using an AWR on the ground:

¾ Prior to start, set the AWR to “off” to avoid any surge currents passing through the
system.

¾ Post start, set the range to SBY and the power to STAB OFF. Selecting STAB OFF
fixes the radar aerial to the aircraft axis and prevents any damage to the gyroscopic
system used in the gimbals of the radar. The radar warms up.

¾ Tilt should be fully up.
¾ When clear of all personnel, buildings, etc., check the timebase for radar returns on

all ranges.
¾ Return to standby.
¾ Before take-off on the runway – power STAB ON, select WEA and tilt as required.
¾ The radar can be switched on before take-off if the aircraft is lined up on the runway.

Radio Navigation 14-9

Chapter 14 Airborne Weather Radar (AWR)

14-10 Radio Navigation

INTRODUCTION

Airborne Doppler Radar systems provide a pilot with ground speed and drift angle information
continuously and automatically. Doppler, unlike most airborne navigation equipment, does not
need any active system outside the aircraft. Although more modern systems have superseded
the use of Doppler, they may still be encountered on older transport aircraft and helicopters.

In 1842, the Austrian scientist, Christian Doppler, noted the changing pitch of the sound
generated by moving vehicles as they approached and then passed beyond a stationary point.
Moving a wave propagation source toward a person, causes the frequency of the sound observed
by that person to be higher than the actual frequency at the source. The shift of frequency
(increasing when the source moves toward the object, decreasing when it is moving away) is
proportional to the velocity of the source. This phenomenon is called Doppler shift. Other names
are Doppler effect or Doppler frequency.

When applying the Doppler principles to an aircraft in flight, with no relative motion between the
radiating source and the receiver (both transmitter and receiver are on board the aircraft), a
Doppler shift still occurs if the transmitted energy returns to the receiver from the Earth. In this
case, the surface of the Earth acts as a reflector.

Frequency
8.8 GHz or 13.5 GHz

DOPPLER EFFECT

Doppler Effect

1

2
3
4

Blueshift +++++ Redshift
P4 P3 P2 P1

As the Transmitter moves to the left, the waves are compressed toward the blueshift. The
intervals between the waves diminish and this apparent shortening in wavelength causes an

Radio Navigation 15-1

Chapter 15 Doppler

increase in frequency or pitch. The frequency appears to increase. As the transmitter passes, the
waves are now stretched. The interval between each wave increases and causes a decrease in
frequency and pitch at the redshift side of the diagram. The frequency appears to decrease.

The diagram above shows a stationary transmitter. Assume that the frequency of transmission is
f Hz. The receiver receives the successive wave fronts at f Hz.

Now assume the transmitter is moving toward the receiver and the velocity is V m/s. The first
wave front is centred on 1, the second on 2, etc. The effect is to decrease the wave front spacing
as the transmitter travels toward the receiver. This appears to the receiver as an increase in the
received frequency. Behind the transmitter, the wave front spacing increases, and a receiver
placed opposite Rx would experience a reduced frequency. This change in frequency is the
Doppler shift.

The change in frequency, the Doppler shift, is represented by fd and is proportional to the
transmitter’s velocity:

fd = Vf = V
cλ

c is the speed of propagation
f is the transmitted frequency
V is the velocity of the transmitter

15-2 Radio Navigation

Doppler Chapter 15

Example: Assume a transmitter (9 GHz) is moving toward a stationary receiver at
300 km/h. What frequency will the receiver record?

First convert 300 km/h to m/s

300 x 1000 ÷ 3600 = 83 m/s

The wavelength for a frequency of 9 GHz is = 0.03 m

fd = V
λ

fd = 83.3 ÷ 0.03 = 2777 Hz

This is the Doppler shift that must be added to the transmitted frequency
to arrive at the received frequency. (If the transmitter is moving away
from the receiver then the Doppler shift would be subtracted from the
transmitted frequency.)

Received frequency = 9 000 000 000 + 2777 = 9 000 002 777 Hz

It is not likely students will have to make any Doppler calculations for the JAR exam. The
example above is probably the style of question that would be asked.

DOPPLER MEASUREMENT OF GROUNDSPEED

Since the aircraft cannot fly directly toward a reflecting surface, the radar beam must tilt
downward. The measured Doppler shift must be mathematically corrected for a tilt angle (θ) in
order to resolve the aircraft’s forward horizontal velocity. The diagram below shows an aircraft
transmitting a narrow radar beam toward the ground at an angle θ (called the depression angle).
The system receives the beam at a different frequency than that transmitted.

The basic Doppler shift formula becomes: 15-3
fd = 2VF cos θ
C

Radio Navigation

Chapter 15 Doppler

The frequency received (Fr) on a forward facing beam will be:

Frequency transmitted (Ft) + 2VF cos θ
C

Where θ is the angle of depression of the beam (the angle between the horizontal and the
direction of the beam). The equation shows that the Doppler shift is at its highest when the angle
θ is shallow, or when the beam faces forward in the flight direction. This is impossible, as there is
no reflecting surface in front of the aircraft. If the pilot makes the beam depression too large, the
Doppler shift is very small, becoming zero when the beam is directed vertically downward. The
choice of beam depression angle is a compromise between these two extremes, a matter of
trying to obtain reliable energy returns with sufficient Doppler shift to be accurately measured. An
angle of approximately 67° normally achieves this.

The system considered here is a single-beam system that has certain disadvantages:

Pitch Error
Changing the aircraft’s pitch changes the θ. If the aeroplane pitches nose down, the
reflected beam in front would depress greater than θ and the Doppler frequency shift
would decrease, even if aircraft speed remains constant. For the system to work
correctly, the aircraft must either fly straight and level at all times, or the aerial must be
stabilised in the pitching plane.

Vertical Motion
Any vertical motion of the aircraft generates a change in the Doppler shift not associated
with the groundspeed.

Drift
If the aerial is fixed, the system measures the speed along the heading. However,
groundspeed is calculated along track. A system design should allow the aerial to move
in azimuth to obtain maximum Doppler shift. This would provide a drift angle as well as a
groundspeed. In practice, this is not done, and the system remains inaccurate.

TWO BEAM JANUS ARRAY

A common way of solving this problem is to transmit a second beam backward from the aircraft,
at the same depression angle as the front beam. The system then algebraically adds the two
sensed Doppler shifts and calculates the ground speed based on this information. This is termed
a Janus system, named after the Roman god, Janus, who could look forward as well as aft at the
same time.

Depression Depression
Angle θ Angle θ

Surface

15-4 Radio Navigation

Doppler Chapter 15

The frequency received from the forward beam (fr) is higher than the frequency transmitted.

fr = f + fd

The frequency received from the rear beam (fr) is lower than the frequency transmitted.

fr = f - fd

Using a two-beam system requires adding the two received frequencies to yield a total received
Doppler shift ft as follows:

ft = (f + fd) - (f - fd) = 2 fd
or

4VF cos θ
C

The total frequency received is double that received from a single beam system.

Pitch Error
Aircraft pitch causes an increase in depression angle for one beam and a decrease in
depression angle in the other beam; thus effectively cancelling each other out.

Vertical Speed
Both front and back beams sense the change in vertical speed and in the summation
process they cancel each other out.

FURTHER JANUS ARRAYS

Four different types of array appear below.

By using any of the systems above, the drift angle will be sensed since drift occurs across the
intended track. To ensure the calculation of drift angle requires using three or four beams, each
pointing in different directions. The configurations have names that reflect their appearance. For
example, the three-beam lambda configuration is similar to the Greek letter λ.

Radio Navigation 15-5

Chapter 15 Doppler

In the four-beam system, the aerials transmit front left/rear right and then front right/rear left. If the
aircraft has no drift, the Doppler shift measured by the two sets of aerials is the same in each
sequence of transmissions. If there is sideways drift, one set of aerials receives a larger Doppler
shift than the other set. The system electronically calculates this difference as an error signal that
rotates the aerial to ensure that the two signals received are the same. The angle of movement is
the drift.

DOPPLER AERIAL

The Doppler aerial system consists of three or four slotted arrays giving shaped beams, each with
a width that varies between 5.5° to 11°. The beams are depressed to an angle of 67° to provide a
measurable Doppler shift. Aerial systems may be hard strapped to the airframe and, if so, cause
some small errors in measured groundspeed during prolonged climbs. Alternatively, they may be
gyro stabilised in pitch to reduce these errors.

SIGNAL CHARACTERISTICS

The sensitivity, and thereby the accuracy, of a Doppler radar increases with frequency. However,
the higher frequency, the more rain reflections, scattering effects, and absorption affect the
signal. A compromise is made, and as a result Doppler airborne radar equipment operates on two
frequency bands: 8.8 GHz and 13.5 GHz. In selecting the actual frequency to use, the pilot must
take into account:

¾ The operational height required
¾ The power output available
¾ The speed of the aircraft

A helicopter Doppler normally uses the higher frequency in order to increase the Doppler shift at
the low speeds.

OUTPUT AND PRESENTATION

Doppler equipment measures drift and groundspeed. If combined with a heading input, it gives
track and ground speed. This can feed into a specific display or, more commonly, be integrated
into a navigation computer, which can add the measured values to a start position to produce
outputs such as:

¾ Position
¾ Distance to go
¾ Bearing and distance to a waypoint
¾ Current W/V

15-6 Radio Navigation

Doppler Chapter 15

As shown in the diagram below, the functions important to the pilot on a Doppler control unit are:

G
F

25 15 5 2 2 5 15 25

TRACK ERROR E
N D
C
WE B

H S A

SEA SLEW
LAND

ST L L
A O
J BY T N
G
DOP WAY
TEST POINT

10 WP
DIM BRG

K LMP DIST
TEST GS

HDG DFT
VAR
FIX POS

L

A Display Switch
B Latitude/Longitude selection switch
C Slew switches
D Light bars E/W or N/S as selected
E Track error indicator
F Warning bars
G Power supply test bars
H Numeric displays
J Function switch
K Display dimmer switch
L Waypoint selection

Certain functions require understanding for the syllabus:

STBY is the setting used during equipment warm-up.

SLEW allows the pilot to set a drift and ground speed if the equipment is operating in
Memory mode.

LAND/SEA creates a mathematical solution to compensate for partial loss of the beam.
When flying over the sea or flat surfaces, the switch should be set to ‘sea’. (See sea
bias.)

Radio Navigation 15-7

Chapter 15 Doppler

ACCURACY AND LIMITATIONS

Modern Doppler radar systems are quite accurate. Expect errors in the order of 0.1 % on ground
speed and 0.15% on drift angle.

SEA BIAS

When flying over the sea, the leading edge of the forward beam and trailing edge of the rear
beam are lost due to increased reflection away from the aircraft. This causes the measured
values of ground speed to be lower than the true ground speed. Selecting the ‘Sea’ position on
the land/sea switch provides a bias, which offers some compensation for this effect. The
Land/Sea switch discriminates between the Doppler frequency over water (fdw) and land (fdl)
when switched to sea. The calibration of the tracker unit is altered to increase the groundspeed
by a nominal 1 – 2%.

Measured fd Correct fd for
Beam Centre Line

Returned Power Over Water
Calibration
Shift

Land
Spectrum

Water
Spectrum

fdw fdl f

MEMORY MODE

If the sea becomes too smooth, (surface wind less than 5 kt) nearly all the energy scatters away
from the aeroplane and no measurements are possible. Under these circumstances the system,
activates a Memory Mode and the drift and ground speed are frozen at the last measured
values. These may not be correct values, but will be used by any associated equipment.
Examples of other situations that cause the system to go into Memory are:

¾ Flight over hot desert regions where the attenuation is high
¾ Flight in adverse weather such as severe thunderstorms. The water content of such

storms causes excessive scattering of the energy.

PITCH AND ROLL ERROR

Errors due to pitch or roll are cancelled by the Janus array. When both pitch and roll occur
simultaneously, an error is likely to occur. Limitations for pitch and roll are typically ± 20° and ±
30°, respectively. Beyond these, the system loses data and enters the Memory Mode.

HEIGHT HOLE ERROR

In a pulse-modulated system, when the Doppler radar is transmitting, the receiver is shut off. Low
flying may introduce an error to the system because the transmitted signals reflect back to the
aircraft before the receiver is functioning again. Likewise, when flying at high altitudes, the time
delay between transmission and reception may cause problems, reception may occur at exactly
the same time that a new transmission is taking place. In such cases, the receiver is short-
circuited.

15-8 Radio Navigation

Doppler Chapter 15

SEA MOVEMENT ERROR

Doppler measures the drift and groundspeed relative to the terrain beneath the aircraft. If the
surface is moving, as is the case with the sea, the following factors may induce errors:

Tidal Stream
Tidal streams normally affect narrow waterways. Since the time the aircraft will be over
this type of feature is small the effect is minimal.

Ocean Currents
Ocean current speeds are slow, and thus have little effect.

Water Transport
The surface wind causes wave movement on the surface of large tracts of water. This
error is complex to understand but the resultant error can affect both drift and
groundspeed.

COMPUTATIONAL ERRORS

Processed Doppler information may be subject to heading input errors that are probably more
significant than those of the measured ground speed. The Doppler is based on an assumption
that 1 nm is equivalent to 1 minute of latitude. This is only correct at 47°42’N/S, and on the
surface of the Earth. As soon as an aircraft is at height, the assumption is again not true. Both
errors are small and are not corrected for.

HEADING ERROR

This is the greatest error in Doppler. The system relies on the accuracy of the heading input
information, which, if wrong, can cause considerable cumulative errors.

SUMMARY OF ERRORS

Error Error Measurement of

Aerial Misalignment Distance Track
Pitch Error
No Yes
Sea Movement
Yes No
Sea Bias
Heading Error A vector error dependent on the time spent over the water,
Altitude and Latitude the direction and movement of the sea and the wind velocity

Yes No

No Yes

Yes No

ADVANTAGES

As mentioned above, the Doppler navigation system is quite accurate. It does not require any
external equipment for basic operation (although updating would require external means). The
system has good, long-term accuracy irrespective of flight time, and the measured ground speed
and drift retain the same accuracy potential. If combined with a short-term accuracy system, the
overall accuracy is excellent.

Radio Navigation 15-9

Chapter 15 Doppler

DISADVANTAGES

Doppler can only give an instantaneous value of drift and ground speed and requires a link to
other equipment. This makes the derived position information dependent upon the accuracy of
such inputs as heading and TAS.

¾ The equipment is very prone to ‘loss of signal’ and entry into Memory Mode.
¾ The equipment is costly to maintain.

15-10 Radio Navigation

INTRODUCTION

Hyperbolic navigation systems determine present position from the intersection of lines of
position. While radials and bearings from VOR and NDBs are straight lines, in hyperbolic
navigation a line of position is based on a curved line - specifically, a hyperbolic curve and, more
precisely, a hyperbolic surface.

A Hyperbola is a line joining all points where the difference of distance between two
fixed points is the same.

HYPERBOLIC FAMILY

¾ Draw a straight line of 4 cm and label the ends M and S.
¾ Draw a line perpendicular to M – S, 2 cm from M. Label it A-B.
¾ Draw circles starting with a radius of 1 cm from M and S.
¾ Do this for 2 cm, 3 cm, 4 cm, etc.
¾ This produces a diagram like the one below:

A

33

MS

B

Look at the line AB. Where AB cuts the line MS is the meeting point of the 2 cm radius circle
drawn from M and the 2 cm circle drawn from S. The difference between the two circles is an
answer to the equation 2 – 2, or zero.

Radio Navigation 16-1

Chapter 16 Hyperbolic Navigation

The 3 cm circles meet at two points on the line AB, one above the line MS and one below. The
same for the 4 cm line, etc. The difference between the circles is zero (3 – 3, 4 – 4, etc). The line
AB is a hyperbola.

Now plot the following points on the diagram. Label each point from X and X1 to Z and Z1.
Distance from S – 5 cm, Distance from M – 3 cm
Distance from S – 4 cm, Distance from M – 2 cm
Distance from S – 3 cm, Distance from M – 1 cm

Each of the circles intersects the others at two points. With the line X-X1 drawn, the diagram
should look like the one below.

A

x
y

z S

M

y1
x1

B

The difference between the circles is 2 (5 - 3, 4 – 2, 3 – 1, etc).

By plotting all the variables, it is possible to establish a family of hyperbolas, shown by the curved
lines. Now assume that these are range lines. By establishing the difference in range between
that from the aircraft’s own position to the M and S points respectively, it becomes possible to use
these hyperbolas as LOPs.

Hyperbolic radio navigation aids operate on the principle that the difference in time of arrival of
signals from two stations is a measure of the difference in distance from the point of observation
to each of the stations. Since radio signals travel at a constant speed (essentially the speed of
light), these measurements can translate the differences in time directly into differences in
distance.

16-2 Radio Navigation

Hyperbolic Navigation Chapter 16

The base line is the line joining M – S, in a hyperbolic navigation system. Note that each
hyperbola cuts the base line at half the difference in range from the central 0 hyperbola.

¾ The 1 cm line cuts the base line at ½.
¾ The 2 cm line cuts the base line at 1.

The lines behind M and S are referred to as base line extensions while the straight line at right
angles to the centre of the base line is called the base line bisector.

Area of Most
Ambiguity Accurate
and Poor
Accuracy

The base line extensions and the base line bisector are straight hyperbolas.

The spacing of the position lines varies. They are constant along the base line but diverge with
increasing distance from the base line. The rate of divergence is least near the base line bisector,
increasing toward the base line extensions. As a result, the accuracy decreases more rapidly in
the vicinity of the base line extensions.

Radio Navigation 16-3

Chapter 16 Hyperbolic Navigation

LINES OF POSITION (LOP)

Due to the curvature of the hyperbola, the same LOP exists on both sides of the base line
extensions. This gives the risk of an ambiguous situation that is not acceptable.

In the application of this principle, one transmitter (M) is used to stabilise or control the
transmissions of the second transmitter (S). The control transmitter is known as the Master and
the other is called the Slave (or Secondary). Since the signals from the Master are used for
control and synchronisation of the master/slave pair it follows that there must be reliable
communication between the two beacons. This communication is normally done by radio so the
two transmitters must be within radio range of each other, ensuring predictable and reliable signal
to noise ratios.

It requires one Master/Slave combination to produce a single LOP (line of position). To determine
a fix requires a minimum of two LOPs and therefore the use of two Master/Slave combinations.
Like all fixes, however, the angle of intersection of the LOPs affects the fix accuracy.

Each LOP has a band of error. If the LOPs intersect at 90° the respective bands of error form a
small rectangle enclosing the position of the receiver. If the LOPs intersect at 30° the area
becomes an elongated diamond. This considerably increases the area of possible positions. In a
hyperbolic system, base line bisectors intersecting at 90° provide the best fixes.

ERRORS OF HYPERBOLIC NAVIGATION

PROPAGATION ERRORS

Since the determination of the LOP depends upon the comparison of time of arrival of the two
signals, anything that affects the accuracy of that timing causes errors. The timing assumes that
the speed of propagation is constant and the same for both signals. If the Master signal travels
over the sea to reach the receiver, and the Slave signal propagates over the land, the
propagation speeds are slightly different and the accuracy of the timing disrupted. Where these
propagation errors are known to exist, either distorting the hyperbola or publishing corrections
generally accounts for them.

16-4 Radio Navigation

Hyperbolic Navigation Chapter 16

HEIGHT ERROR

Note that the hyperbolas are indeed three-dimensional. A constant range differential is
observable not only along the Earth’s surface but also on a three-dimensional curved hyperbolic
surface. The hyperbolic surface is a vertical plane over the base line bisector but, in the vicinity of
the transmitters, it becomes noticeably more curved. For charts drawn for a hyperbolic navigation
system, it is normal for the hyperbola to be drawn for Sea Level.

Although an aircraft at altitude may be on the same curved hyperbolic surface represented by a
plotted LOP, the plotted LOP shows only the line where this curved surface intersects Sea Level.
Using the plotted LOP introduces an error, termed Height Error.

SIMPLE HYPERBOLIC CALCULATION

Example: A hyperbola cuts the base line 100 km from the Master end and 150 km
from the Slave end. When on the same hyperbola at a range of 120 km
from the Master, the range from the slave will be:

Draw the situation.

Hyperbola is a line joining all points where the difference of distance from two fixed points
is the same.

The difference between the M – S base line cut is 50 km.

Therefore the difference in distance must be the same at 120 km.

Therefore the distance from the slave is 120 + 50 km = 170 km if the aircraft is on the
same hyperbola.

Radio Navigation 16-5

Chapter 16 Hyperbolic Navigation

Example: An aircraft is 120 km from the Master and 170 km from the Slave. How
far from the Slave will the hyperbola cut the base line? Master – Slave
distance is 250 km:

Draw the situation.

120 km 170 km

M S
125 km
125 km

Look at the range difference between the Master and the Slave = 50 km.
Halve this difference = 25 km.
This is the distance the hyperbola crosses the base line from the central perpendicular.
Therefore, the hyperbola cuts 125 + 25 km = 150 km from the Slave.

16-6 Radio Navigation

INTRODUCTION

LORAN is an acronym for LOng RAge Navigation, developed in the Second World War with the
intention of providing aircraft with a hyperbolic navigation system that did not suffer from sky
wave contamination.

PRINCIPLE OF OPERATION

LORAN C is a hyperbolic navigation system. The system was scheduled to be discontinued in
2000 but some users made strong moves to retain its operational status. LORAN C stations are
grouped in a network or chain, with one station as a master and the other stations arranged
around it as secondary (slave) stations. Secondary stations are identified by the letters W, X, Y or
Z. The chains covering the North Atlantic appear below. The distance between master and slave
is between 600 and 1000 nm.

Z 7970 X 7970

SWEDEN

GREENLAND NORWAY

X 7930 7970 Y ICELAND X 9980
W 9980 M
M 7970
9980
U. K.
IRELAND W GERMANY
7970

M 7930

LABRADOR Icelandic Chain Norwegian Sea Chain
Loran GRI 9980 Loran GRI 7970
W 7930
M Sandur ICLD M Eidhi Faroes DNK
Labrador Sea Chain W Angissog GRLD W Sylt GER
Loran GRI 7930 X Eidhi Faroes DNK X Bo NRWY
M Fox Harbour LB Y Sandur ICLD
W Cape Race NFLD Z Jan Mayen NRWY
X Angissog GRLD

Coverage extends from Asia, over the USA, North Atlantic Region and Europe.

Radio Navigation 17-1

Chapter 17 Loran C

LORAN C uses the principle of differential range by pulse technique.

Frequency
90 to 110 kHz

TYPICAL LORAN C CHAIN

In the diagram above, three chains cover the North Atlantic. Looking at the individual composition
of the Norwegian Sea Chain:

Station Position Designation Location
Eidhi Faroes Master 7970 Faroe Islands
Slave 7970W
Sylt Slave 7970X Germany
Bo Slave 7970Y Norway
Sandur Slave 7970Z Iceland
Jan Mayen Arctic

The coverage requirements determine a station’s location with respect to other stations in the
chain. A monitor station is also part of a LORAN C chain, and is usually located at one of the
transmitting stations.

The distance between master and secondary stations (base line) varies between 600 and
1000 nm. LORAN C signals are transmitted at low frequencies, around 100 kHz. Low frequency
signals travel as surface waves over the earth giving LORAN C a theoretical range of up to 1500
nm over water. The master station in a chain transmits first, and its signal is used as a reference.
Each secondary station has a unique emission delay that allows the aircraft to receive the signal
from the master before the secondary transmits. LORAN signals are transmitted in pulses in the
shape of an elongated pear. Signals from the master stations are sent out in groups of nine
pulses with a pause after the eighth. Signals from the secondary stations are sent out in groups of
eight pulses.

LORAN C TRANSMISSION

All the masters and slaves transmit on the same frequency of 100 kHz. To ensure that there are
no chain identification problems, each chain is allocated a specific PRI. The time that elapses
from the beginning of one master pulse group to the beginning of the next is different for each
LORAN chain. The name for this time interval is the group repetition interval (GRI). For each
chain, a minimum GRI is selected. It must be of sufficient length to allow time for the transmission
of the pulse group from each station, plus the time between each pulse group. The GRI is also
used as a way to label LORAN chains. For example, a GRI of 79 700 microseconds becomes
chain 7970 by dropping the last significant zero. Chain 7970 corresponds to the Norwegian Sea
Chain. Permissible GRIs are multiples of 10 microseconds from 40 000 through 99 990
microseconds.

OPERATION

Using the principle of differential range by pulse technique, it is necessary to look at the way an
operator deciphers the system. Weather or static build up can affect transmissions. The multi-
pulse system minimises the problems of low frequency long-range transmissions. The master and
slave pulses are sent in groups of eight, with each pulse separated by 1000 microseconds. The
master pulse is identified by an extra pulse which is transmitted 2000 microseconds after the
main group. The pulses are then summated to give one strong pulse. Each pulse is between 180
and 270 microseconds long.

17-2 Radio Navigation

Loran C Chapter 17

The operator will receive a train of pulses as shown above. The master transmits first followed by,
the slaves in the order W, X, Y, and Z.

The operator looks at the chain and chooses the two slaves that will give the most accurate fix. At
the same time the master is gated to align the timing of the system.

Once the system has been gated, the matching of the received signals is carried out to match the
cycles.

The measurement of time difference between the master and relevant slave is obtained from the
pulse. The third cycle is always used, as this is never contaminated by returning sky waves once
it has been identified. For a 100 kHz radio frequency, the period (1/T) is 10 microseconds which is
also the accuracy obtainable.

The LORAN C receiver uses the pulse envelope to find the timing point on a particular cycle. If
the pulse is distorted, due to a poor signal for instance, the cycle tracking point may jump to the
next pulse causing an error. This is how the 10 microsecond accuracy is determined. An error of
10 microseconds in the time measurements equals a typical 1 nm position error in the area
around the base line. The position error increases away from the base line.

Radio Navigation 17-3

Chapter 17 Loran C

W

Baseline 1 nm 10 M sec

3 nm 10 M sec
M

10 nm 10 M sec

To minimise this error, LORAN C determines the time intervals by calculating the time between
pulses and cycle matching within the pulses by comparing the cycles of radio frequency within
the master signal with those within the slave signal. This makes it possible to determine the time
interval to an accuracy of 10 microseconds.

The measured difference depends on the aircraft’s position in relation to the master slave pair.
On the slave base line extension, the total time difference is base-line propagation time plus slave
delay and will be the smallest value.

On the master base line extension, the delay is twice base line propagation time plus slave delay
and will be the largest value. The area in the vicinity of the base line extensions is not a good
LOP area due to spread of the hyperbola and high risk of ambiguity. Each pulse within a group
may have its radio frequency cycles organised in a pattern of regular or inverted pulses. This
phase coding enables the receivers to distinguish the master signal from the secondary stations
and also helps to reduce sky wave and noise interference.

In summary, the propagation delay between the pulses from the master and slave at every chain
are:

Baseline Extension from the Slave is the time delay at the slave.

Baseline Extension from the Master is twice the time the pulse takes to travel from the
master to the slave plus the time delay at the slave.

The Baseline Bisector is the time of travel of the pulse to the slave plus the time delay
at the slave.

All other propagation delays are between the above figures.

17-4 Radio Navigation

Loran C Chapter 17

COVERAGE, LIMITATIONS, AND ACCURACY

LORAN C COVERAGE

Even though most of the major avionics manufacturers now produce LORAN C systems, LORAN
C’s nautical origin is still evident in several ways. One of the most obvious is LORAN C’s
continuing orientation toward over-water areas. Most of the existing chains exist in coastal areas
or on islands, providing fairly complete North Atlantic and Pacific coverage. The western part of
northern Europe and the Mediterranean Sea has almost complete LORAN C coverage. No
coverage currently exists in the Southern Hemisphere. This limits LORAN C capability as an
international long-range navigational system.

SKY WAVES

LORAN C is a low frequency system, which means that LORAN C unit sometimes receives the
sky wave in addition to the normal and preferred ground wave. Since the sky wave travels a
greater distance than the ground wave, it takes a longer period of time to reach the receiver. At
certain distances from the transmitter this may adversely affect the accuracy of the plotted LOP.

Up to distances of 1000 nm from the transmitter, the ground wave dominates and the receiver
can fairly easily distinguish ground waves from sky waves. At greater distances however, usually
over 1500 nm, the ground wave becomes so weak that the sky wave begins to dominate. The
ground wave is more stable and reliable than the sky wave and is preferred for navigation
purposes. Modern LORAN C receivers have circuits that enable them to differentiate between the
stronger ground wave and the weaker sky wave. Most LORAN C receivers are able to make use
of the sky wave signals at this range, but accuracy is reduced.

The problems arise at the intermediate distances, from approximately 1000 to 1500 nm. At these
distances, the sky wave and the ground wave are about equal in strength. This may confuse the
receiver and distort the results, particularly at night. The phase coding of the transmitted pulses
helps the receiver in solving the problems of interfering sky waves.

STATIC DISTURBANCES

The modern LORAN C receiver constantly monitors the signal to noise ratio (SNR). The SNR is a
ratio given by the signal received, divided by noise due to static or other disturbances.
Precipitation static causes electronic noise, and this noise causes the SNR to drop. LORAN C
receivers normally issue a warning when the ratio drops below a certain minimum level. The
static charge picked up when flying through precipitation can seldom be completely eliminated but
it can be reduced by airframe grounding and by installing static discharge wicks. Other electrical
disturbances, such as those produced by thunderstorms, may also have an influence on
LORAN C accuracy.

RADIO PROPAGATION SPEED

The accuracy of LORAN C is related to the propagation speed of the radio waves, which is
assumed to be constant. This is not the case. The speed of radio waves is influenced by factors
such as the time of day, the season, and especially by different kinds of terrain. LORAN C was
designed as an over-water system, and all of the assumptions as to the signal speed and
transmission are based on signals propagated over seawater. Position calculations based on
signals propagated over land have slight errors. Modern LORAN C receivers have compensating
circuits, called additional secondary factors (ASF) circuits, to correct for this condition. These
propagation errors are usually not significant. They only affect the accuracy slightly and within
predictable limits. The pilot should be aware that peak accuracy can only be obtained with
LORAN C when operating over oceanic areas using signals from transmitters situated on islands
or coastal regions.

Radio Navigation 17-5

Chapter 17 Loran C

GEOMETRY OF CROSSING ANGLES

A factor that affects accuracy is the angle at which the hyperbolic LOPs intersect each other.
LOPs that cross at 90° angles produce small, square areas of position between hyperbolae.
LOPs that cross at oblique angles produce large, diamond shaped areas between hyperbolae.
Furthermore, the distance from the LORAN C station affects accuracy by increasing the distance
between LOPs. A 10-microsecond change represents approximately one nautical mile close to
the base line, compared to eight nautical miles near the base line extension. These distances
both increase since the hyperbolae spread with a distance increase from the base line. Due to
ambiguity, the area around the base line extension is not usable for LORAN C navigation even
though the aircraft may be close to the station. Interference from navigation and communication
stations within the same band as LORAN C may influence LORAN C accuracy. To cope with
interference from other transmitters within the same band, the manufacturers install notch filters
at the receiver input, which reduce interference from transmissions on adjacent frequencies.

USE OF LORAN C

The use of LORAN was more complex before the introduction of new electronic processors.
Signals from the master and secondary stations were presented on the horizontal trace of a
cathode ray tube and appeared initially as vertical blips on the linear time base (as shown on
page 17-3). The cathode ray tube displayed the master and secondary pulses. The operators
electronically moved the two pulses and compared them, reading the time difference between the
master and secondary pulses as the distance X. This distance corresponded to a hyperbolic LOP.
The operator plotted this line of position on a LORAN chart that had all the hyperbolic lines drawn
on it. The process was repeated to find another line of position by using another
master/secondary pair in the chain.

With the introduction of electronic processors came the development of a new generation of
LORAN C receivers. These very sophisticated receivers perform the same operations the
navigator did, but in a much shorter time. They also provide present position instantaneously. The
new LORAN receivers not only provide position fixes, but can also provide additional information
such as bearing, track, groundspeed, and drift angle without the need of any manual operation.
There are many different types of control display units (CDU) for LORAN C operations.

There are also more expensive and sophisticated CDUs, which are intended for larger aircraft
and are generally designed to fit in the control stand on the pilot’s side with the bulk of the
electronics located in a remote unit. Although still found in many aircraft, it is unlikely that newer
aircraft will have them since the future of the system is very much in doubt.

There are currently 15 chains worldwide. Most LORAN C units require the pilot to manually select
a specific LORAN C chain. The pilot uses the GRI to tune the desired chain. For instance, the
GRI for the Norwegian Sea is 79 700 microseconds. The identification for this station is 7970. In
more sophisticated units, an internal memory remembers the unit’s position when the set is shut
off. This position is used to find the applicable chain when power is switched on again. Once a
GRI has been selected, the unit then goes into search, identify, and track cycle. During the cycle,
the unit attempts to receive the master station for the GRI selected and then looks for the two
best secondary signals in that chain.

17-6 Radio Navigation

Loran C Chapter 17

LORAN C NAVIGATION

LORAN C can provide point-to-point navigational guidance with great reliability and accuracy and
is easy to use. Specific procedures vary from unit to unit, but essentially, once the unit
establishes the point of origin, the pilot can determine a route between waypoints and follow it,
using the additional information provided by the unit. Several methods of entering the point of
origin into the unit exist. For instance, latitude and longitude co-ordinates can be entered
manually. Selecting a previously stored airport identifier or waypoint can also enter Latitude and
Longitude. Finally, selecting position mode and allowing the LORAN C to determine the present
position by itself can automatically enter the point of origin. This last procedure is usually
accomplished in flight.

Once present position has been entered, the destination waypoint is entered, either by
LAT/LONG co-ordinates, or by waypoint identifier. At this point, the unit can be switched to
navigation mode and track guidance is provided on a CDI, an HSI, or on a dedicated LORAN
instrument. The most direct method of tracking using LORAN C is by using the cross track
distance (XTD,) or the distance between the desired track and the actual track. Directly displayed
on the receiver’s monitor, XTD is the actual distance the aircraft is off track. If, for instance, the
LORAN C unit indicates a cross track distance 1.2 nm right of track, the pilot can simply correct to
the left and monitor the cross track distance decreasing as the aircraft approaches the desired
track. Many modern sophisticated Navigation Processing units accept LORAN C position
information for use alongside that from other sources.

The development of GNSS would appear to mean that the use of LORAN will become rapidly
less important in aviation.

TRANSMITTER FAULT INDICATION

The ninth pulse in the master transmission can be coded to indicate a variety of problems.
Normally the blink is the Morse code letter R (• – •). A slave station can also blink the first two
pulses of transmission to indicate a fault. Using a blink code indicates:

¾ Station not transmitting
¾ Incorrect phase coding
¾ Incorrect number of pulses
¾ Incorrect pulse spacing
¾ Incorrect pulse shape
¾ Time difference outside specified limits

Both master and slave stations blink if either station is operating incorrectly.

Radio Navigation 17-7

Chapter 17 Loran C

RANGE AND ACCURACY

Both range and accuracy can be affected by the following:

¾ The power and accuracy of the ground transmission
¾ The path of the surface wave
¾ The propagation conditions for sky waves
¾ The receiver and aerial employed
¾ Noise in the reception area
¾ The aircraft’s position in the chain

The approximate daytime ranges of LORAN are:

Ground Wave 1200 nm over water
900 nm over land

Sky Wave Up to 2500 nm by day or night

ACCURACY LIMITS

Ground Wave ± 0.5 nm on 95% of occasions, up to 1000 nm over the sea

Sky Wave Sky wave should only be used when out of ground wave range.
10 – 15 nm at 1500 nm. Up to 20 nm at 2500 nm

17-8 Radio Navigation